# CVE-2026-73548: Envoy HTTP upgrade connection poisoning

Source: https://junglewise.ai/threats/cve-2026-73548-envoy-is-an-open-source-edge-and-service-proxy-designed-for-cloud  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-21. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-73548 · Severity: high · CVSS 7.5 · Published 2026-09-21

Envoy, an open-source edge and service proxy for cloud-native applications, allows unauthenticated HTTP/2 clients to inject smuggled HTTP/1.1 requests that poison shared upstream connections. An attacker can craft a request that causes Envoy to forward data before the upstream accepts the upgrade, resulting in the smuggled response being received by a different legitimate client. This enables response interception and potential credential or data theft.

Technologies: [Envoy Proxy Envoy](https://junglewise.ai/threats/technologies/envoy). Vendors: [Envoy Proxy](https://junglewise.ai/threats/vendors/envoy-proxy).

## Quick answers

**Is CVE-2026-73548 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-73548 being exploited in the wild.

**How severe is CVE-2026-73548?** High severity, with a CVSS score of 7.5, published 21 September 2026.

**Which products and versions are affected?** Envoy Proxy Envoy Before 1.36.10, before 1.37.6, before 1.38.4, before 1.39.1.

**Is there a fix for CVE-2026-73548?** 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-2026-73548 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-73548?** 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

The vulnerability occurs in Envoy's handling of generic HTTP upgrades: data sent before the upstream accepts the upgrade is written unframed to keep-alive HTTP/1.1 upstream connections and interpreted as pipelined HTTP/1 requests. An unauthenticated network attacker can place a complete HTTP/1.1 request in extended CONNECT data; Envoy downgrades and forwards it, returning the socket to the shared pool while the smuggled response remains queued. Unaffected configurations include WebSocket upgrades, plain CONNECT, disabled backend keep-alive, per-downstream pools, and max_requests_per_connection set to 1. The fix pauses generic upgrade payload until the upstream accepts the upgrade.

## Affected products

- Envoy Proxy Envoy Before 1.36.10, before 1.37.6, before 1.38.4, before 1.39.1

## Timeline

- 2026-09-21: disclosed

## References

- [https://github.com/envoyproxy/envoy/commit/309855626966cff176155c821043ed3b44671361](https://github.com/envoyproxy/envoy/commit/309855626966cff176155c821043ed3b44671361)
- [https://github.com/envoyproxy/envoy/commit/3c7998545a9c9fb3933a4bc1907d92f0e752bc2b](https://github.com/envoyproxy/envoy/commit/3c7998545a9c9fb3933a4bc1907d92f0e752bc2b)

## Related threats

- [Envoy use-after-free in HTTP OAuth2 filter](https://junglewise.ai/threats/cve-2026-48090-envoy-use-after-free-in-http-oauth2-filter)
- [Envoy denial of service via NULL pointer dereference in log formatting](https://junglewise.ai/threats/cve-2026-47220-envoy-denial-of-service-via-null-pointer-dereference-in-log)
- [Envoy Use-After-Free in ext_authz filter during stream teardown](https://junglewise.ai/threats/cve-2026-47205-envoy-use-after-free-in-ext-authz-filter-during-stream-teardown)
- [Envoy HTTP/3 to HTTP/1 request smuggling in downstream translation](https://junglewise.ai/threats/cve-2026-48743-envoy-http-3-to-http-1-request-smuggling-in-downstream)
- [Envoy heap buffer overflow in TcpStatsdSink](https://junglewise.ai/threats/cve-2026-48706-envoy-heap-buffer-overflow-in-tcpstatsdsink)