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A GitHub-reviewed security advisory, not necessarily under active exploitation the way our CISA KEV advisories are, but a real, disclosed vulnerability in an open-source dependency.
Affected packages
Vulnerable: < 5.12.5
Patched in 5.12.5
Details
fastify crashes with an uncaught ERR_HTTP2_INVALID_CONNECTION_HEADERS exception when a route that registers a response trailer via reply.trailer() is served over HTTP/2. Fastify unconditionally adds the Transfer-Encoding: chunked header when a trailer is set, which is forbidden on HTTP/2, so Node.js throws while serializing the response headers. The exception is not caught and becomes an uncaughtException, terminating the Node.js process.
One unauthenticated HTTP/2 request to any route that uses trailers is enough to crash the server, dropping all in-flight requests, and the request can be repeated to keep the process down. Applications are affected only when HTTP/2 is enabled (http2: true) and at least one route registers a trailer. HTTP/1.x responses are not affected.
Upgrade to fastify 5.12.5 or later.
Avoid registering response trailers with reply.trailer() on routes served over HTTP/2 until upgrading.
EPSS, exploit probability
Low0.41%
estimated chance of real-world exploitation in the next 30 days, higher than 36.0% of every CVE FIRST.org scores
Refreshed 11/15/2026, via FIRST.org's EPSS model, not CVSS, this measures likelihood of exploitation, not how severe it would be.
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