Vulnerability · Published Jul 29, 2026 · Assigned by openjs
CVE-2026-18174@fastify/forwarded vulnerable to improper input validation via unstripped tab characters in X-Forwarded-For
Medium 5.3
In plain English
A medium-severity improper input validation in @fastify/forwarded from @fastify/forwarded. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 25% of all known vulnerabilities).
Official description
@fastify/forwarded resolves client addresses from the X-Forwarded-For header. In versions before 3.0.2, when the header contains two or more comma separated entries, the parser trims only space characters and does not strip horizontal tabs, even though RFC 7230 defines optional whitespace as both space and tab. As a result, an entry padded with a tab keeps the literal tab in the resolved address string. Applications that make exact string match security decisions on the resolved client IP, such as an allowlist, a blocklist, a per IP rate limit key, or audit log correlation, can be evaded because the tab corrupted string no longer matches the expected value. This does not cross the trust boundary, since a tab corrupted string is not a valid IP and cannot be mistaken for a trusted proxy. The issue is fixed in @fastify/forwarded 3.0.2.
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| @fastify/forwarded | @fastify/forwarded | 0 – before 3.0.2 |
Weakness: CWE-20, CWE-184 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N