CVE-2026-52906
HighCVSS 7.7Exploitation Probability (EPSS)
Low risk2th percentile - higher than 2% of all known CVEs
Summary
In the Linux kernel's 9p filesystem, a bug was found where access mode flags were ORed instead of replaced, causing multiple bits to be set simultaneously. This prevented correct access mode matching and led to the use of an invalid user ID (nobody/65534) for file operations.
Risk Assessment
The organization faces issues with permission management in 9p filesystems, including the inability for root to change file ownership (chown) and potential misassignment of operations to an incorrect user, disrupting applications relying on this filesystem.
Recommendation
Immediately update the Linux kernel to a version containing the fix (commit that corrects the ORing of flags). After updating, remount 9p partitions to ensure the new settings are applied correctly.
Original NVD description (English source)
In the Linux kernel, the following vulnerability has been resolved: 9p: fix access mode flags being ORed instead of replaced Since commit 1f3e4142c0eb ("9p: convert to the new mount API"), v9fs_apply_options() applies parsed mount flags with |= onto flags already set by v9fs_session_init(). For 9P2000.L, session_init sets V9FS_ACCESS_CLIENT as the default, so when the user mounts with "access=user", both bits end up set. Access mode checks compare against exact values, so having both bits set matches neither mode. This causes v9fs_fid_lookup() to fall through to the default switch case, using INVALID_UID (nobody/65534) instead of current_fsuid() for all fid lookups. Root is then unable to chown or perform other privileged operations. Fix by clearing the access mask before applying the user's choice.