CVE-2026-18174: fastify fastify\/forwarded Improper Input Validation

Vulnerability summary

CVE-2026-18174 is a medium (CVSS 5.3) Improper Input Validation vulnerability in fastify fastify\/forwarded. EPSS estimates a 0.32% chance of exploitation in the next 30 days, higher than 23.8% of scored vulnerabilities.

Timeline

1. 29-07-2026
   PublishedVulnerability first disclosed.
2. 29-07-2026
   Last modifiedVulnerability information updated.

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.

• CWE-20Improper Input ValidationThe product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
• CWE-184Incomplete List of Disallowed InputsThe product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are not allowed by policy or otherwise require other action to neutralize before additional processing takes place, but the list is incomplete.