CVE-2026-63795
CRITICALCVE-2026-63795 is a critical-severity (CVSS 10) Use After Free vulnerability in Kernel. O3 Security confirms whether CVE-2026-63795 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
9p: avoid putting oldfid in p9_client_walk() error path
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-63795 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 367,996 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
KernelReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Linux packages — download data is not available via public APIs for these ecosystems.
Description
In the Linux kernel, the following vulnerability has been resolved:
9p: avoid putting oldfid in p9_client_walk() error path
When p9_client_walk() is called with clone set to false, fid aliases oldfid. If the walk subsequently fails after the request has been sent, the error path jumps to clunk_fid, which currently calls p9_fid_put(fid) unconditionally.
This drops a reference to oldfid even though ownership of oldfid remains with the caller. If this is the last reference, oldfid can be clunked and destroyed while the caller still expects it to be valid. A later use or put of oldfid can then trigger a use-after-free or refcount underflow.
Fix this by only putting fid in the clunk_fid error path when it does not alias oldfid, matching the existing guard in the error path below.
This can be triggered when a multi-component walk is split into multiple p9_client_walk() calls and a later non-cloning walk fails. A reproducer and refcount warning logs are available on request.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐧Linux | Kernel | ≥ 6.0.0&&< 6.1.177 | 6.1.177 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Kernel. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update Kernel to 6.1.177 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-63795 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-63795 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-63795. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-63795 in your dependencies?
O3 detects CVE-2026-63795 across Linux dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.