CVE-2026-73256
Received Received - Intake

HTTP Request Smuggling in Mongoose Web Server

Publication date: 2026-08-20

Last updated on: 2026-08-20

Assigner: GitHub, Inc.

Description
Mongoose is an embedded web server and network library. Prior to 7.22, a remote unauthenticated attacker can exploit an HTTP/1.0 reverse-proxy deployment by sending a request with Transfer-Encoding: chunked and conflicting framing. The http_cb() function in src/http.c tests hm.proto.len with an impossible greater-than-eight condition even though mg_http_parse() requires an eight-byte protocol string, so is_http_1_0 is never set. Mongoose consequently processes chunked encoding that an HTTP/1.0 proxy can ignore, enabling request smuggling and unauthorized access or state changes. This issue is fixed in version 7.22.

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Meta Information

Published
2026-08-20
Last Modified
2026-08-20
Generated
2026-08-20
AI Q&A
2026-08-20
EPSS Evaluated
N/A
NVD
CVE-2026-73256
EUVD
EUVD-2026-63497

Affected Vendors & Products

Showing 3 associated CPEs
Vendor Product Version / Range
mongoose mongoose 7.22
cesanta mongoose to 7.22 (exc)
cesanta mongoose 7.22

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-2026-73256 is a critical HTTP request smuggling vulnerability in Mongoose versions prior to 7.22. It occurs due to an off-by-one error in HTTP/1.0 request detection where a flawed length check (`hm.proto.len > 8`) fails to recognize valid HTTP/1.0 requests. This causes Mongoose to incorrectly accept `Transfer-Encoding: chunked` headers, which are only valid for HTTP/1.1 per RFC 7230. When behind an HTTP/1.0 reverse proxy, the proxy ignores the chunked encoding while Mongoose processes it, leading to request smuggling and potential unauthorized access or state changes.

Detection Guidance

To detect this vulnerability, monitor for HTTP request smuggling attempts by analyzing network traffic for conflicting Transfer-Encoding and Content-Length headers. Check Mongoose server logs for malformed requests or unexpected request parsing. Use tools like Wireshark or tcpdump to inspect HTTP/1.0 traffic with chunked encoding.

Impact Analysis

This vulnerability allows remote unauthenticated attackers to smuggle malicious HTTP requests through Mongoose. This could lead to unauthorized access to sensitive data, manipulation of application state, or bypassing security controls. Systems using Mongoose as a reverse proxy or embedded web server in versions before 7.22 are particularly at risk of such attacks.

Compliance Impact

This vulnerability could lead to unauthorized data access or modification, violating confidentiality and integrity requirements under GDPR and HIPAA. Organizations using vulnerable Mongoose versions may face compliance violations, legal penalties, and reputational damage due to potential data breaches or unauthorized state changes.

Mitigation Strategies

Upgrade Mongoose to version 7.22 or later immediately. If upgrading is not possible, disable HTTP/1.0 support in your reverse proxy configuration or add strict validation for Transfer-Encoding headers. Monitor for suspicious traffic patterns indicating exploitation attempts.

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