Envoy Proxy EUVD-2026-39826

The ext_proc (External Processing) filter must be explicitly enabled in the Envoy configuration - it is not active by default and requires deliberate opt-in. An attacker must control or compromise the ext_proc gRPC server that the Envoy instance is configured to contact, as the malicious payload must originate from that server-side position in the gRPC exchange. The CVSS PR:L metric confirms that some level of access or privilege is required, consistent with the requirement to control the ext_proc server. Exploitation is further bounded by: ext_proc being a non-default, intentionally deployed feature; the need for a supply-chain, insider, or network pivot position at the gRPC server; and the impact being strictly limited to process availability with no data exposure. Envoy deployments that do not use the ext_proc filter are entirely unaffected.

The CVSS 6.5 vector (AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H) is well-calibrated for this vulnerability. The network attack vector and low complexity are accurate - once an attacker controls the ext_proc gRPC endpoint, crafting the triggering payload is straightforward. The PR:L requirement is the primary risk gate: exploitation requires the attacker to occupy the ext_proc server role, which implies either supply-chain compromise of the ext_proc service, insider access, or SSRF pivot to a position where gRPC responses can be injected. No EPSS score was provided in the input, so exploitation probability cannot be quantified numerically. No CISA KEV listing exists and no public POC has been identified, suggesting this has not yet seen opportunistic exploitation. The impact is strictly limited to availability (process crash, potential service disruption); there is zero confidentiality or integrity exposure. For organizations actively using ext_proc, this warrants prompt patching given low post-exploitation complexity; for those not using ext_proc, real-world risk is nil regardless of CVSS score.

An attacker who has compromised or controls a configured ext_proc gRPC server crafts a single gRPC protocol message that bundles multiple ProcessingResponse frames, where the first frame is designed to trigger destruction of Envoy's gRPC stream object. When Envoy receives and processes the batched message, the iterator accesses the freed stream pointer for subsequent frames, causing a use-after-free crash and taking the proxy process down. No public proof-of-concept has been identified, but the trigger condition is mechanically straightforward for an attacker with ext_proc server control.