Envoy, a widely used service proxy, experiences a use‑after‑free crash when an ext_proc server sends a single gRPC message that contains multiple specially crafted ProcessingResponse messages. The first response in that batch causes the gRPC stream object to be torn down, so when Envoy later attempts to process subsequent responses it dereferences freed memory. This flaw manifests as a crash, rendering the affected Envoy instance unable to process further traffic and effectively denying service. The weakness is a classic use‑after‑free, catalogued as CWE‑416.

The vulnerability affects the Envoy project (envoyproxy:envoy). Versions from 1.34.0 up to and including 1.35.13, plus the isolated releases 1.36.9, 1.37.5, and 1.38.3, are impacted. All other tracked Envoy releases are not affected.

The CVSS score of 6.5 indicates medium severity. No EPSS data is available, so the likelihood of proactive exploitation cannot be quantified. The vulnerability is not in CISA KEV, so there is no public evidence of active exploitation. The attack requires an ext_proc server capable of sending a crafted gRPC response; therefore, a compromised or malicious ext_proc service, or a malicious user with control over such traffic, would need to be the vector.