Versions Affected : All versions prior to TrueNAS 12.0-U3.1

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Description

Negative idmap cache entries can cause incorrect group entries in the Samba file server process token.

The Samba smbd file server must map Windows group identities (SIDs) into unix group ids (gids). The code that performs this had a flaw that could allow it to read data beyond the end of the array in the case where a negative cache entry had been added to the mapping cache. This could cause the calling code to return those values into the process token that stores the group membership for a user. Most commonly this flaw caused the calling code to crash, but an alert user (Peter Eriksson, IT Department, Linköping University) found this flaw by noticing an unprivileged user was able to delete a file within a network share that they should have been disallowed access to. Analysis of the code paths has not allowed us to discover a way for a remote user to be able to trigger this flaw reproducibly or on demand, but this CVE has been issued out of an abundance of caution.

Workaround

No workaround is available. However, systems that do not use Samba are not vulnerable.

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Mitigation

• Upgrade to 12.0-U3.1 or later

Commit

• TrueNAS Jira Ticket NAS-110461
• TrueNAS PR
• Samba commit

Further information

• 

• NIST CVE-2021-20254 Entry

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• MITRE CVE-2021-20254 Entry

• SAMBA Project Announcement