CVE-2026-48743: Envoy HTTP/3 to HTTP/1 request smuggling in downstream translation

CVE-2026-48743 · Severity: high · CVSS 7.5 · Published 2026-06-26

Envoy, a popular open-source service proxy used to manage network traffic, is vulnerable to a request smuggling flaw when translating traffic between different web protocols (HTTP/3 to HTTP/1). An attacker can craft a specific type of request that tricks the backend server into misinterpreting where one request ends and the next begins. This allows the attacker to bypass security rules, potentially accessing restricted data or performing unauthorized actions on the backend system.

Technologies: Envoy Proxy Envoy. Vendors: Envoy Proxy.

Quick answers

Is CVE-2026-48743 exploited in the wild?
No. Junglewise Threat Intelligence has no report of CVE-2026-48743 being exploited in the wild.
How severe is CVE-2026-48743?
High severity, with a CVSS score of 7.5, published 26 June 2026.
Which products and versions are affected?
Envoy Proxy Envoy >= 1.35.0, < 1.35.11; >= 1.36.0, < 1.36.7; >= 1.37.0, < 1.37.3; >= 1.38.0, < 1.38.1.
Is there a fix for CVE-2026-48743?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-48743 attacked?
It is attacked over the network, in a hosted service.
Am I affected by CVE-2026-48743?
Check whether you run Envoy Proxy Envoy at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A request smuggling vulnerability exists in Envoy's HTTP/3 to HTTP/1 translation boundary. When a downstream HTTP/3 request is received with a 'HEADERS with FIN' (headers-only close) but contains a non-zero Content-Length header, Envoy fails to reconcile the declared body size with the actual received bytes before forwarding the request. The upstream HTTP/1 codec emits the Content-Length and finalizes the request, leaving the backend origin with 'unresolved body debt.' If the backend origin replies before reading the full declared body and keeps the connection alive, it may interpret the start of a subsequent request as the body of the first, and the remainder of that subsequent request as a new, unsanitized command. This allows attackers to bypass Envoy's routing policies and authorization checks. The issue is fixed in versions 1.35.11, 1.36.7, 1.37.3, and 1.38.1.

Affected products

  • Envoy Proxy Envoy >= 1.35.0, < 1.35.11; >= 1.36.0, < 1.36.7; >= 1.37.0, < 1.37.3; >= 1.38.0, < 1.38.1

Timeline

  • 2026-06-23: advisory: GitHub Security Advisory published by Envoy maintainers
  • 2026-06-26: disclosed: CVE published to NVD

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.