CVE-2026-16158: Fastify Fastify/reply-From

Critical severity, CVSS 10.0. EPSS: 0.4% chance of exploitation in the next 30 days.

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.

Affected products

• Fastify Fastify/reply-From: from 8.3.1, before 12.6.4 (fixed in 12.6.4)

Published 2026-07-18. Last modified 2026-08-05.