PatchTheCVECVE-2026-3635: Summary When trustProxy is configured with a restrictive trust function (e.g., a specific IP like trustProxy:
CVE-2026-3635 is a medium-severity flaw (CVSS 6.1) affecting fastify. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
Summary When trustProxy is configured with a restrictive trust function (e.g., a specific IP like trustProxy: '10.0.0.1', a subnet, a hop count, or a custom function), the request.protocol and request.host getters read X-Forwarded-Proto and X-Forwarded-Host headers from any connection — including connections from untrusted IPs. This allows an attacker connecting directly to Fastify (bypassing the proxy) to spoof both the protocol and host seen by
Why this priority
- No elevated risk signals yet
A fixed release or distro patch exists.
Risk facts
| CVSS score | 6.1 (Medium) |
| Attack vector | Adjacent network |
| Attack complexity | High |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 0.2% (top 95%) |
| CISA exploited list | No |
| Weakness | CWE-348 |
| Published | 2026-03-23 |
| Last modified | 2026-06-17 |
Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2026-3635?
fastify: upgrade to 5.8.3. Exact commands for each OS are on this page.
Is CVE-2026-3635 being exploited?
CVE-2026-3635 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2026-3635 affect?
CVE-2026-3635 affects fastify. Severity is medium (CVSS 6.1). Summary When trustProxy is configured with a restrictive trust function (e.g., a specific IP like trustProxy: '10.0.0.1', a subnet, a hop count, or a custom function), the request.protocol and request.host getters read X-Forwarded-Proto and
How urgent is CVE-2026-3635?
Priority 23/100. No elevated risk signals yet.
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