CVE-2026-48743 in envoy
Summary
by MITRE • 06/26/2026
Envoy is an open source edge and service proxy designed for cloud-native applications. Prior to 1.35.11, 1.36.7, 1.37.3, and 1.38.1, Envoy can translate a downstream HTTP/3 request that is complete at the transport layer (HEADERS with FIN / headers-only close) but still carries a nonzero Content-Length into a complete upstream HTTP/1 request with unresolved body debt. In an HTTP/1 upstream deployment where the origin replies before reading the declared body and keeps the connection reusable, the beginning of the next Envoy-generated upstream request can be consumed as the first request's body. The remaining bytes are then parsed by the origin as a new HTTP/1 request. This was reproduced as a route-bypass/desync: direct /pwn was denied by Envoy, but the second downstream H3 stream received the response for backend-parsed GET /pwn HTTP/1.1. This vulnerability is fixed in 1.35.11, 1.36.7, 1.37.3, and 1.38.1.
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Analysis
by VulDB Data Team • 06/26/2026
This vulnerability resides within Envoy's HTTP/3 to HTTP/1 translation mechanism, representing a sophisticated protocol mismatch that can lead to request smuggling and route bypass conditions. The flaw manifests when Envoy processes downstream HTTP/3 requests that arrive with complete headers but include a nonzero Content-Length field, creating a scenario where the proxy translates these requests into HTTP/1 format while maintaining unresolved body debt. This translation process fails to properly account for the discrepancy between the transport layer completeness of the original request and the HTTP/1 protocol's expectations regarding body handling.
The technical implementation of this vulnerability leverages the fundamental differences between HTTP/3 and HTTP/1 connection handling, particularly around how body data is managed during request processing. When an HTTP/3 request with a HEADERS frame containing FIN flag arrives complete at the transport layer but still carries a Content-Length header indicating additional body data, Envoy's proxy logic incorrectly interprets this as a request that requires body buffering. The system maintains this body debt throughout the translation process, creating a state where the upstream HTTP/1 connection remains in an inconsistent condition. This is particularly problematic when the upstream origin server responds before fully reading the declared body content, leaving the connection in a reusable state with partial data consumption.
The operational impact of this vulnerability extends beyond simple protocol translation failures to create serious security implications including route bypass and potential request smuggling attacks. The specific scenario described demonstrates how an attacker could exploit this condition to achieve unauthorized access through a route bypass mechanism. When Envoy processes the initial request and the origin server responds before consuming the complete body, the subsequent processing of the next upstream request causes data corruption at the connection level. The beginning of the next Envoy-generated HTTP/1 request gets consumed as the body of the previous request, effectively causing the remaining bytes to be parsed by the origin server as a completely new HTTP/1 request. This creates a desynchronization between what Envoy intended to send and what the origin server actually receives, allowing for potential bypass of access controls and unauthorized resource access.
This vulnerability aligns with CWE-1245 "Improper Handling of HTTP/3 to HTTP/1 Translation" and represents a specific instance of protocol confusion attacks within the broader ATT&CK framework under T1071.004 Application Layer Protocol. The flaw demonstrates how proxy systems can fail when translating between different protocol versions, creating conditions where connection state management becomes inconsistent across protocol boundaries. Attackers could potentially exploit this to perform unauthorized operations by manipulating the request flow such that legitimate responses are delivered to unintended requests, effectively bypassing authentication and authorization mechanisms that should have prevented access to resources like the /pwn endpoint mentioned in the reproduction case.
The fix implemented in versions 1.35.11, 1.36.7, 1.37.3, and 1.38.1 addresses this issue by enhancing the HTTP/3 to HTTP/1 translation logic to properly handle cases where transport layer requests arrive complete but carry Content-Length headers indicating additional data. The mitigation strategy involves stricter validation of body debt during request translation and improved connection state management when dealing with protocol transitions. Organizations should ensure immediate deployment of these patched versions across all Envoy deployments, particularly in cloud-native environments where HTTP/3 support is actively utilized. Security teams should also implement monitoring for unusual patterns in HTTP/1 request processing that might indicate this vulnerability's exploitation attempts, as the effects can be subtle and may not immediately manifest in obvious error conditions but rather in more insidious routing bypass scenarios.