GHSA-3m5p-2c4r-xxw2 is a medium-severity (CVSS 6.1) CWE-348 vulnerability in fastify. O3 Security confirms whether GHSA-3m5p-2c4r-xxw2 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-3m5p-2c4r-xxw2.
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
GHSA-3m5p-2c4r-xxw2 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 0 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
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
fastifynpmDescription
Impact
The fix for CVE-2026-3635 (GHSA-444r-cwp2-x5xf) added a proxyFn(socket.remoteAddress, 0) guard on the X-Forwarded-* reads in request.host, request.protocol, request.hostname, request.ip, and request.ips. That guard closes the IP, CIDR, and custom-function forms of trustProxy correctly because those forms compile to predicates that inspect the connecting address. The hop-count form (trustProxy: <number>) compiles to a predicate that structurally ignores the address argument, so the guard reduces to 0 < tp, always true for any tp >= 1.
Applications configured with trustProxy: <number> (documented as "behind N reverse proxies", trustProxy: 1 being the canonical single-proxy setting) remain vulnerable. An attacker who can reach the Fastify origin directly, bypassing the front-facing proxy, can spoof the request fields exactly as in the unpatched version. Impact class matches the parent CVE-2026-3635: host injection in generated URLs, HTTPS-enforcement bypass, secure-cookie / CSRF-origin bypass, host-based routing and cache poisoning.
Patches
Patched in fastify 5.12.1. The numeric form of trustProxy is now disabled at runtime and removed from the TypeScript type union.
Workarounds
- Migrate to an IP / CIDR / custom-function
trustProxyvalue that validates the connecting address. Custom functions must inspect theaddressargument, not only the hop index. - Ensure the Fastify origin is only reachable through the trusted proxy chain (no direct network path).
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | fastify | ≥ 5.8.3&&< 5.12.1 | 5.12.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastify. 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 fastify to 5.12.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-3m5p-2c4r-xxw2 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 GHSA-3m5p-2c4r-xxw2 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 GHSA-3m5p-2c4r-xxw2. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-3m5p-2c4r-xxw2 in your dependencies?
O3 detects GHSA-3m5p-2c4r-xxw2 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.