Protecting BitLocker From Pre-Boot Attacks

CountermeasuresProtecting BitLocker-encrypted devices from attacks

January 2014

Table of contents

3 Attacks3 Bootkit and rootkit attacks5 Brute-force sign-in attacks5 Direct memory access attacks7 Hyberfil.sysattacks8 Memory remanence attacks

10 Countermeasures10 Protection before startup14 Protection during pre-boot: pre-boot authentication16 Protection during startup17 Protection after startup: DMA attack protection

18 Choosing the right countermeasures

21 Summary

1PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

CountermeasuresProtecting BitLocker-encrypted devices from attacks

Full-volumeencryptionusingBitLockerDriveEncryptionisvitalforprotectingdataandsystemintegrityondevicesrunningtheWindows8.1,Windows8,orWindows7operatingsystem.ItisequallyimportanttoprotecttheBitLockerencryptionkey.OnWindows7devices,sufficientlyprotectingthatkeyoftenrequiredpre-bootauthentication,whichmanyusersfindinconvenientandcomplicatesdevicemanagement.

MicrosofthasmadeimprovementsinWindows8.1andworkedcloselywithhardwaremanufacturerstodeliverWindows8.1andWindows8devicesthatarefundamentallyresistanttoknownattacksagainsttheBitLockerencryptionkey.Asaresult,many organizations can now meet their security requirementswithoutusingpre-bootauthentication,reducingcomplexityandinconvenience.

Thispaperprovidesdetailedinformationthatwillhelpyou understand the circumstances under which the use of pre-boot authentication is recommended and when it canbesafelyomittedfromadevice’sconfiguration.

NOTE

Forthelatestinformation,pleaseseehttp://aka.ms/bitlockerinfo.

2PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

BitLockerusesencryptiontoprotectthedataonyourdrive,butBitLockersecurityisonlyeffectivewhentheencryptionkeyisprotected.Manyusershavereliedonpre-bootauthenticationtoprotecttheoperatingsystem’sintegrity,diskencryptionsolution(e.g.,encryptionkeys),andthePC’sdatafromofflineattacks.Withpre-bootauthentication,usersmustprovidesomeformofcredentialbeforeunlockingencryptedvolumesandstartingWindows.Typically,theyauthenticatethemselvesusingaPINoraUSBflashdriveasakey.

Pre-bootauthenticationprovidesexcellentstartupsecurity,butitinconveniencesusersandincreasesITmanagementcosts.EverytimethePCisunattended,thedevicemustbesettohibernate(i.e.,shutdownandpoweredoff);whenthecomputerrestarts,usersmustauthenticatebeforetheencryptedvolumesareunlocked.ThisrequirementincreasesrestarttimesandpreventsusersfromaccessingremotePCsuntiltheycanphysicallyaccessthecomputertoauthenticate,makingpre-bootauthenticationunacceptableinthemodernITworld,whereusersexpecttheirdevicestoturnoninstantlyandITrequiresPCstobeconstantlyconnectedtothenetwork.

IfuserslosetheirUSBkeyorforgettheirPIN,theycan’taccesstheirPCwithoutarecoverykey.Withaproperlyconfiguredinfrastructure,theorganization’ssupportwillbeabletoprovidetherecoverykey,butdoingsoincreasessupportcosts,andusersmightlosehoursofproductiveworktime.

Windows8andnewdevicesdesignedforWindows8changeeverything.TheUnifiedExtensibleFirmwareInterface(UEFI)SecureBoot and Windows Trusted Boot startup process ensures operating systemintegrity,allowingWindowstostartautomaticallywhileminimizingtheriskofmaliciousstartuptoolsandrootkits.Inaddition,manymodernmobiledevicesarefundamentallyphysicallyresistanttosophisticatedattacksagainstthecomputer’smemory,andnowWindowsauthenticatestheuserbeforemakingdevicesthatmayrepresentathreattothedeviceandencryptionkeysavailableforuse.

ThesectionsthatfollowhelpyouunderstandwhichPCsstillneed pre-boot authentication and which can meet your security requirementswithouttheinconvenienceofit.

TheUnifiedExtensibleFirmwareInterface(UEFI)SecureBootandWindows Trusted Boot startup process ensures operating system integrity,allowingWindows to start automaticallywhileminimizing the risk of maliciousstartuptoolsandrootkits.

3PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

AttacksThenextfewsectionsdescribeeachtypeofattackthatcouldbeusedtocompromiseavolumeencryptionkey,whetherforBitLockeroranon-Microsoftencryptionsolution.Afteranattackerhascompromisedavolumeencryptionkey,theattackercanreaddatafromyoursystemdriveoreveninstallmalwarewhileWindowsisoffline.Eachsectionbeginswithagraphicaloverviewoftheattack’sstrengthsandweaknessesaswellassuggestedmitigationsforWindows8–andWindows7–certifieddevices.

Bootkit and rootkit attacks

Rootkitsareasophisticatedanddangeroustypeofmalwarethatrunsinkernelmode,usingthesameprivilegesastheoperatingsystem.Becauserootkitshavethesameorpossiblyevenmorerightsthantheoperatingsystem,theycancompletelyhidethemselvesfromWindowsandevenanantimalwaresolution.Often,rootkitsarepartofanentiresuiteofmalwarethatcanbypasslocallogins,recordpasswords,transferprivatefiles,andcapturecryptographykeys.

Differenttypesofbootkitsandrootkitsloadatdifferentsoftwarelevels:

• Kernellevel Rootkitsrunningatthekernellevelhavethehighestprivilegeintheoperatingsystem.Theymaybeabletoinjectmaliciouscodeorreplaceportionsofthecoreoperatingsystem,includingboththekernelanddevicedrivers.

• Applicationlevel Theserootkitsareaimedtoreplaceapplicationbinarieswithmaliciouscode,suchasaTrojan,andcanevenmodifythebehaviorofexistingapplications.

• Librarylevel Thepurposeoflibrary-levelrootkitsistohook,patch,orreplacesystemcallswithmaliciouscodethatcanhidethemalware’spresence.

• Hypervisorlevel Hypervisorrootkitstargetthebootsequence.Theirprimarypurposeistomodifythebootsequencetoloadthemselvesasahypervisor.

• Firmwarelevel TheserootkitsoverwritethePC’sBIOSfirmware,givingthemalwarelow-levelaccessandpotentiallytheabilitytoinstallorhidemalware,evenifit’scleanedorremovedfromtheharddisk.

Regardlessoftheoperatingsystemorencryptionmethod,rootkitshaveaccesstoconfidentialdataonceinstalled.Application-levelrootkitscanreadanyfilestheusercanaccess,bypassingvolume-levelencryption.Kernel-,library-,hypervisor-,andfirmware-levelrootkitshavedirectaccesstosystemfilesonencryptedvolumesandcanalsoretrieveanencryptionkeyfrommemory.

4PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

Windows7offerssubstantialprotectionfrombootkitsandrootkits,butitispossibletobypassoperatingsystemsecuritywhenanattackerhasphysicalaccesstothedeviceandcaninstallthemalwaretothedevicewhileWindowsisoffline.Forexample,anattackermightbootaPCfromaUSBflashdrivecontainingmalwarethatstartsbeforeWindows.ThemalwarecanreplacesystemfilesorthePC’sfirmwareorsimplystartWindowsunderitscontrol.

TosufficientlyprotectaPCfrombootandrootkits,devicesmustusepre-bootauthenticationorUEFI-basedSecureBoot,ortheencryptionsolutionmustusethedevice’sTrustedPlatformModule(TPM)asameansofmonitoringtheintegrityoftheend-to-endbootprocess.Pre-bootauthenticationisavailableforanydevice,regardlessofthehardware,butbecauseitisinconvenienttousers,itshouldbeusedonlytomitigatethreatsthatareapplicabletothedevice.UEFI-basedSecureBootisrequiredforallWindows8.1–andWindows8–certifieddevices.Onthosedevices,youdonotneedtousepre-bootauthenticationtoprotectagainstbootandrootkitattacks.

AlthoughpasswordprotectionoftheUEFIconfigurationisimportantforprotectingadevice’sconfigurationandpreventinganattackerfromdisablingUEFI’sSecureBootfeature,useofaTPManditsPlatformConfigurationRegister(PCR)measurements(PCR7)toensurethatthesystem’sbootloader(whetheraWindowsornon-Microsoftencryptionsolution)istamperfreeandthefirstcodetostartonthedeviceiscritical.Anencryptionsolutionthatdoesn’tuseadevice’sTPMtoprotectitscomponentsfromtamperingmaybeunabletoprotectitselffrombootkit-levelinfectionsthatcouldlogauser’spasswordoracquireencryptionkeys.Forthisreason,whenBitLockerisconfiguredonWindows8–andWindows7–certifieddevicesthatincludeaTPM,theTPManditsPCRsarealwaysusedtosecureandconfirmtheintegrityofthepre–operatingsystemenvironmentbeforemakingencryptedvolumesaccessible.

AnychangestotheUEFIconfigurationinvalidatesthePCR7andrequiretheusertoentertheBitLockerrecoverykey.Becauseofthisfeature,it’snotcriticaltopassword-protectyourUEFIconfiguration.IfanattackersuccessfullyturnsoffSecureBootorotherwisechangestheUEFIconfiguration,theywillneedtoentertheBitLockerrecovery

UEFI-basedSecureBootisrequiredforallWindows8.1–andWindows8–certifieddevices.Onthosedevices,youdonotneed to use pre-boot authentication to protect against boot androotkitattacks.

5PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

key,butUEFIpasswordprotectionisabestpracticeandisstillrequiredforsystemsnotusingaTPM(suchasnon-Microsoftalternatives).

Brute-force sign-in attacks

Attackerscanfindanypasswordifyouallowthemtoguessenoughtimes.Theprocessoftryingmillionsofdifferentpasswordsuntilyoufindtherightoneisknownasabrute-force sign-in attack. Intheory,anattackercouldobtainanypasswordbyusingthismethod.

Threeopportunitiesforbrute-forceattacksexist:

• Against the pre-boot authenticator Anattackercouldattackthedevicedirectlybyattemptingtoguesstheuser’sBitLockerPINoranequivalentauthenticator.TheTPMmitigatesthisapproachbyinvokinganantihammeringlockoutcapabilitythatrequirestheusertowaituntilthelockoutperiodendsorentertheBitLockerrecoverykey.

• Against the recovery key Anattackercouldattempttoguessthe48-digitBitLockerrecoverykey.Evenwithoutalockoutperiod,thekeyislongenoughtomakebrute-forceattacksimpractical.Specifically,theBitLockerrecoverykeyhas128bitsofentropy;thus,theaveragebrute-forceattackwouldsucceedafter18,446,744,073,709,551,616guesses.Ifanattackercouldguess1millionpasswordspersecond,theaveragebrute-forceattackwouldrequiremorethan580,000yearstobesuccessful.

• Against the operating system sign-in authenticator An attacker can attempt to guess a validusernameandpassword.Windowsimplementsadelaybetweenpasswordguesses,slowingdownbrute-forceattacks.Inaddition,allrecentversionsofWindowsallowadministratorstorequirecomplexpasswordsandpasswordlockouts.Similarly,administratorscanuseMicrosoftExchangeActiveSyncpolicyorGroupPolicytoconfigureWindows8.1andWindows8toautomaticallyrestartandrequiretheusertoentertheBitLocker48-digitrecoverykeyafteraspecifiednumberofinvalidpasswordattempts.Whenthesesettingsareenabledandusersfollowbestpracticesforcomplexpasswords,brute-forceattacksagainsttheoperatingsystemsign-inareimpractical.

Ingeneral,brute-forcesign-inattacksarenotpracticalagainstWindowswhenadministratorsenforcecomplexpasswordsandaccountlockouts.

Direct memory access attacks

Directmemoryaccess(DMA)allowscertaintypesofhardwaredevicestocommunicatedirectlywithadevice’ssystemmemory.Forexample,ifyouuseThunderbolttoconnectanotherdeviceto

6PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

yourcomputer,theseconddeviceautomaticallyhasReadandWriteaccesstothetargetcomputer’smemory.

Unfortunately,DMAportsdon’tuseauthenticationandaccesscontroltoprotectthecontentsofthecomputer’smemory.WhereasWindowspreventssystemcomponentsandappsfromreadingandwritingtoprotectedpartsofmemory,adevicecanuseDMAtoreadanylocationinmemory,includingthelocationofanyencryptionkeys.

DMAattacksarerelativelyeasytoexecuteandrequirelittletechnicalskills.AnyonecandownloadatoolfromtheInternet,suchasthosemade by Passware,ElcomSoft,andothers,andthenuseaDMAattacktoreadconfidentialdatafromaPC’smemory.Becauseencryptionsolutionsstoretheirencryptionkeysinmemory,theycanbeaccessedbyaDMAattack.

ToperformaDMAattack,attackerstypicallyconnectasecondPCthatisrunningamemory-scanningtool(e.g.,Passware,ElcomSoft) totheFireWireorThunderboltportofthetargetcomputer.Whenconnected,thesoftwarescansthesystemmemoryofthetargetandlocatestheencryptionkey.Onceacquired,thekeycanbeusedtodecryptthedriveandreadormodifyitscontents.

Amuchmoreefficientformofthisattackexistsintheory:AnattackercraftsacustomFireWireorThunderboltdevicethathastheDMAattacklogicprogrammedonit.Now,theattackersimplyneedstophysicallyconnectthedevice.Iftheattackerdoesnothavephysicalaccess,theycoulddisguiseitasafreeUSBflashdriveanddistributeittoemployeesofatargetorganization.Whenconnected,theattackingdevicecoulduseaDMAattacktoscanthePC’smemoryfortheencryptionkey.Itcouldthentransmitthekey(oranydatainthePC’smemory)usingthePC’sInternetconnectionoritsownwirelessconnection.Thistypeofattackwouldrequireanextremelyhighlevelofsophistication,becauseitrequiresthattheattackercreateacustomdevice(devicesofthesetypesarenotreadilyavailableinthemarketplaceatthistime).

Themostcommon,legitimateuseforDMAportsisdeveloperdebugging,ataskthatsomedevelopersneedtoperformandonethatfewconsumerswilleverperform.BecauseUSB;DisplayPort;andother,moresecureporttypessatisfyconsumers,mostnewmobilePCsdonotincludeDMAports.Microsoft’sviewisthatbecauseof

NOTE

NotallporttypesarevulnerabletoDMAattacks.USBinparticulardoesnotallowDMA,butdevicesthathaveanyofthefollowingporttypesarevulnerable:

• FireWire

• Thunderbolt

• ExpressCard

• PCMCIA

• PCI

• PCI-X

• PCIExpress

7PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

theinherentsecurityrisksofDMAports,theydonotbelongonmobiledevices,andMicrosofthasprohibitedtheirinclusiononanyInstantGo-certifieddevices.InstantGodevicesoffermobilephone–likepowermanagementandinstant-oncapabilities;atthetimeofwriting,theyareprimarilyfoundinWindowstablets.In2014,MicrosoftexpectstoseeInstantGotrickledownintomoremobiledevicetypes,suchasconvertiblesandtraditionallaptops.

DMA-basedexpansionslotsareanotheravenueofattack,buttheseslotsgenerallyappearonlyondesktopPCsthataredesignedforexpansion.OrganizationscanusephysicalsecuritytopreventoutsideattacksagainsttheirdesktopPCs.Inaddition,aDMAattackontheexpansionslotwouldrequireacustomdevice;asaresult,anattackerwouldmostlikelyinsertaninterfacewithatraditionalDMAport(forexample,FireWire)intotheslottoattackthePC.

NewtoWindows8.1isacapabilitybywhichWindowswon’tenablenewlyattachedDMAdevicesuntiltheoperatingsystemstartsandausersignsin.EverytimethePCswitchestosuspend,hibernation,orsleepmode,WindowswaitsfortheusertosigninbeforegrantingnewdevicesDMAaccess.ThisdelayhelpspreventDMAattackswhenanauthorizeduserisn’tpresent.ThisnewWindows8.1behaviorsuccessfullymitigatestheDMAattackvectorandeliminatestheneedforpre-bootauthenticationinmostscenarios.AnotheroptionisforadministratorstoconfigurepolicysettingstodisableFireWireandotherdevicetypesthathaveDMA;manyPCsallowthosedevicestobedisabledbyusingfirmwaresettings.Althoughtheneedforpre-bootauthenticationcanbeeliminatedatthedevicelevelorthroughWindowsconfiguration,theBitLockerpre-bootauthenticationfeatureisstillavailablewhenneeded.Whenused,itsuccessfullymitigatesalltypesofDMAportandexpansionslotattacksonanytypeofdevice.

Hyberfil.sys attacks

Thehyberfil.sysfileistheWindowshibernationfile.ItcontainsasnapshotofsystemmemorythatisgeneratedwhenadevicegoesintohibernationandincludestheencryptionkeyforBitLockerandotherencryptiontechnologies.Attackershaveclaimedthattheyhavesuccessfullyextractedencryptionkeysfromthehyberfil.sysfile.

Windows8.1waitsfor the user to sign in before granting new devicesDMAaccess.ThisnewbehaviorsuccessfullymitigatestheDMAattackvector.

8PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

LiketheDMAportattackdiscussedintheprevioussection,toolsareavailablethatcanscanthehyberfile.sysfileandlocatetheencryptionkey,includingatoolmadebyPassware.MicrosoftdoesnotconsiderWindowstobevulnerabletothistypeofattack,becauseWindowsstoresthehyberfil.sysfilewithintheencryptedsystemvolume.Asaresult,thefilewouldbeaccessibleonlyiftheattackerhadbothphysicalandsign-inaccesstothePC.Whenanattackerhassign-inaccesstothePC,therearefewreasonsfortheattackertodecryptthedrive,becausetheywouldalreadyhavefullaccesstothedatawithinit.

Inpractice,theonlyreasonanattackonhyberfil.syswouldgrantanattackeradditionalaccessisifanadministratorhadchangedthedefaultWindowsconfigurationandstoredthehyberfil.sysfileonanunencrypteddrive.Bydefault,bothWindows8andWindows7aredesignedtobesecureagainstthistypeofattack.

Memory remanence attacks

A memory remanence attackisaside-channelattackthatreadstheencryptionkeyfrommemoryafterrestartingaPC.AlthoughaPC’smemoryisoftenconsideredtobeclearedwhenthePCisrestarted,memorychipsdon’timmediatelylosetheirmemorywhenyoudisconnectpower.Therefore,anattackerwhohasphysicalaccesstothePC’smemorymightbeabletoreaddatadirectlyfromthememory—includingtheencryptionkey.

Whenperformingthistypeofcoldbootattack,theattackeraccessesthePC’sphysicalmemoryandrecoverstheencryptionkeywithinafewsecondsorminutesofdisconnectingpower.Thistypeof attack was demonstrated by researchers at PrincetonUniversity.Withtheencryptionkey,theattackerwouldbeabletodecryptthedriveandaccessitsfiles.

Toacquirethekeys,attackersfollowthisprocess:

1. FreezethePC’smemory.Forexample,anattackercanfreezethememoryto−50°Cbysprayingitwithaerosolairdusterspray.

2.RestartthePC.

3. InsteadofrestartingWindows,boottoanotheroperatingsystem.Typically,thisisdonebyconnectingabootableflashdriveorloadingabootableDVD.

4.Thebootablemedialoadsthememoryremanenceattacktools,whichtheattackerusestoscanthesystemmemoryandlocatetheencryptionkeys.

5.Theattackerusestheencryptionkeystoaccessthedrive’sdata.

9PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

Iftheattackerisunabletobootthedevicetoanotheroperatingsystem(forexample,ifbootableflashdriveshavebeendisabledorUEFISecureBootisenabled),theattackercanattempttophysicallyremovethefrozenmemoryfromthedeviceandattachittoadifferent,possiblyidenticaldevice.Fortunately,thisprocesshasprovenextremelyunreliable,asevidencedbytheDefenceResearchandDevelopmentCanada(DRDC)Valcartiergroup’sanalysis(see“Anin-depthanalysisofthecoldbootattack”athttp://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA545078).Onanincreasingportionofmoderndevices,thistypeofattackisnotevenpossible,becausememoryissoldereddirectlytothemotherboard.

AlthoughPrinceton’sresearchprovedthatthistypeofattackwaspossibleondevicesthathaveremovablememory,devicehardwarehaschangedsincetheresearchwaspublishedin2008:

• Windows8–certifieddevicesincludeUEFI-basedSecureBoot,whichpreventsthemalicioustoolsthatthePrincetonattackdependsonfromrunningonthetargetdevice.

• Windows8andWindows7systemswithBIOSorUEFIcanbelockeddownwithapassword,andbootingtoaUSBdrivecanbeprevented.

• IfbootingtoUSBisrequiredonthedevice,itcanbelimitedtostartingtrustedoperatingsystemsonWindows8–certifieddevices(UEFI-basedSecureBoot).

• Thedischargeratesofmemoryarehighlyvariableamongdevices,andmanydeviceshavememorythatiscompletelyimmunetomemoryremanenceattacks.

• Increaseddensityofmemorydiminishestheirremanencepropertiesandreducesthelikelihoodthattheattackcanbesuccessfullyexecuted,evenwhenmemoryisphysicallyremovedandplacedinanidenticalsystemwherethesystem’sconfigurationmayenablebootingtothemalicioustools.

Becauseofthesefactors,thistypeofattackisrarelypossibleonmoderndevices.Evenincaseswheretheriskfactorsexistonlegacydevices,attackerswillfindtheattackunreliable.Fordetailedinformationaboutthepracticalusesforforensicmemoryacquisitionandthefactorsthatmakeacomputervulnerableorresistanttomemoryremanenceattacks,read“Anin-depthanalysisofthecoldbootattack”athttp://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA545078.

TheBitLockerpre-bootauthenticationfeaturecansuccessfullymitigatememoryremanenceattacksonmostdevices,butyoucanalsomitigatesuchattacksbyprotectingthesystemUEFIorBIOSandpreventthePCfrombootingfromexternalmedia(suchasaUSBflashdriveorDVD).Thelatteroptionisoftenabetterchoice,becauseitprovidessufficientprotectionwithoutinconveniencinguserswithpre-bootauthentication.

10PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

CountermeasuresBitLockerwasintroducedintheWindowsVistaoperatingsystemaspartofastrategicapproachtosecuringmobiledatathroughencryptiontechnology.Dataonalostorstolencomputerisvulnerabletounauthorizedaccess,eitherbyrunningasoftwareattacktoolagainstitorbytransferringthecomputer’sharddisktoadifferentcomputer.Today,BitLockerhelpsmitigateunauthorizeddataaccessonlostorstolencomputersbeforetheoperatingsystemisstartedby:

• Encrypting the hard drives on your computer Forexample,youcanturnonBitLockerforyouroperatingsystemdrive(thedriveonwhichWindowsisinstalled),afixeddatadrive(suchasadifferentvolumeonthesystemdriveoraseparateinternalharddrive),oraremovabledatadrive(suchasaUSBflashdrive).TurningonBitLockerforyouroperatingsystemdriveencryptsallsystemfilesontheoperatingsystemdrive,includingtheswapfilesandhibernationfiles.

• Ensuring the integrity of early boot components and boot configuration data OnWindows7–certifieddevicesthathaveaTPMversion2.0or1.2,BitLockerusestheenhancedsecuritycapabilitiesoftheTPMtohelpensurethatyourdataisaccessibleonlyifthecomputer’sbootcomponentsappearunalteredandtheencrypteddiskislocatedintheoriginalcomputer.OnWindows8–certifieddevices,acombinationofUEFIandTPMhelpsensureintegrity.

ThesectionsthatfollowprovidemoredetailedinformationaboutthedifferenttechnologiesthatWindowsusestoprotectagainstattacksontheBitLockerencryptionkeyinfourdifferentbootphases:beforestartup,duringpre-boot,duringstartup,andfinallyafterstartup.

Protection before startup

BeforeWindowsstarts,youmustrelyonsecurityfeaturesimplementedaspartofthedevicehardware,includingTPMandUEFISecureBoot.Fortunately,manymoderncomputersfeatureTPM,andallWindows8.1–andWindows8–certifieddevicessupportallofthesefeatures.

Trusted Platform Module

Softwarealoneisn’tsufficienttoprotectasystem.Afteranattackerhascompromisedsoftware,thesoftwaremightbeunabletodetectthecompromise.Therefore,asinglesuccessfulsoftwarecompromiseresultsinanuntrustedsystemthatmightneverbedetected.Hardware,however,ismuchmoredifficulttomodify.

11PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

A TPMisamicrochipdesignedtoprovidebasicsecurity-relatedfunctions,primarilyinvolvingencryptionkeys.TheTPMisusuallyinstalledonthemotherboardofacomputerand communicates with the rest of the system through a hardwarebus.Physically,TPMsaredesignedtobetamper-proof.Ifanattackertriestophysicallyretrievedatadirectlyfromthechip,they’llprobablydestroythechipintheprocess.

BybindingtheBitLockerencryptionkeywiththeTPMandproperlyconfiguringthedevice,it’snearlyimpossibleforanattackertogainaccesstotheBitLocker-encrypteddatawithoutobtaininganauthorizeduser’scredentials.Therefore,computerswithaTPMcanprovideahighlevelofprotectionagainstattacksthatattempttodirectlyretrievetheBitLockerencryptionkey.

OndevicesrunningWindows8,thecombinationofaTPMandUEFISecureBootprovidessufficientdeviceintegrity–relatedsecurity.OndevicesrunningWindows8orWindows7withoutUEFI-basedSecureBoot,theTPMwillbeusedtoprotectthesystem’sboot-relatedcomponents.

UEFI and Secure Boot

Nooperatingsystemcanprotectadevicewhentheoperatingsystemisoffline.Forthatreason,Microsoftworkedcloselywithhardwarevendorstorequirefirmware-levelprotectionagainstbootandrootkitsthatmightcompromiseanencryptionsolution’sencryptionkeysinallWindows8–certifieddevices.

TheUEFIisaprogrammablebootenvironmentintroducedasareplacementforBIOS,whichhasforthemostpartremainedunchangedforthepast30years.LikeBIOS,PCsstartUEFIbeforeanyothersoftware;itinitializesdevices,andUEFIthenstartstheoperatingsystem’sbootloader.Aspartofitsintroductionintothepre–operatingsystemenvironment,UEFIservesanumberofpurposes,butoneofthekeybenefitsistoprotectnewerdevicesagainstasophisticatedtypeofmalwarecalledabootkit through the useofitsSecureBootfeature.

RecentimplementationsofUEFI(startingwithversion2.3.1,whichisinallWindows8–certifieddevices)canverifythedigitalsignatures

By binding the BitLockerencryptionkey with the TPM and properlyconfiguringthedevice,it’snearlyimpossibleforanattacker to gain access totheBitLocker-encrypted data without obtaining anauthorizeduser’scredentials.

12PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

ofthedevice’sfirmwarebeforerunningit.BecauseonlythePC’shardwaremanufacturerhasaccesstothedigitalcertificaterequiredtocreateavalidfirmwaresignature,UEFIcanpreventfirmware-basedbootkits.Thus,UEFIisthefirstlinkinthechainoftrust.

TheUEFI-basedSecureBootfeatureisthefoundationofplatformandfirmwaresecurityandwascreatedtoenhancesecurityinthepre-bootenvironmentregardlessofdevicearchitecture.Usingsignaturestovalidatetheintegrityoffirmwareimagesbeforetheyareallowedtoexecute,SecureBoothelpsreducetheriskofbootloaderattacks.ThepurposeofSecureBootistoblockuntrustedfirmwareandbootloaders(signedorunsigned)frombeingabletostartonthesystem.

WiththelegacyBIOSbootprocess,thepre–operatingsystemenvironmentisvulnerabletoattacksbyredirectingbootloaderhandofftopossiblemaliciousloaders.Theseloaderscouldremainundetectedtooperatingsystemandantimalwaresoftware.Thediagram in Figure 1contraststheBIOSandUEFIstartupprocesses.

WithSecureBootenabled,UEFI,incoordinationwiththeTPM,canexaminethebootloaderanddeterminewhetherit’strustworthy.Todeterminewhetherthebootloaderistrustworthy,UEFIexaminesthebootloader’sdigitalsignature.Usingthedigitalsignature,theUEFI:

• Verifiesthatthebootloaderhasn’tbeenmodifiedsinceitwassigned

• Verifiesthatthebootloaderwassignedusingatrustedcertificate(inthecaseofWindows8,Microsoft’scertificate)

Ifthebootloaderpassesthesetwotests,theUEFIknowsthatthebootloaderisn’tabootkitandstarts

FigurE 1 The BIOS and UEFI startup processesBIOS

UEFI VerifiedOS loader

Any OS loader(including malware)

OS Start

OS Start

13PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

it.Atthispoint,Windows8.1’sTrustedBootfeaturetakesover,andtheWindows8bootloader,usingthesamecryptographictechnologiesthatUEFIusedtoverifythebootloader,thenverifiesthattheWindowssystemfileshaven’tbeenchanged.

AllWindows8–certifieddevicesmustmeetseveralrequirementsrelatedtoUEFI-basedSecureBoot:

• TheymusthaveSecureBootenabledbydefault.

• TheymusttrustMicrosoft’scertificate(andthusanybootloaderMicrosofthassigned).

• TheymustallowtheusertoconfigureSecureBoottotrustothersignedbootloaders.

• ExceptforWindowsRTdevices,theymustallowtheusertocompletelydisableSecureBoot.

Theserequirementshelpprotectyoufromrootkitswhileallowingyoutorunanyoperatingsystemyouwant.Youhavethreeoptionsforrunningnon-Microsoftoperatingsystems:

• Use an operating system with a certified bootloader BecauseallCertifiedforWindows8PCsmusttrustMicrosoft’scertificate,Microsoftoffersaservicetoanalyzeandsignnon-MicrosoftbootloaderssothattheycanbetrustedbyallCertifiedforWindows8PCs.TheLinuxcommunityisusingthisprocesstoenableLinuxtotakeadvantageofUEFISecureBootonWindows-certifieddevices.

• Configure UEFI to trust your custom bootloader AllCertifiedforWindows8PCsallowyoutotrustasigned,noncertifiedbootloaderthatyouspecifyintheUEFIdatabase,allowingyoutorunanyoperatingsystem,includinghomemadeoperatingsystems.

• Turn off Secure Boot AllCertifiedforWindows8PCsallowyoutoturnoffSecureBootsoyoucanrunanysoftware.Thisdoesnothelpprotectyoufrombootkits,however.

Topreventmalwarefromabusingtheseoptions,theuserhastomanuallyconfiguretheUEFIfirmwaretotrustanoncertifiedbootloaderortoturnoffSecureBoot.SoftwarecannotchangetheSecureBootsettings.

Anydevicethatdoesn’trequireSecureBootorasimilarbootloader-verificationtechnology,regardlessofthearchitectureoroperatingsystem,isvulnerabletobootkits,whichcanbeusedtocompromisetheencryptionsolution.Bydefault,allWindows8–certifieddeviceshaveUEFI-basedSecureBootenabled.

UEFIissecurebydesign,butit’scriticaltoprotecttheSecureBootconfigurationbyusingpasswordprotection.Inaddition,althoughseveralwell-publicizedattacksagainstUEFIhaveoccurred,

14PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

theywereexploitingfaultyUEFIimplementations.ThoseattacksareineffectivewhenUEFIisimplementedproperly.

FormoreinformationaboutSecureBoot,referto“SecuringtheWindows8BootProcess”athttp://technet.microsoft.com/en-US/windows/dn168167.aspx.

Protection during pre-boot: pre-boot authentication

Pre-bootauthenticationwithfull-diskencryptionproducts(includingBitLocker)isaprocessthatrequiresausertoauthenticatepriortomakingthecontentsofthesystemdriveaccessible.InthecaseofBitLocker,BitLockerencryptstheentiredrive,includingallsystemfiles.BitLockeraccessesandstorestheencryptionkeyinmemoryonlyafterauserprovidesaspecificPINorUSBstartupkey.

IfWindowscan’taccesstheencryptionkey,thedevicecan’treadoreditthefilesonthesystemdrive.EvenifanattackertakesthediskoutofthePCorstealstheentirePC,theywon’tbeabletoreadoreditthefileswithouttheencryptionkey.Theonlyoptionforbypassingpre-bootauthenticationisenteringthehighlycomplex,48-digitrecoverykey.

TheBitLockerpre-bootauthenticationcapabilityisnotspecificallydesignedtopreventtheoperatingsystemfromstarting:That’smerelyasideeffectofhowBitLockerprotectsdataconfidentialityandsystemintegrity.Pre-bootauthenticationisdesignedtopreventtheencryptionkeyfrombeingloadedtosystemmemoryondevicesthatarevulnerabletocertaintypesofcoldbootattacks.Manymoderndevicespreventanattackerfromeasilyremovingthememory,andMicrosoftexpectsthosedevicestobecomeevenmorecommoninthefuture.

OncomputerswithacompatibleTPM,operatingsystemdrivesthatareBitLocker-protectedcanbeunlockedinfourways:

• TPM-only UsingTPM-onlyvalidationdoesnotrequireanyinteractionwiththeusertodecryptandprovideaccesstothedrive.IftheTPMvalidationsucceeds,theuserlogonexperienceisthesameasastandardlogon.IftheTPMismissingorchangedoriftheTPMdetectschangestocriticaloperatingsystemstartupfiles,BitLockerentersitsrecoverymode,andtheusermustenterarecoverypasswordtoregainaccesstothedata.

• TPM with startup key InadditiontotheprotectionthattheTPMprovides,partoftheencryptionkeyisstoredonaUSBflashdrive,referredtoasastartup key. Data on the encryptedvolumecannotbeaccessedwithoutthestartupkey.

• TPM with PiN InadditiontotheprotectionthattheTPMprovides,BitLockerrequiresthattheuserenteraPIN.DataontheencryptedvolumecannotbeaccessedwithoutenteringthePIN.

15PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

• TPM with startup key and PiN InadditiontothecorecomponentprotectionthattheTPMprovides,partoftheencryptionkeyisstoredonaUSBflashdrive,andaPINisrequiredtoauthenticatetheusertotheTPM.ThisconfigurationprovidesMultifactorAuthenticationsothatiftheUSBkeyislostorstolen,itcannotbeusedforaccesstothedrive,becausethecorrectPINisalsorequired.

Formanyyears,Microsofthasrecommendedusingpre-bootauthenticationtoprotectagainstDMAandmemoryremanenceattacks.Today,Microsoftrecommendsusingpre-bootauthenticationonlyonPCsrunningWindows7thathaveanenabledDMAportoranydevicethatissusceptibletomemoryremanenceattacks.

Althougheffective,pre-bootauthenticationisinconvenienttousers.Inaddition,ifauserforgetstheirPINorlosestheirstartupkey,they’redeniedaccesstotheirdatauntiltheycancontacttheirorganization’ssupportteamtoobtainarecoverykey.Today,mostnewPCsrunningWindows8.1orWindows8providesufficientprotectionagainstDMAattackswithoutrequiringpre-bootauthentication.Forexample,mostmodernPCsincludeUSBportoptions(whicharenotvulnerabletoDMAattacks)butdonotincludeFireWireorThunderboltports(whicharevulnerabletoDMAattacks).

Infact,toachieveaWindows8InstantGo(formerlyConnectedStandby)certificationfromMicrosoft,newdevicescan’tincludeaDMAport,eliminatingtheneedforpre-bootauthenticationtomitigateagainstaDMAportattackinmosttabletsandotherWindows8–certifieddevices.Althoughthiscertificationiscurrentlyimplementedonlyontabletdevices,startingin2014,MicrosoftexpectstoseedevicessuchasconvertiblesandlaptopscertifiedforInstantGo.

BitLocker-encrypteddeviceswithDMAportsenabled,includingFireWireorThunderboltports,shouldbeconfiguredwithpre-bootauthenticationiftheyarerunningWindows7.Windows8.1devicesdonotneedpre-bootauthenticationtoprotectagainstthemostcommonlyusedDMAattackvectorsbecausenewlyattachedDMAdevicesgetDMAaccessonlyafterauserauthenticatesandsignsintoWindows.ManycustomersfindthattheDMAportsontheirdevicesareneverused,andtheychoosetoeliminatethepossibilityofanattackbydisablingtheDMAportsthemselves,eitheratthehardwarelevelorthroughGroupPolicy.

Manynewmobiledeviceshavethesystemmemorysolderedtothemotherboard,whichhelpspreventthecoldboot–styleattack,wherethesystemmemoryisfrozen,removed,andthenplacedintoanotherdevice.Thosedevices,andmostPCs,canstillbevulnerablewhenbootingtoamaliciousoperatingsystem,however.

16PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

Youcanmitigatetheriskofbootingtoamaliciousoperatingsystem:

• Windows 8.1 (without Secure Boot), Windows 8 (without uEFi-based Secure Boot), or Windows 7 (with or without a TPM) Disablebootingfromexternalmedia,andrequireafirmwarepasswordtopreventtheattackerfromchangingthatoption.

• Windows 8.1 or Windows 8 (certified or with Secure Boot) Passwordprotectthefirmware,anddonotdisableSecureBoot.

Protection during startup

Duringthestartupprocess,Windows8.1andWindows8useTrustedBootandEarlyLaunchAnti-Malware(ELAM)toexaminetheintegrityofeverycomponent.Thesectionsthatfollowdescribethesetechnologiesinmoredetail.

Trusted Boot

TrustedBoottakesoverwhereUEFI-basedSecureBootleavesoff—duringtheoperatingsysteminitializationphase.ThebootloaderverifiesthedigitalsignatureoftheWindows8kernelbeforeloadingit.TheWindows8kernel,inturn,verifieseveryothercomponentoftheWindowsstartupprocess,includingthebootdrivers,startupfiles,andELAMdriver.IfafilehasbeenmodifiedorisnotproperlysignedwithaMicrosoftsignature,Windowsdetectstheproblemandrefusestoloadthecorruptedcomponent.Often,Windows8canautomaticallyrepairthecorruptedcomponent,restoringtheintegrityofWindowsandallowingthePCtostartnormally.

Windows8usesTrustedBootonanyhardwareplatform:ItrequiresneitherUEFInoraTPM.However,withoutSecureBoot,it’spossibleformalwaretocompromisethestartupprocesspriortoWindowsstarting,atwhichpointTrustedBootprotectionscouldbebypassedorpotentiallydisabled.

Early Launch Anti-Malware

BecauseUEFI-basedSecureBoothasprotectedthebootloaderandTrustedBoothasprotectedtheWindowskernelorotherWindowsstartupcomponents,thenextopportunityformalwaretostartisbyinfectinganon-Microsoftboot-relateddriver.Traditionalantimalwareappsdon’tstartuntilaftertheboot-relateddrivershavebeenloaded,givingarootkitdisguisedasadrivertheopportunitytowork.

ThepurposeofELAMistoloadanantimalwaredriverbeforedriversthatareflaggedasboot-startcanbeexecuted.Thisapproachprovidestheabilityforanantimalwaredrivertoregisterasa

17PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

trustedboot-criticaldriver.ItislaunchedduringtheTrustedBootprocess,andwiththat,Windowsensuresthatitisloadedbeforeanyothernon-Microsoftsoftware.

Withthissolutioninplace,bootdriversareinitializedbasedontheclassificationthattheELAMdriverreturnsaccordingtoaninitializationpolicy.ITproshavetheabilitytochangethispolicythroughGroupPolicy.

ELAMclassifiesdriversasfollows:

• good Thedriverhasbeensignedandhasnotbeentamperedwith.

• Bad Thedriverhasbeenidentifiedasmalware.Itisrecommendedthatyounotallowknownbaddriverstobeinitialized.

• Bad but required for boot Thedriverhasbeenidentifiedasmalware,butthecomputercannotsuccessfullybootwithoutloadingthisdriver.

• unknown Thisdriverhasnotbeenattestedtobyyourmalware-detectionapplicationorclassifiedbytheELAMboot-startdriver.

Protection after startup: DMA attack protection

Windows8.1minimizestheriskofDMAattacksbypreventingnewlyattachedDMAdevicesfromgainingDMAuntilauserauthenticatesbysigning-in.Thisdoesn’teliminatetherisk,butitdoesreducetheriskofanattackerconnectingaDMAdevicetoaPCandretrievingtheencryptionkeywhiletheuserisawayfromthePC.

TosuccessfullyperformaDMAattackonaWindows8.1device,theattackerwouldneedamaliciousDMAdeviceconnectedtothePCwhiletheuserwasloggedon.TheattackerwouldnotsimplybeabletoattachaDMAdevicewhentheuserwasatthePC,retrievetheencryptionkey,andthenleavewiththedevice.Theattackerwouldeitherneedto:

• Attachthedevicewhiletheuserwasloggedon

• Attachthedeviceatanytime,waitfortheusertologon,andthenreturntoretrievethedevice

Windows8InstantGo–certifieddevicesdonothaveDMAports,eliminatingtheriskofDMAattacks.Onotherdevices,youmightbeabletodisableFireWire,Thunderbolt,orotherportsthatsupportDMA.

18PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

Choosing the right countermeasuresFigure 2,Figure3onpage19,andFigure 4 on page 20 summarize the recommended mitigationsfordifferenttypesofattacksagainstPCsrunningrecentversionsofWindows.Theorangeblocksindicatethatthesystemrequiresadditionalconfigurationfromthedefaultsettings.

FigurE 2 How to choose the best countermeasures for Windows 7

Windows 7without TPM

Bootkits and Rootkits

Without TPM, bootintegrity checking isnot available

Secure by default, and canbe improved with accountlockout Group Policy

Check devices for DMAports. Consider disablingports if not in use or requireBitLocker with pre-boot authentication

Secure by default,hyberfil.sys secured onencrypted volume

Require a BIOS password and disable booting from external media. If an attack is viable, consider pre-boot authentication

Secure by default. RequireBitLocker with TPM for bootintegrity validation

Secure by default, and canbe improved with accountlockout Group Policy

Secure by default,hyberfil.sys secured onencrypted volume

Require a BIOS password and disable booting from external media. If an attack is viable, consider pre-boot authentication

Brute ForceSign-in

DMA Attacks

Hyberfil.sys Attacks

Memory Remanence Attacks

Windows 7with TPM

Check devices for DMAports. Consider disablingports if not in use or requireBitLocker with pre-boot authentication

19PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

FigurE 3 How to choose the best countermeasures for Windows 8

Bootkits and Rootkits

Without TPM, bootintegrity checking isnot available

Secure by default, and canbe improved with accountlockout Group Policy

Check devices for DMAports. Consider disablingports if not in use or requireBitLocker with pre-boot authentication

Secure by default,hyberfil.sys secured onencrypted volume

Require a BIOS passwordand disable booting fromexternal media. If an attack is viable, consider pre-boot authentication

Secure by default whenUEFI-based Secure Bootis enabled and a password is required to change settings

Secure by default, and canbe improved with accountlockout and device lockoutGroup Policy settings

Check devices for DMAports. Consider disablingports if not in use or requireBitLocker with pre-boot authentication

Secure by default,hyberfil.sys secured onencrypted volume

Password protect thefirmware and ensureSecure Boot is enabled. If an attack is viable, consider pre-boot authentication

Brute ForceSign-in

DMA Attacks

Hyberfil.sys Attacks

Memory Remanence Attacks

Windows 8 without TPM

Windows 8 Certified

20PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

FigurE 4 How to choose the best countermeasures for Windows 8.1

Bootkits and Rootkits

Secure by default, and canbe improved with accountlockout Group Policy

Secure by default for all lostor stolen devices becausenew DMA devices aregranted access only when anauthorized user is signed in

Secure by default,hyberfil.sys secured onencrypted volume

Require a BIOS passwordand disable booting fromexternal media. If an attack is viable, consider pre-boot authentication

Secure by default, and canbe improved with accountlockout and device lockoutGroup Policy settings

Secure by default for all lostor stolen devices becausenew DMA devices aregranted access only when anauthorized user is signed in

Secure by default,hyberfil.sys secured onencrypted volume

Password protect thefirmware and ensureSecure Boot is enabled. If an attack is viable, consider pre-boot authentication

Brute ForceSign-in

DMA Attacks

Hyberfil.sys Attacks

Memory Remanence Attacks

Windows 8.1 without TPM

Windows 8.1 Certified

Without TPM, bootintegrity checking isnot available

Secure by default whenUEFI-based Secure Bootis enabled and a password is required to change settings

21PROTECTING BITLOCKER-ENCRYPTED DEVICES FROM ATTACKS

SummaryYoucanuseBitLockertoprotectyourWindows8.1,Windows8,andWindows6clientPCs.Whicheveroperatingsystemyou’reusing,MicrosoftandWindows-certifieddevicesprovidecountermeasurestoaddressattacksandimproveyourdatasecurity.Inmostcases,particularlyonWindows8devices,thisprotectioncanbeimplementedwithouttheneedforpre-bootauthentication.

ThelatestWindows8.1InstantGodevices,primarilytablets,aredesignedtobesecurebydefaultagainstallattacksthatmightcompromisetheBitLockerencryptionkey.OtherWindows8.1devicescanbe,too.DMAport–basedattacks,whichrepresenttheattackvectorofchoice,arenotpossibleonInstantGodevices,onwhichtheseporttypesareprohibited.DMAportsonevennon-InstantGodevicesisincreasinglyrare,particularlyonmobiledevices.Regardlessofthehardwareconfiguration,theriskofDMAattackshasbeenaddressedinWindows8.1itself,whichhasbeenupdatedtopreventnewDMAdevicesthathavebeenattachedtoadevicefromgainingDMAuntilanauthorizedusersigns-in.DMAportscanevenbedisabledentirely,whichisincreasinglypopularoptionbecausetheuseofDMAportsisrareinthenon-developerspace.

Memoryremanenceattackscanbemitigatedwithproperconfiguration;incaseswherethesystemmemoryisfixedandnon-removable,theyarenotpossibleusingpublishedtechniques.Evenincaseswheresystemmemorycanberemovedandloadedintoanotherdevice,attackerswillfindtheattackvectorextremelyunreliable,ashasbeenshownintheDRDCValcartiergroup’sanalysis(see“Anin-depthanalysisofthecoldbootattack”athttp://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA545078).

Windows7PCssharethesamesecurityrisksasWindows8devicesbutarefarmorevulnerabletoDMAandmemoryremanenceattacks,becauseWindows7devicesaremorelikelytoincludeDMAports,lacksupportforUEFI-basedSecureBoot,andrarelyhavefixedmemory.Toeliminatetheneedforpre-bootauthenticationonWindows7devices,disabletheabilitytoboottoexternalmedia,password-protecttheBIOSconfiguration,anddisabletheDMAports.Ifyoubelievethatyourdevicesmaybeatargetofamemoryremanenceattack,wherethesystemmemorymayberemovedandputintoanothermachinetogainaccesstoitscontents,considertestingyourdevicestodeterminewhethertheyaresusceptibletothistypeofattack.

Intheend,manycustomerswillfindthatpre-bootauthenticationimprovessecurityonlyforashrinkingsubsetofdeviceswithintheirorganization.Microsoftrecommendsacarefulexaminationoftheattackvectorsandmitigationsoutlinedinthisdocumentalongwithanevaluationofyourdevicesbeforechoosingtoimplementpre-bootauthentication,whichmaynotenhancethesecurityofyourdevicesandinsteadwillonlycompromisetheuserexperienceandaddtosupportcosts.

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