CVE-2026-16732
mediumpriority · track · 0EPSS 0.002 · 5 %ileUSEPSS · scored 2026-10-27
fastify is a fast and low overhead web framework for Node.js. Impact: the fix for CVE-2026-3635 added a guard on the forwarded-header reads used to derive the request host, protocol, hostname, ip, and ips values, checking the connecting address. 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, where trustProxy is set to a number, compiles to a predicate that structurally ignores the address, so the guard is always satisfied for any hop count of one or more. Applications configured with a numeric trustProxy value, such as trustProxy set to 1 for a single reverse proxy, remain vulnerable: an attacker who can reach the Fastify origin directly, bypassing the front-facing proxy, can spoof the forwarded request fields exactly as in the unpatched version. The impact class matches the parent CVE-2026-3635, including host injection in generated URLs, HTTPS-enforcement bypass, secure-cookie and CSRF-origin bypass, and host-based routing and cache poisoning. Affected versions are fastify from 5.8.3 up to but not including 5.12.1. Patches: patched in fastify 5.12.1, where the numeric form of trustProxy is disabled at runtime and removed from the TypeScript type union. Workarounds: migrate to an IP, CIDR, or custom-function trustProxy value that validates the connecting address, and ensure the Fastify origin is only reachable through the trusted proxy chain.
cvss
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base score 0–10 · higher = more severe · hover a vector to decode
ssvc
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weaknesses
The CWE weakness classes this flaw belongs to.
affected products (2)
Products and version ranges known to be affected.
advisories (1)
Vendor and ecosystem advisories describing this.
- GHSA-3m5p-2c4r-xxw2fastify vulnerable to X-Forwarded-* spoofing under trustProxy hop-count2026-09-02ghsa · osvUS
## Impact The fix for [CVE-2026-3635](https://www.cve.org/CVERecord?id=CVE-2026-3635) ([GHSA-444r-cwp2-x5xf](https://github.com/fastify/fastify/security/advisories/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 `trustProxy` value that validates the connecting address. Custom functions must inspect the `address` argument, not only the hop index. - Ensure the Fastify origin is only reachable through the trusted proxy chain (no direct network path).
sources
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references (2) nvd
External citations — patches, advisories, analysis.
- https://cna.openjsf.org/security-advisories.htmlThird Party Advisory
- https://github.com/fastify/fastify/security/advisories/GHSA-3m5p-2c4r-xxw2MitigationVendor Advisory
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