Banned APIs and Sin Within! Michael Howard mikehow@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 APIs strcpy, strcpyA, strcpyW, wcscpy, _tcscpy, _mbscpy, StrCpy, StrCpyA, StrCpyW, lstrcpy, lstrcpyA, lstrcpyW, _tccpy, _mbccpy strcat, strcatA, strcatW, wcscat, _tcscat, _mbscat, StrCat, StrCatA, StrCatW, lstrcat, lstrcatA, lstrcatW, StrCatBuff, StrCatBuffA, StrCatBuffW, StrCatChainW, _tccat, _mbccat strncpy, wcsncpy, _tcsncpy, _mbsncpy, _mbsnbcpy, StrCpyN, StrCpyNA, StrCpyNW, StrNCpy, strcpynA, StrNCpyA, StrNCpyW, lstrcpyn, lstrcpynA, lstrcpynW strncat, wcsncat, _tcsncat, _mbsncat, _mbsnbcat, StrCatN, StrCatNA, StrCatNW, StrNCat, StrNCatA, StrNCatW, lstrncat, lstrcatnA, lstrcatnW, lstrcatn CharToOem, 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, wvnsprintfW strtok, _tcstok, wcstok, _mbstok makepath, _tmakepath, _makepath, _wmakepath, _splitpath, _tsplitpath, _wsplitpath scanf, wscanf, _tscanf, sscanf, swscanf, _stscanf, snscanf, snwscanf, _sntscanf _itoa, _itow, _i64toa, _i64tow, _ui64toa, _ui64tot, _ui64tow, _ultoa, _ultot, _ultow gets, _getts, _gettws IsBadWritePtr, IsBadHugeWritePtr, IsBadReadPtr, IsBadHugeReadPtr, IsBadCodePtr, IsBadStringPtr memcpy

PnP MS05-039 Zotob CONFIGRET ResDesToNtResource( 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);

PCT SChannel MS04-011 #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); http://www.microsoft.com/technet/security/Bulletin/MS04-011.mspx

NNTP MS05-030 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); Last Updated 20060103

LSASS MS04-011 Sasser VOID 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); }

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, Murder Important: Bearing False Witness Moderate: Stealing Low: 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 execution Important: Server DoS Moderate: Temporary Server DoS Low: 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. How do you remove banned APIs? By replacing them with something less dangerous! Easy to say, easy 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

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 Within Summary 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!?