CVE-2026-16158: @fastify/reply-from vulnerable to cross-upstream request routing via URL cache key collision

Severity: critical (CVSS 10.0 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-18 · updated 2026-08-05
Weakness: CWE-441

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.

Affected products

@fastify/reply-from: from 8.3.1 before 12.6.4 — fixed in 12.6.4

How to fix

Upgrade to a fixed version: @fastify/reply-from 12.6.4. Check the vendor advisory for mitigations.

References

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