CVE-2026-92487 exfat: fix valid_size extension over a shared writable mapping
A race condition in the Linux kernel exfat filesystem allows local attackers to overwrite data in shared writable mappings.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-92487?
CVE-2026-92487 is a vulnerability in Linux, published on 17 Sep 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
exfat: fix valid_size extension over a shared writable mapping
When a shared writable mapping has its valid_size extended by a buffered write or a page fault, exfat zeroes the page-cache gap below the new valid_size. A store through the mapping can race with this zeroing and be overwritten.
Fix this by zeroing the gap lazily. Drop ->map_pages so that every first write fault goes through exfat_page_mkwrite(), which advances valid_size to cover the faulting page. With fault-around enabled, a store could install a writable PTE, skip ->page_mkwrite(), and land past valid_size without advancing it. Extending valid_size one faulting page at a time also leaves never-written pages in a large mapping alone.
The gap is filled with block granularity, zeroing only the not-uptodate blocks and preserving blocks that may hold data stored through the mapping. On the buffered-write path the invalidate lock is held and the gap is unmapped before zeroing, so a racing store re-faults and, under the inode lock, completes only after the gap has been zeroed and valid_size covers it.
Which versions are affected by CVE-2026-92487?
How do I fix CVE-2026-92487?
Apply the vendor's patch.
Is CVE-2026-92487 being exploited?
There is no known exploitation of CVE-2026-92487 in the wild and no public exploit code.
Sources
Also known as GHSA-5qr9-x6vc-9rx2, UBUNTU-CVE-2026-92487.