CVE-2026-73548
A different downstream client can then receive the attacker's response.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.46%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
Description
Envoy is an open source edge and service proxy designed for cloud-native applications. Prior to 1.36.10, 1.37.6, 1.38.4, and 1.39.1, Envoy forwards data for a configured non-WebSocket HTTP upgrade before the upstream accepts the upgrade. An unauthenticated HTTP/2 client can place a complete HTTP/1.1 request in extended CONNECT data; Envoy downgrades the request, writes the data unframed to a keep-alive HTTP/1.1 upstream, and returns the socket to the shared pool while the smuggled response remains queued. A different downstream client can then receive the attacker's response. The relevant scope boundary is that webSocket upgrades, plain CONNECT, disabled backend keep-alive, per-downstream pools, and max_requests_per_connection set to 1 are not affected by the demonstrated path. This issue is fixed in versions 1.36.10, 1.37.6, 1.38.4, and 1.39.1.
- CVSS 3.1
- 7.5 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
- EPSS
- 0.46% probability · 40th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Not listed
- Weakness
- CWE-444
- Affected
- envoyproxy/envoy
- Source
- security-advisories@github.com
EPSS moved from 0.7% on 22 September 2026 to 0.5% on 26 October 2026.
References
- https://github.com/envoyproxy/envoy/commit/309855626966cff176155c821043ed3b44671361Patch
- https://github.com/envoyproxy/envoy/commit/3c7998545a9c9fb3933a4bc1907d92f0e752bc2bPatch
- https://github.com/envoyproxy/envoy/commit/4633b8cce3d15b8734eefe232e30c12b0140b91cPatch
- https://github.com/envoyproxy/envoy/commit/bd6711f2617658e28dfa3df3dace6bfe3cfc0766Patch
- https://github.com/envoyproxy/envoy/releases/tag/v1.36.10Release Notes
- https://github.com/envoyproxy/envoy/releases/tag/v1.37.6Release Notes
- https://github.com/envoyproxy/envoy/releases/tag/v1.38.4Release Notes
- https://github.com/envoyproxy/envoy/releases/tag/v1.39.1Release Notes
- https://github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-jqc2Exploit, Vendor Advisory
- https://github.com/envoyproxy/envoy/security/advisories/GHSA-3vhp-c83q-jqc2Exploit, Vendor Advisory
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.