CVE-2026-74583
CVE-2026-74583 is a security vulnerability in Kernel. O3 Security confirms whether CVE-2026-74583 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
net/sched: cls_route: fix fastmap use-after-free on filter
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:
net/sched: cls_route: fix fastmap use-after-free on filter
The route4 classifier maintains a 16-slot fastmap cache that stores raw struct route4_filter pointers indexed by (id, iif). The reader (route4_classify) populates this cache via route4_set_fastmap() for every classified packet that hits a filter. The writer (route4_delete, route4_change) clears the cache via route4_reset_fastmap() before RCU-deferred kfree of the filter.
This creates a UAF race:
- Reader walks the RCU-protected bucket chain, finds filter f
- Writer unlinks f, calls route4_reset_fastmap(), then tcf_queue_work()
- Reader calls route4_set_fastmap() and writes f into the cache after the writer's reset, caching a pointer about to be freed
- After the RCU grace period, kfree(f) executes
- Next classified packet on the same (id, iif) tuple hits the stale fastmap entry and reads f->res from freed memory
Reproduced with an mdelay(100) accelerator in route4_set_fastmap() and a concurrent add/delete stress test (provided by both zdi and Santosh). Both triggered KASAN slab-use-after-free reports in the route4 fastmap paths.
Fix: Introduce a per-filter boolean dying flag to suppress stale fastmap republishing by in-flight readers.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐧Linux | Kernel | ≥ 3.18.0&&< 5.10.265 | 5.10.265 |
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 5.10.265 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-74583 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-2026-74583 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-2026-74583. 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-2026-74583 in your dependencies?
O3 detects CVE-2026-74583 across Linux dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.