CVE-2026-16158: fastify fastify\/reply-from Confused Deputy

Vulnerability summary

CVE-2026-16158 is a critical (CVSS 10.0) Confused Deputy vulnerability in fastify fastify\/reply-from. EPSS estimates a 0.38% chance of exploitation in the next 30 days, higher than 30.6% of scored vulnerabilities.

Timeline

1. 18-07-2026
   PublishedVulnerability first disclosed.
2. 20-07-2026
   Last modifiedVulnerability information updated.

Description

Impact: @fastify/reply-from versions from 8.3.1 up to but not including 12.6.4 build the internal URL cache key by concatenating the destination and source path without a delimiter. Different destination and source pairs can therefore produce the same key while resolving to different upstream URLs. When getUpstream selects an upstream from request data, a URL cached for one upstream can be reused for a request intended for another upstream, causing cross-upstream data access and modification. The default configuration is affected. Setting disableCache to true prevents the behavior. Patches: upgrade to @fastify/reply-from 12.6.4. Workarounds: pass disableCache: true when registering the plugin.

• CWE-441Unintended Proxy or Intermediary ('Confused Deputy')The product receives a request, message, or directive from an upstream component, but the product does not sufficiently preserve the original source of the request before forwarding the request to an external actor that is outside of the product's control sphere. This causes the product to appear to be the source of the request, leading it to act as a proxy or other intermediary between the upstream component and the external actor.