CVE-2026-68149 in Linux
Summary
by MITRE • 08/10/2026
In the Linux kernel, the following vulnerability has been resolved:
fs: preserve ACL_DONT_CACHE state in forget_cached_acl()
The ACL_DONT_CACHE state is meant to be a constant state for the inode for filesystems that want to opt out of posix acl caching.
Commit facd61053cff1 ("fuse: fixes after adapting to new posix acl api") used this facility to opt out of posix acl caching for fuse inodes with fuse server that does not negotiate FUSE_POSIX_ACL (fc->posix_acl).
The commit also takes care to gate the forget_all_cached_acls() call in fuse_set_acl() on fc->posix_acl because there is no need for it, but there are other placed in fuse code which call forget_all_cached_acls() unconditional to fc->posix_acl and those cause the loss of the ACL_DONT_CACHE state.
This is not only a functional bug. Properly timed, a get_acl() from this fuse filesystem can return a stale cached value, as was observed in tests, because set_acl() does not invalidate the unintentional acl cache.
We could fix this in fuse, but it actually makes no sense for the vfs helper forget_cached_acl() to invalidate the ACL_DONT_CACHE state, so let it not do that to fix fuse and future users of ACL_DONT_CACHE.
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Analysis
by VulDB Data Team • 08/10/2026
The vulnerability described represents a critical flaw in the Linux kernel's virtual file system (VFS) layer concerning Access Control List (ACL) caching mechanisms. This issue specifically impacts how the kernel handles the ACL_DONT_CACHE state flag, which is designed to prevent POSIX ACL caching for certain filesystems that wish to maintain explicit control over their access control management. The vulnerability stems from an improper interaction between the FUSE (Filesystem in Userspace) implementation and the generic VFS caching infrastructure, creating a scenario where cached ACL values become stale and potentially invalid.
The technical flaw manifests when the forget_cached_acl() helper function in the VFS layer inadvertently clears the ACL_DONT_CACHE state flag during cache invalidation operations. This occurs because the function does not properly distinguish between regular cached ACL entries and those explicitly marked with the DONT_CACHE flag, leading to a fundamental violation of the intended caching behavior for filesystems that opt out of POSIX ACL caching. The problem was introduced through commit facd61053cff1 which modified the FUSE implementation to handle POSIX ACL API changes, but failed to account for all code paths that might trigger cache invalidation.
The operational impact of this vulnerability extends beyond simple functional failure to encompass potential security implications and data integrity concerns. When the ACL_DONT_CACHE state is incorrectly cleared, filesystem operations can return stale cached ACL values that no longer reflect the actual access permissions, potentially allowing unauthorized access or denial of service conditions. This is particularly problematic in FUSE-based filesystems where the server component may not support POSIX ACL negotiation but the client-side kernel code still attempts to manage caching behavior appropriately. The vulnerability affects not only FUSE filesystems but any future implementations that rely on the ACL_DONT_CACHE mechanism, as the fix must address the core VFS helper function rather than specific filesystem implementations.
The solution requires modifying the forget_cached_acl() helper function to preserve the ACL_DONT_CACHE state flag during cache invalidation operations, ensuring that filesystems which explicitly opt out of POSIX ACL caching maintain their intended behavior. This approach aligns with established security practices and follows the principle of least privilege by preventing unintended cache invalidation that could compromise access control policies. The fix addresses the root cause rather than symptomatically patching individual filesystem implementations, making it applicable to all users of the ACL_DONT_CACHE facility. This vulnerability classification corresponds to CWE-284 Access Control Bypass and relates to ATT&CK technique T1068 Privilege Escalation through improper access control enforcement in kernel space operations.
The broader implications of this vulnerability highlight the complexity of kernel-level caching mechanisms and the importance of maintaining explicit state flags for filesystem-specific behaviors. It demonstrates how seemingly minor changes in API adoption can introduce subtle but significant security flaws that affect multiple subsystems, emphasizing the critical need for comprehensive testing of caching behaviors across different filesystem implementations and the importance of preserving intended behavioral contracts between kernel components and filesystem drivers.