CVE-2026-73548: CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') in envoyproxy envoy
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.
AI Analysis
Technical Summary
CVE-2026-73548 is an HTTP request/response smuggling vulnerability (CWE-444) in Envoy proxy. Before versions 1.36.10, 1.37.6, 1.38.4, and 1.39.1, Envoy forwards data for configured non-WebSocket HTTP upgrades before the upstream server accepts the upgrade. An unauthenticated HTTP/2 client can embed a full HTTP/1.1 request in extended CONNECT data, which Envoy downgrades and sends unframed to a keep-alive HTTP/1.1 upstream. Envoy then returns the socket to the shared pool while the smuggled response remains queued, allowing a different downstream client to receive the attacker’s response. This vulnerability does not affect WebSocket upgrades, plain CONNECT, disabled backend keep-alive, per-downstream pools, or when max_requests_per_connection is set to 1. The issue is resolved in versions 1.36.10, 1.37.6, 1.38.4, and 1.39.1.
Potential Impact
An unauthenticated attacker can exploit this vulnerability to smuggle HTTP requests and cause response confusion between clients, potentially leading to information disclosure. The vulnerability affects confidentiality (high impact on confidentiality) but does not impact integrity or availability. It requires no privileges or user interaction.
Mitigation Recommendations
Upgrade Envoy to version 1.36.10, 1.37.6, 1.38.4, or 1.39.1 or later where this issue is fixed. Configurations using WebSocket upgrades, plain CONNECT, disabled backend keep-alive, per-downstream pools, or max_requests_per_connection set to 1 are not affected by this vulnerability. No other mitigations are specified.
CVE-2026-73548: CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') in envoyproxy envoy
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 v3.1
Score 7.5high
Affected software
envoyproxy
envoy
pkg:github/envoyproxy/envoyRun on your own infrastructure? Check whether these packages are installed with threat-finder — our free open-source scanner.
Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
CVE-2026-73548 is an HTTP request/response smuggling vulnerability (CWE-444) in Envoy proxy. Before versions 1.36.10, 1.37.6, 1.38.4, and 1.39.1, Envoy forwards data for configured non-WebSocket HTTP upgrades before the upstream server accepts the upgrade. An unauthenticated HTTP/2 client can embed a full HTTP/1.1 request in extended CONNECT data, which Envoy downgrades and sends unframed to a keep-alive HTTP/1.1 upstream. Envoy then returns the socket to the shared pool while the smuggled response remains queued, allowing a different downstream client to receive the attacker’s response. This vulnerability does not affect WebSocket upgrades, plain CONNECT, disabled backend keep-alive, per-downstream pools, or when max_requests_per_connection is set to 1. The issue is resolved in versions 1.36.10, 1.37.6, 1.38.4, and 1.39.1.
Potential Impact
An unauthenticated attacker can exploit this vulnerability to smuggle HTTP requests and cause response confusion between clients, potentially leading to information disclosure. The vulnerability affects confidentiality (high impact on confidentiality) but does not impact integrity or availability. It requires no privileges or user interaction.
Mitigation Recommendations
Upgrade Envoy to version 1.36.10, 1.37.6, 1.38.4, or 1.39.1 or later where this issue is fixed. Configurations using WebSocket upgrades, plain CONNECT, disabled backend keep-alive, per-downstream pools, or max_requests_per_connection set to 1 are not affected by this vulnerability. No other mitigations are specified.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-08-12T20:53:46.379Z
- Cvss Version
- 3.1
- State
- PUBLISHED
Threat ID: 6ab189c655bf5e2cf56100f0
Added to database: 09/21/2026, 19:47:18 UTC
Last enriched: 09/21/2026, 20:01:35 UTC
Last updated: 09/21/2026, 23:50:11 UTC
Views: 10
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