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CVE-2021-36955

Description

Windows Common Log File System Driver Elevation of Privilege Vulnerability This CVE ID is unique from CVE-2021-36963, CVE-2021-38633. **Recent assessments:** **gwillcox-r7** at September 18, 2021 12:23am UTC reported: Hmm so this is quite an interesting one. This is similar to CVE-2021-36963 and CVE-2021-38633, both of which are marked as low complexity for being exploited and which will likely get you SYSTEM access, however if you look at the advisory for this bug at <https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-36955> it is the only one where the “Exploit Code Maturity” is marked as “Functional” in other words Microsoft has verified that code exists to exploit this vulnerability and it works under most scenarios. Again this is still only local privilege escalation so this isn’t triggerable remotely, hence why the severity is high due to M.S saying this is easy to form an exploit for and the fact that functional exploit code exists according to M.S, however it isn’t Very High since you still need access to an account on the target to exploit this. Given that this gives SYSTEM level code access and its not hard to make an exploit for it according to Microsoft I would patch this sooner rather than later along with CVE-2021-36963 and CVE-2021-38633. I am still investigating this deeper to figure out what was patched but here is the list of functions within clfs.sys that I believe were patched as part of fixing this bug: 00012 1c00299b4 private: long CClfsBaseFilePersisted::CreateContainer(struct _UNICODE_STRING const &,unsigned __int64 const &,unsigned long,unsigned char,unsigned char,class CClfsContainer * &) 1c0028824 private: long CClfsBaseFilePersisted::CreateContainer(struct _UNICODE_STRING const &,unsigned __int64 const &,unsigned long,unsigned char,unsigned char,class CClfsContainer * &) 0.990 62 61 Perfect match, same name 00013 1c0029d74 public: long CClfsContainer::Create(struct _UNICODE_STRING &,unsigned __int64 const &,struct _CLFS_FILTER_CONTEXT const &,void * const,unsigned char,unsigned char &) 1c0028bdc public: long CClfsContainer::Create(struct _UNICODE_STRING &,unsigned __int64 const &,struct _CLFS_FILTER_CONTEXT const &,void * const,unsigned char,unsigned char &) 0.950 50 49 Perfect match, same name 00024 1c0031a68 public: long CClfsBaseFile::GetSymbol(long,unsigned char,struct _CLFS_CLIENT_CONTEXT * *) 1c00308c8 public: long CClfsBaseFile::GetSymbol(long,unsigned char,struct _CLFS_CLIENT_CONTEXT * *) 0.950 14 17 Perfect match, same name 00026 1c0032550 ClfsCreateLogFile 1c00313d0 ClfsCreateLogFile 0.910 169 164 Perfect match, same name 00025 1c0032420 public: long CClfsBaseFile::GetSymbol(long,unsigned long,struct _CLFS_CONTAINER_CONTEXT * *) 1c0031290 public: long CClfsBaseFile::GetSymbol(long,unsigned long,struct _CLFS_CONTAINER_CONTEXT * *) 0.870 17 20 Perfect match, same name 00047 1c004f3d8 private: long CClfsBaseFilePersisted::ExtendMetadataBlockDescriptor(unsigned long,unsigned long) 1c004e238 private: long CClfsBaseFilePersisted::ExtendMetadataBlockDescriptor(unsigned long,unsigned long) 0.740 41 46 Perfect match, same name More details to come when I get the analysis finished some more. So far that the function `Feature_Servicing_2103c_ClfsStatusPrivilegeNotHeld_31093721__private_IsEnabled()` was removed from the new build of `clfs.sys`, and also from the `CClfsBaseFilePersisted::CreateContainer` call where it was called from. Now it directly checks the return code from `CClfsContainer::Create(_UNICODE_STRING &,unsigned __int64 const &,_CLFS_FILTER_CONTEXT const &,void * const,uchar,uchar &)` to see if it returned the status code `STATUS_PRIVILEGE_NOT_HELD` whereas before it would check the return code of `Feature_Servicing_2103c_ClfsStatusPrivilegeNotHeld_31093721__private_IsEnabled()` for this status. Assessed Attacker Value: 4 Assessed Attacker Value: 4Assessed Attacker Value: 0


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