CVE-2026-47692: Envoy PROXY Protocol v2 Request Smuggling via Oversized TLV Headers

Technical summary

CVE-2026-47692 stems from improper length field handling in the PROXY Protocol v2 (PP2) header generator within Envoy. When a PP2 header's Type-Length-Value (TLV) extensions exceed the 16-bit maximum of 65,535 bytes, the code fails to properly constrain the output or reject the oversized header. Instead, it writes all TLV data while the length field in the PP2 header reflects only a truncated value. This creates a buffer overflow condition where trailing bytes—intended as part of the PP2 frame—are left unaccounted for in the header's declared length, allowing them to be interpreted as part of the upstream request body. An attacker with network-level access who can influence request metadata or trigger high-cardinality logging/tracing data could exploit this to inject additional request content.

Business impact

Organizations running Envoy as an ingress proxy or service mesh component face a moderate risk of request smuggling attacks that could bypass security controls or corrupt backend service logic. The practical impact depends on deployment topology: if Envoy sits between untrusted clients and backend servers, an attacker could craft requests that appear legitimate at the proxy but deliver unexpected payloads to the backend. This is particularly concerning in zero-trust or multi-tenant environments where request isolation is a security boundary. The vulnerability requires authenticated or network-adjacent attacker positioning, limiting its exploitability in fully cloud-managed scenarios but posing real risk in hybrid or on-premises deployments with shared network infrastructure.

Affected systems

Envoy versions 1.34.0 through 1.34.x (all), 1.35.0 through 1.35.12, 1.36.0 through 1.36.8, 1.37.0 through 1.37.4, and 1.38.0 through 1.38.2 are vulnerable. The fix is available in Envoy 1.35.13, 1.36.9, 1.37.5, and 1.38.3. Organizations should verify their deployed version and check their update cadence; LTS release lines may differ, so consult the Envoy release notes for your configured branch.

Exploitability

Exploitation requires network-adjacent or authenticated attacker access (AV:A per CVSS) and elevated privileges (PR:H), making opportunistic remote exploitation unlikely. However, an insider, compromised application, or attacker already on the network perimeter could reliably trigger this by sending requests with large TLV payloads or by inducing Envoy to generate bulky trace/metadata headers. No public exploit code is known at this time, and the vulnerability does not appear on the KEV catalog, suggesting limited in-the-wild exploitation to date.

Remediation

Upgrade Envoy to a patched version: 1.35.13 or later, 1.36.9 or later, 1.37.5 or later, or 1.38.3 or later. For organizations on unsupported versions, upgrade to the latest stable release in your chosen branch. Interim mitigations include disabling or limiting PP2 header generation if not required for your proxy topology, and implementing network-layer access controls to restrict who can send large or malformed metadata to Envoy.

Patch guidance

Apply the patched releases as soon as operationally feasible, prioritizing instances that handle untrusted traffic or sit at network boundaries. Test patches in a staging environment that mirrors your production PROXY Protocol setup (particularly if using extended TLV fields for tracing or custom metadata) before rolling out. Most organizations on regular update cycles should encounter these patches within their normal maintenance windows.

Detection guidance

Monitor Envoy access logs for signs of oversized PP2 headers or TLV fields approaching the 65KB boundary. Enable verbose Envoy admin logging to capture PP2 frame generation. Upstream servers may log malformed or unexpected request data; correlate backend errors or security events with client IP ranges known to communicate through the affected Envoy instance. Network-based detection is difficult without deep protocol analysis, but packet captures during suspected attacks should reveal PP2 headers with length fields that do not match actual frame size.

Why prioritize this

Although the CVSS score is moderate (4.8), this vulnerability merits swift attention due to its request smuggling nature, which can undermine security controls and application logic. Organizations with strict request validation boundaries, those enforving zero-trust policies, or those processing sensitive data should treat this as high priority. The low barrier to exploitation for network-adjacent threats and the potential for supply-chain or multi-tenant attacks in shared infrastructure make it a medium-to-high business priority depending on your threat model, even though random internet scanning is not a realistic vector.

Risk score, explained

The CVSS 3.1 score of 4.8 reflects the requirement for authenticated or network-adjacent attacker position and elevated privileges, capping the base severity at MEDIUM. However, the impact—smuggled bytes on the upstream request—introduces integrity concerns (I:L) and potential availability issues if malformed requests crash backends (A:L). The score does not fully capture the insidious nature of request smuggling, which can enable authorization bypass or cache poisoning. Organizations should consider the CVSS as a floor, not a ceiling, especially in architectures where Envoy enforces critical security boundaries.

Frequently asked questions