banned apis and sin within! michael howard mikehow@microsoft.com

Banned APIs and Sin Within!

Michael Howardmikehow@microsoft.com

Who Is This Guy?

• mikehow@microsoft.com• Christian (imperfect in every possible way!)• Microsoft employee for 20 years• Always in security• Worked on the Microsoft SDL since inception

Goals and Non-Goals

• I am not one for drawing analogies– “Security Analogies are usually Wrong”

http://blogs.msdn.com/b/michael_howard/archive/2006/03/09/547575.aspx

• I use quotes from the Bible to compare/contrast software security

• “The Bible is correct, your code is not.” :-)

If cars operated in an environment like the Internet, they would…

• Be driven by people with little regard for safe automobile operation.• Have their windshields shot out every 60 secs.• Once you have bullet-proof glass, the bad guys place nails at freeway

off-ramps next to signs like, “free coffee this way”– and someone is always trying to steal your keys– and pull out your sparkplugs– and siphon your gas

• Talking of gas, you fill up at a Shell station, only to realize the gas really isn’t gas, it’s vegetable oil and sand

• Oh, that gas station isn’t a Shell station, it certainly looked like one, but they took your credit card details anyway

• As this all goes on, you can’t see the adversary• And the adversaries are sharing new weapons with each other

The SDL

• A set of process changes that help improve software security

• Over 100 requirements and recommendations• About 30 deal with memory corruption• Removing banned APIs is one such

requirement

What Are The Banned APIs?

• Mostly memory corruption APIs– strcpy …– strcat …– strncpy …– strncat …– sprintf …– gets …

Banned APIsstrcpy, strcpyA, strcpyW, wcscpy, _tcscpy, _mbscpy, StrCpy, StrCpyA, StrCpyW, lstrcpy, lstrcpyA, lstrcpyW, _tccpy, _mbccpystrcat, strcatA, strcatW, wcscat, _tcscat, _mbscat, StrCat, StrCatA, StrCatW, lstrcat, lstrcatA, lstrcatW, StrCatBuff, StrCatBuffA, StrCatBuffW, StrCatChainW, _tccat, _mbccatstrncpy, wcsncpy, _tcsncpy, _mbsncpy, _mbsnbcpy, StrCpyN, StrCpyNA, StrCpyNW, StrNCpy, strcpynA, StrNCpyA, StrNCpyW, lstrcpyn, lstrcpynA, lstrcpynWstrncat, wcsncat, _tcsncat, _mbsncat, _mbsnbcat, StrCatN, StrCatNA, StrCatNW, StrNCat, StrNCatA, StrNCatW, lstrncat, lstrcatnA, lstrcatnW, lstrcatnCharToOem, CharToOemA, CharToOemW, OemToChar, OemToCharA, OemToCharW, CharToOemBuffA, CharToOemBuffW

wnsprintf, wnsprintfA, wnsprintfW, sprintfW, sprintfA, wsprintf, wsprintfW, wsprintfA, sprintf, swprintf, _stprintf, _snwprintf, _snprintf, _sntprintf, wvsprintf, wvsprintfA, wvsprintfW, vsprintf, _vstprintf, vswprintf, _vsnprintf, _vsnwprintf, _vsntprintf, wvnsprintf, wvnsprintfA, wvnsprintfWstrtok, _tcstok, wcstok, _mbstokmakepath, _tmakepath, _makepath, _wmakepath, _splitpath, _tsplitpath, _wsplitpathscanf, wscanf, _tscanf, sscanf, swscanf, _stscanf, snscanf, snwscanf, _sntscanf_itoa, _itow, _i64toa, _i64tow, _ui64toa, _ui64tot, _ui64tow, _ultoa, _ultot, _ultowgets, _getts, _gettwsIsBadWritePtr, IsBadHugeWritePtr, IsBadReadPtr, IsBadHugeReadPtr, IsBadCodePtr, IsBadStringPtrmemcpy

CONFIGRETResDesToNtResource( IN PCVOID ResourceData, IN RESOURCEID ResourceType, IN ULONG ResourceLen, IN PCM_PARTIAL_RESOURCE_DESCRIPTOR pResDes, IN ULONG ulTag ){ case ResType_ClassSpecific: { PCS_RESOURCE pCsData = (PCS_RESOURCE)ResourceData; LPBYTE ptr = NULL; ptr = (LPBYTE)((LPBYTE)pResDes + sizeof(CM_PARTIAL_RESOURCE_DESCRIPTOR)); memcpy(ptr, pCsData->CS_Header.CSD_Signature + pCsData->CS_Header.CSD_LegacyDataOffset, pCsData->CS_Header.CSD_LegacyDataSize);

PnP MS05-039Zotob

#define SSL2_MAX_CHALLENGE_LEN 32

typedef struct _Ssl2_Client_Hello {DWORD dwVer;DWORD cCipherSpecs;DWORD cbSessionID;DWORD cbChallenge;UCHAR SessionID[SSL3_SESSION_ID_LEN]; UCHAR Challenge[SSL2_MAX_CHALLENGE_LEN];Ssl2_Cipher_Kind CipherSpecs[MAX_UNI_CIPHERS];

} Ssl2_Client_Hello, * PSsl2_Client_Hello;

SP_STATUS Pct1SrvHandleUniHello(..., PSsl2_Client_Hello pHello,...) { Pct1_Client_Hello ClientHello;

...

CopyMemory( ClientHello.Challenge,pHello->Challenge,pHello->cbChallenge);

PCT SChannel MS04-011

NNTP MS05-030

Last Updated 20060103

HRESULT CNewsStore::OnResponse(LPNNTPRESPONSE pResponse) {...if (pResponse->state == NS_LIST)

hr = _HandleListResponse(pResponse, FALSE)...

}

HRESULT CNewsStore::_HandleListResponse(LPNNTPRESPONSE pResp, BOOL fNew) { LPSTR psz, pszCount; int nSize; char szGroupName[CCHMAX_FOLDER_NAME]; LPNNTPLIST pnl = &pResp->rList; for (DWORD i = 0; i < pnl->cLines; i++, m_op.dwProgress++) { psz = pnl->rgszLines[i]; while (*psz && !IsSpace(psz)) psz = CharNext(psz); nSize = (int)(psz - pnl->rgszLines[i]); if (nSize >= CCHMAX_FOLDER_NAME) nSize = CCHMAX_FOLDER_NAME - 1; CopyMemory(szGroupName, pnl->rgszLines[i], nSize);

LSASS MS04-011VOID DsRolepDebugDumpRoutine( IN DWORD DebugFlag, IN LPWSTR Format, va_list arglist ) {

#define DsRolepDebugDumpRoutine_BUFFERSIZE 1024 WCHAR OutputBuffer[DsRolepDebugDumpRoutine_BUFFERSIZE]; ... length += (ULONG) wvsprintfW(&OutputBuffer[length], Format, arglist); ...}

Sasser

How Do you Find Them?

• #include <banned.h>• C4996 warnings

The Replacements

• Don’t use C++ as a glorified C!• Use std::string• Use strsafe.h• Use strcpy_s etc

Auto-replacement of Banned Functions

• If the compiler knows the destination buffer size at compile time, it can automatically generate secure code

• Add the following to auto-migrate functions to

safe functions• #define _CRT_SECURE_CPP_OVERLOAD_STANDARD_NAMES (1)

char buf[32];strcpy(buf,src);

char buf[32];strcpy_s(buf,src,32);

But Isn’t C dead?

http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html

The Leap of Faith

• What about regressions?• In ten years, I have seen only one regression at

Microsoft

Effectiveness?

• Over 25% of MSRC memory corruption vulns did not affect newer products simply because we banned the API(s) in question and replaced them with a more secure version

• That’s low cost engineering at its best!

Pop Quiz

• What’s in an 8oz glass of wine?• What’s in an 8oz glass of poison?• What’s in an 8oz glass of wine with a drop of

poison?

Sin and Insecure Code

Righteous Man+ One Sin

Sinful Man

Well-Written Code+ One Vulnerability

Insecure System

All Sin is the Same …• There is no “good” or “bad” sin, it’s all sin in

God’s eyes.• There is no “Security Bulletin” scale for sin

Critical: Adultery, MurderImportant: Bearing False WitnessModerate: StealingLow: Coveting

… but insecure Code is not the Same

• An anonymously accessible remote code execution vulnerability that gives you root is *way* worse than a local information disclosure vuln accessibly only by admins

Critical: Remote code executionImportant: Server DoSModerate: Temporary Server DoSLow: Client DoS

Banned APIs

• We have banned over 120 APIs at Microsoft• They are great examples of “One-line” Sins

Removing Sin• How do you remove Sin?• By replacing Sin with something not Sinful!• Easy to say, very hard to do.

And I know that nothing good lives in me, that is, in my sinful nature. I want to do what is right, but I can't.Romans 7:18

• How do you remove banned APIs?• By replacing them with something less dangerous!• Easy to say, easy to do.

Removal takes a Leap of Faith

• Trust that God forgives your Sins• Trust that the banned API replacements don’t

introduce regressions!

Praise the Lord, … who forgives all your sins.Psalm 103:3

How Do you Remove Banned APIs?

• Admit you have banned APIs (admit you sin!)• Do something about it (admit the Lord into

your heart)• Don’t repeat!

Banned APIs and the Sin WithinSummary

• Admit you sin– In life and in code

• Do something about it – Study Romans– Remove Banned APIs

• Put steps in place to help prevent Sin and banned APIs– Think!!– Use banned.h in all your C/C++ code

Questions!?