Security in Software

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Listing 720
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void LazyInitialization() { HKEY softwareKey = NULL; LONG retCode = RegOpenKeyEx(HKEY_LOCAL_MACHINE, L Software , 0, MAXIMUM_ALLOWED, &softwareKey); HKEY bookKey = NULL; retCode = RegCreateKey(bookKey, L Advanced Windows Debugging , &bookKey); RegCloseKey(bookKey); RegCloseKey(softwareKey); BOOL otherCode = ImpersonateSelf(SecurityImpersonation); HANDLE threadToken = NULL; otherCode = OpenThreadToken(GetCurrentThread(), TOKEN_QUERY, FALSE, &threadToken); if (threadToken) CloseHandle(threadToken); HANDLE event = CreateEvent(NULL, FALSE, FALSE, L 07sample ); CloseHandle(event); HANDLE threadTokenAsSelf = NULL; otherCode = OpenThreadToken(GetCurrentThread(), TOKEN_QUERY |TOKEN_IMPERSONATE , TRUE, &threadTokenAsSelf); RevertToSelf(); otherCode = ImpersonateLoggedOnUser(threadTokenAsSelf); if (threadTokenAsSelf) CloseHandle(threadTokenAsSelf); RevertToSelf(); }
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Because the product tests are good and no apparent bugs exist in this code, this code is incorporated into a product and then released Soon after, the customer reports that the application fails with one of the following errors in the log file, printed on the screen by the sample as follows:
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RegCreateKeyW failedLast error = 6 ImpersonateSelf failedLast error = 5 OpenThreadToken failedLast error = 5
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Investigating Security Failures
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Along with the known access denied error code 5, we can see an unexpected invalid handle error 6 coming from the registry API By correlating all the places where the key is used or created, we figure out the faulting code is in the lazy initialization path It is triggered by the client request, which executes in the client request thread while the thread impersonates the user We have simulated the impersonation in a simple client application by logging in a specific user, impersonating it, and calling the LazyInitialization function, as shown in the following:
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void Sample2() { HANDLE userToken = NULL; BOOL retCode = LogonUser(L Test1 , NULL, L TestUser1 , LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT, &userToken); ImpersonateLoggedOnUser(userToken); LazyInitialization(); RevertToSelf(); CloseHandle(userToken); }
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Because the code review does not reveal the failure source, we will run this code under a user mode debugger to fully understand what s going wrong Immediately after the first failure line executes, that is, the advapi32!RegCreateKey API, we examine the handle value passed in as the first parameter using the !handle extension command We pick that parameter because the registry API returns invalid handle error
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0:000> !handle poi(softwareKey) 7 Handle 58 Type Key Attributes 0 GrantedAccess 0x20019: ReadControl QueryValue,EnumSubKey,Notify HandleCount 2 PointerCount 3 Name \REGISTRY\MACHINE\SOFTWARE 0:000> * The !handle command decodes the rights granted to the caller
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We notice that the registry API was not granting rights to create any new key in the softwareKey The security manager grants rights to objects when the object is opened,
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based on its security descriptor and requested access mask The access granted and stored in the handle table, along with the handle, is checked by every operation using the handle for validity The access mask associated with the handle is displayed by the !handle extension command, as shown in the previous listing In this case, the key was opened while impersonating a low-privilege user Reading the code once again, we can see the requested mask used to open the registry key as MAXIMUM_ALLOWED, which is a convenient access mask definition that everybody uses Perhaps the developer had no time or desire to find out the necessary rights, and was not willing to justify the use of GENERIC_ALL The system indeed returns what the code asks for, but the granted access is different from what the developer intended As a side note, MAXIMUM_ALLOWED should be used only for probing the object allowed access Using it anywhere else is a code defect waiting to show up After we found one defect, two more errors are waiting Looking back to the trace log, advapi32!ImpersonateSelf fails with an access denied As discussed in the earlier section Local Security Failures, we should first understand the operation and identify the security of all components involved in the operation It is clear by now that advapi32!ImpersonateSelf opens the process handle, duplicates the primary access token, and sets it on the calling thread We set a breakpoint at advapi32!ImpersonateSelf in the user mode debugger, but we continue our investigation using a kernel mode debugger while the user mode debugger is stopped at the breakpoint We start by checking the security information of the process object, as shown in Listing 721 Listing 721
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lkd> !process 0 1 07Sampleexe Peb: 7ffde000 ParentCid: 0284 PROCESS ffb36020 SessionId: 0 Cid: 0784 DirBase: 0a257000 ObjectTable: e183bbb0 HandleCount: 22 Image: 07sampleexe VadRoot ffa7c978 Vads 33 Clone 0 Private 66 Modified 0 Locked 0 DeviceMap e1798128 Token e196a3f0 lkd> !process 0 2 07sampleexe Peb: 7ffde000 ParentCid: 0284 PROCESS ffb36020 SessionId: 0 Cid: 0784 DirBase: 0a257000 ObjectTable: e183bbb0 HandleCount: 22 Image: 07sampleexe THREAD 82f408a8 Cid 078404f8 Teb: 7ffdf000 Win32Thread: e17a5d28 WAIT : (Executive) KernelMode Non-Alertable SuspendCount 1 f3ad77d4 SynchronizationEvent
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