CVE-2026-73548
Received Received - Intake

HTTP/2 Request Smuggling in Envoy Proxy

Publication date: 2026-09-21

Last updated on: 2026-09-21

Assigner: GitHub, Inc.

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.

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Meta Information

Published
2026-09-21
Last Modified
2026-09-21
Generated
2026-09-22
AI Q&A
2026-09-21
EPSS Evaluated
N/A
NVD
CVE-2026-73548
EUVD
EUVD-2026-84103

Affected Vendors & Products

Showing 13 associated CPEs
Vendor Product Version / Range
envoy envoy 1.36.10
envoy envoy 1.37.6
envoy envoy 1.38.4
envoy envoy 1.39.1
envoy envoy to 1.36.10 (exc)
envoy envoy to 1.37.6 (exc)
envoy envoy to 1.38.4 (exc)
envoy envoy to 1.39.1 (exc)
envoyproxy envoy 1.36.10
envoyproxy envoy 1.37.6
envoyproxy envoy 1.38.4
envoyproxy envoy 1.39.1
envoyproxy envoy to 1.40.0 (exc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-444 The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination.

Attack-Flow Graph

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Executive Summary

CVE-2026-73548 is a vulnerability in Envoy Proxy where an unauthenticated HTTP/2 client can send a malicious payload before a generic HTTP upgrade. Envoy downgrades this to an HTTP/1.1 request and writes the payload unframed to a keep-alive backend connection. If the backend keeps the connection alive, the socket is reused from a shared pool. A different downstream client may then receive the attacker's response instead of their own, leading to potential data exposure or response poisoning.

Detection Guidance

Detecting this vulnerability requires checking Envoy Proxy versions and configurations. First, verify your Envoy version with `envoy --version`. If it is below 1.36.10, 1.37.6, 1.38.4, or 1.39.1, the system is vulnerable. Check for the presence of `http2_protocol_options.allow_connect: true` and non-WebSocket `upgrade_configs` in the configuration. Monitor logs for unexpected HTTP response poisoning or connection reuse issues.

Impact Analysis

This vulnerability allows an attacker to poison shared upstream connections, causing other users to receive incorrect or malicious responses. This could expose sensitive data, disrupt services, or lead to unauthorized access to information intended for other users. The impact depends on the configuration and whether the backend uses keep-alive connections.

Compliance Impact

This vulnerability could lead to unauthorized access to sensitive data, which may violate compliance requirements under GDPR (data protection) and HIPAA (healthcare data privacy). Unauthenticated HTTP response poisoning may expose private or confidential information, potentially resulting in regulatory penalties or breaches of data protection laws.

Mitigation Strategies
  • Upgrade Envoy Proxy to versions 1.36.10, 1.37.6, 1.38.4, 1.39.1, or later to address the vulnerability.
  • Disable the affected feature temporarily by setting the runtime feature `envoy.reloadable_features.http_pause_generic_upgrade_request_body` to false if immediate upgrade is not possible.
  • Review configurations to ensure WebSocket upgrades, plain CONNECT, disabled backend keep-alive, per-downstream pools, or max_requests_per_connection set to 1 are in place to reduce exposure.

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