software security: buﬀer overﬂow attacks€¦ · 10/5/16 cse 484 / cse m 584 - fall 2016 4...

CSE484/CSEM584:ComputerSecurityandPrivacy

SoftwareSecurity:BufferOverflowAttacks

Fall2016

Adam(Ada)[email protected]

ThankstoFranziRoesner,DanBoneh,DieterGollmann,DanHalperin,YoshiKohno,JohnManferdelli,JohnMitchell,VitalyShmatikov,BennetYee,andmanyothersforsampleslidesandmaterials...

Announcements

•  Signtheethicsformbytodayat5!•  Homework1isdueonMonday.

•  Pleasestartforminggroupsforlab1– Youcanusetheforumtofindgroupmembers

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Announcements

•  TAofficehourshavebeenmovedtoMondaysat4:30(afterclass),inthesecondfloorbreakout.– Sorryfortheconfusion!

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Security:NotJustforPCs

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smartphones

wearables

gameplatforms

cars

medicaldevicesEEGheadsetsvotingmachines

RFID mobilesensingplatforms

airplanes

SoftwareProblemsareUbiquitous

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•  Otherseriousbugs(manyothersexist)– USVincennestrackingsoftware– MV-22Osprey

– MedtronicModel8870SoftwareApplicationCard

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AdversarialFailures

•  Softwarebugsarebad–  Consequencescanbeserious

•  Evenworsewhenanintelligentadversarywishestoexploitthem!–  Intelligentadversaries:Forcebugsinto“worstpossible”conditions/states

–  Intelligentadversaries:Picktheirtargets

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BUFFEROVERFLOWS

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AdversarialFailures

•  Bufferoverflowsbugs:Bigclassofbugs– Normalconditions:Cansometimescausesystemstofail

– Adversarialconditions:Attackerabletoviolatesecurityofyoursystem(control,obtainprivateinformation,...)

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ReferenceforQ1

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Text region Heap StackAddr 0x00...0 Addr 0xFF...F

Top Bottom

ret/IP Caller’s frame

Addr 0xFF...F

Saved FP

Executecodeatthisaddressafterfunc()finishes

buf

Localvariables

str

Args

ABitofHistory:MorrisWorm

•  Wormwasreleasedin1988byRobertMorris–  GraduatestudentatCornell,sonofNSAchiefscientist–  ConvictedunderComputerFraudandAbuseAct,

sentencedto3yearsofprobationand400hoursofcommunityservice

–  NowanEECSprofessoratMIT•  Wormwasintendedtopropagateslowlyand

harmlesslymeasurethesizeoftheInternet•  Duetoacodingerror,itcreatednewcopiesasfast

asitcouldandoverloadedinfectedmachines•  $10-100Mworthofdamage

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MorrisWormandBufferOverflow

•  Oneoftheworm’spropagationtechniqueswasabufferoverflowattackagainstavulnerableversionoffingerdonVAXsystems–  Bysendingspecialstringtofingerdaemon,worm

causedittoexecutecodecreatinganewwormcopy–  UnabletodetermineremoteOSversion,wormalso

attackedfingerdonSunsrunningBSD,causingthemtocrash(insteadofspawninganewcopy)

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FamousInternetWorms

•  Bufferoverflows:verycommoncauseofInternetattacks–  In1998,over50%ofadvisoriespublishedbyCERT(computer

securityincidentreportteam)werecausedbybufferoverflows

•  Morrisworm(1988):overflowinfingerd–  6,000machinesinfected

•  CodeRed(2001):overflowinMS-IISserver–  300,000machinesinfectedin14hours

•  SQLSlammer(2003):overflowinMS-SQLserver–  75,000machinesinfectedin10minutes(!!)

•  Sasser(2005):overflowinWindowsLSASS–  Around500,000machinesinfected

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…AndMore

•  Conficker(2008-08):overflowinWindowsRPC–  Around10millionmachinesinfected(estimatesvary)

•  Stuxnet(2009-10):severalzero-dayoverflows+sameWindowsRPCoverflowasConficker–  Windowsprintspoolerservice–  WindowsLNKshortcutdisplay–  Windowstaskscheduler

•  Flame(2010-12):sameprintspoolerandLNKoverflowsasStuxnet–  Targetedcyperespionagevirus

•  Stillubiquitous,especiallyinembeddedsystems

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AttacksonMemoryBuffers

•  Bufferisapre-defineddatastorageareainsidecomputermemory(stackorheap)

•  Typicalsituation:–  Afunctiontakessomeinputthatitwritesintoapre-

allocatedbuffer.–  Thedeveloperforgetstocheckthatthesizeoftheinput

isn’tlargerthanthesizeofthebuffer.–  Uhoh.

•  “Normal”badinput:crash•  “Adversarial”badinput:takecontrolofexecution

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StackBuffers

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•  SupposeWebservercontainsthisfunction void func(char *str) {

char buf[126]; ... strcpy(buf,str); ... }

•  Noboundscheckingonstrcpy()•  Ifstrislongerthan126bytes–  Programmaycrash–  Attackermaychangeprogrambehavior

buf uh oh!

AnswerQ2

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•  SupposeWebservercontainsthisfunction void func(char *str) {

char buf[126]; ... strcpy(buf,str); ... }

•  Noboundscheckingonstrcpy()•  Ifstrislongerthan126bytes–  Programmaycrash–  Attackermaychangeprogrambehavior

buf uh oh!

Example:ChangingFlags

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• authenticated variable

buf authenticated11 ( :-) ! )

Example:ChangingFlags

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•  authenticated variable

• Morriswormalsooverflowedabuffertooverwriteanauthenticatedflaginfingerd

buf authenticated11 ( :-) ! )

MemoryLayout

•  Textregion:Executablecodeoftheprogram•  Heap:Dynamicallyallocateddata•  Stack:Localvariables,functionreturnaddresses;

growsandshrinksasfunctionsarecalledandreturn

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Text region Heap StackAddr 0x00...0 Addr 0xFF...F

Top Bottom

RedirectingProgramFlow

•  Insteadof“normal”string,attackersends2thingsasinput:– Assemblycodeshewantstoexecute– Theaddresswheresheexpectsthatcodetoappear

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RedirectingProgramFlow

•  Insteadof“normal”string,attackersends2thingsasinput:– Assemblycodeshewantstoexecute– Theaddresswheresheexpectsthatcodetoappear

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“Shellcode”

StackBuffers

•  SupposeWebservercontainsthisfunc3on: void func(char *str) {

char buf[126]; strcpy(buf,str); }

•  Whenthisfunc3onisinvoked,anewframe(ac3va3onrecord)ispushedontothestack.

Allocatelocalbuffer(126bytesreservedonstack)

Copyargumentintolocalbuffer


Addr 0xFF...F

Saved FP

Executecodeatthisaddressafterfunc()finishes

buf

Localvariables

str

Args

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WhatifBufferisOverstuffed?

•  Memorypointedtobystriscopiedontostack… void func(char *str) {

char buf[126]; strcpy(buf,str); }

•  Ifastringlongerthan126bytesiscopiedintobuffer,itwilloverwriteadjacentstacklocations.

strcpydoesNOTcheckwhetherthestringat*strcontainsfewerthan126characters

Thiswillbeinterpretedasreturnaddress!


Addr 0xFF...F

Saved FPbuf

Localvariables

str

Args

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• Whatifthestringisreadinfromanattackeronthenetwork?



Addr 0xFF...F

Saved FPbuf

Localvariables

str

Args

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exec(“/bin/sh”) asdf…asdf 0xFFFFFFA2



Addr 0xFF...F

Saved FPbuf

Localvariables

str

Args

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ExecutingAttackCode

•  Whenfunc3onexits,codeinthebufferwillbeexecuted,givingaAackerashell–  Rootshellifthevic3mprogramissetuidroot

ret/IPSaved FPbuf Caller’s stack frame

Addr 0xFF...F

Attackerputsactualassemblyinstructionsintohisinputstring,e.g.,binarycodeofexecve(“/bin/sh”)

exec(“/bin/sh”)

Intheoverflow,apointerbackintothebufferappearsinthelocationwherethesystemexpectstofindreturnaddress

Caller’s framestr

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StretchBreak

•  Whenfunc3onexits,codeinthebufferwillbeexecuted,givingaAackerashell–  Rootshellifthevic3mprogramissetuidroot

ret/IPSaved FPbuf Caller’s stack frame

Addr 0xFF...F

Attackerputsactualassemblyinstructionsintohisinputstring,e.g.,binarycodeofexecve(“/bin/sh”)

exec(“/bin/sh”)

Intheoverflow,apointerbackintothebufferappearsinthelocationwherethesystemexpectstofindreturnaddress

Caller’s framestr

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BufferOverflowscanbeHard

•  OverflowportionofthebuffermustcontaincorrectaddressofattackcodeintheRETposition–  ThevalueintheRETpositionmustpointtothebeginningofattackassemblycodeinthebuffer•  Otherwiseapplicationwill(probably)crashwithsegmentationviolation

– Attackermustcorrectlyguessinwhichstackpositionhis/herbufferwillbewhenthefunctioniscalled

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Problem:NoBoundsChecking

•  strcpydoesnotcheckinputsize–  strcpy(buf,str)simplycopiesmemorycontentsintobuf

startingfrom*struntil“\0”isencountered,ignoringthesizeofareaallocatedtobuf

•  ManyClibraryfunctionsareunsafe–  strcpy(char*dest,constchar*src)–  strcat(char*dest,constchar*src)–  gets(char*s)–  scanf(constchar*format,…)–  printf(constchar*format,…)

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•  strncpy(char*dest,constchar*src,size_tn)–  Ifstrncpyisusedinsteadofstrcpy,nomorethanncharacterswill

becopiedfrom*srcto*dest•  Programmerhastosupplytherightvalueofn

•  Potentialoverflowinhtpasswd.c(Apache1.3):strcpy(record,user); strcat(record,”:”); strcat(record,cpw);

•  Publishedfix:strncpy(record,user,MAX_STRING_LEN-1); strcat(record,”:”) strncat(record,cpw,MAX_STRING_LEN-1);

DoesBoundsCheckingHelp?

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Copiesusername(“user”)intobuffer(“record”),thenappends“:”andhashedpassword(“cpw”)

•  strncpy(char*dest,constchar*src,size_tn)–  Ifstrncpyisusedinsteadofstrcpy,nomorethanncharacterswill

becopiedfrom*srcto*dest•  Programmerhastosupplytherightvalueofn

•  Potentialoverflowinhtpasswd.c(Apache1.3):strcpy(record,user); strcat(record,”:”); strcat(record,cpw);

•  Publishedfix:strncpy(record,user,MAX_STRING_LEN-1); strcat(record,”:”) strncat(record,cpw,MAX_STRING_LEN-1);

AnswerQ3

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Copiesusername(“user”)intobuffer(“record”),thenappends“:”andhashedpassword(“cpw”)

Misuseofstrncpyinhtpasswd“Fix”

•  Published“fix”forApachehtpasswdoverflow:strncpy(record,user,MAX_STRING_LEN-1); strcat(record,”:”) strncat(record,cpw,MAX_STRING_LEN-1);

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MAX_STRING_LENbytesallocatedforrecordbuffer

contentsof*user

PutuptoMAX_STRING_LEN-1charactersintobuffer

:

Put“:”

contentsof*cpw

AgainputuptoMAX_STRING_LEN-1charactersintobuffer

WhatAboutThis?

•  Home-brewedrange-checkingstringcopy

void mycopy(char *input) { char buffer[512]; int i; for (i=0; i<=512; i++) buffer[i] = input[i]; } void main(int argc, char *argv[]) { if (argc==2) mycopy(argv[1]); }

•  1-byteoverflow:can’tchangeRET,butcanchangepointertopreviousstackframe–  Onlittle-endianarchitecture,makeitpointintobuffer–  RETforpreviousfunctionwillbereadfrombuffer!

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Off-By-OneOverflow

•  Home-brewedrange-checkingstringcopy

void mycopy(char *input) { char buffer[512]; int i; for (i=0; i<=512; i++) buffer[i] = input[i]; } void main(int argc, char *argv[]) { if (argc==2) mycopy(argv[1]); }

•  1-byteoverflow:can’tchangeRET,butcanchangepointertopreviousstackframe–  Onlittle-endianarchitecture,makeitpointintobuffer–  RETforpreviousfunctionwillbereadfrombuffer!

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Thiswillcopy513charactersintobuffer.Oops!

FramePointerOverflow


Addr 0xFF...F

Saved FPbuf

Localvariables

str

Args

Fake RETFake FPATTACKCODE

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AnotherVariant:FunctionPointerOverflow

•  Cusesfunctionpointersforcallbacks:ifpointertoFisstoredinmemorylocationP,thenanotherfunctionGcancallFas(*P)(…)

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attackcode

Bufferwithattacker-suppliedinputstring

Callbackpointer

Heap

LegitimatefunctionF

overflow

(elsewhereinmemory)

OtherOverflowTargets

•  FormatstringsinC– Moredetailsnexttime

•  Heapmanagementstructuresusedbymalloc()– Moredetailsinsection

•  TheseareallattacksyoucanlookforwardtoinLab#1J

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LookingForward

•  Ethicsformdueat5!•  Homework#1dueMonday,Oct10•  Nextfewclasses:– Friday:guestlecturebyDavidAucsmith– Monday:morebufferoverflows– Wednesday:guestlecturebyEmilyMcReynolds

•  SectiontomorrowaboutLab1

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software security: buﬀer overﬂow attacks€¦ · 10/5/16 cse 484 / cse m 584 - fall 2016 4...

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