CVE-2026-73256 in Mongoose
Summary
by MITRE • 08/20/2026
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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Analysis
by VulDB Data Team • 08/20/2026
Mongoose serves as a widely adopted embedded web server and network library designed for resource-constrained environments such as IoT devices and microcontrollers. The vulnerability identified prior to version 7.22 resides within the HTTP parsing logic, specifically affecting deployments that utilize Mongoose behind an HTTP/1.0 reverse proxy. This architectural setup creates a specific attack surface where protocol discrepancies between the front-end proxy and the back-end server can be exploited by remote unauthenticated attackers. The core of the issue lies in how Mongoose handles the Transfer-Encoding header when combined with chunked transfer encoding, particularly under conditions where framing conflicts arise due to the older HTTP/1.0 standard's limitations regarding persistent connections and certain headers.
The technical flaw originates from a logic error within the http_cb function located in src/http.c. During the parsing of incoming HTTP requests, Mongoose invokes mg_http_parse to determine the protocol version. This parser requires an eight-byte protocol string to correctly identify HTTP/1.0 or higher versions. However, the subsequent validation step checks if hm.proto.len is greater than eight using a condition that can never be true because the maximum length for a standard HTTP version identifier like "HTTP/1.0" is exactly nine characters including the null terminator, but the specific check logic fails to correctly set the internal flag is_http_1_0 even when an HTTP/1.0 request is received. This misclassification means that Mongoose incorrectly assumes it is not communicating with an HTTP/1.0 client or proxy in a way that would restrict certain behaviors. Consequently, the server proceeds to process requests containing Transfer-Encoding: chunked headers without applying the necessary restrictions that should apply when interacting with legacy proxies that do not support this encoding method natively.
This misconfiguration enables HTTP request smuggling attacks, specifically type T2 (CL.TE) or TE.CL depending on the specific framing conflict exploited by the attacker. By sending a crafted request with chunked transfer encoding to an HTTP/1.0 reverse proxy, the attacker can exploit the fact that many older proxies ignore the Transfer-Encoding header and instead rely on Content-Length for message boundaries. If Mongoose processes the chunked data while the proxy interprets the message based on content length or ignores the chunking entirely, it creates a desynchronization between what the proxy believes has been sent and what the backend server actually parses. This allows an attacker to smuggle malicious requests behind legitimate traffic, potentially bypassing access controls, injecting unauthorized commands, or manipulating application state in ways that were not intended by the developer.
The operational impact of this vulnerability is significant for any system relying on Mongoose as a back-end service behind legacy infrastructure. Unauthorized access can occur if an attacker successfully smuggles a request to an administrative endpoint that was previously protected from direct external access due to network segmentation or proxy rules. Furthermore, state changes induced by smuggled requests could lead to data integrity issues, denial of service through resource exhaustion via malformed chunked streams, or privilege escalation within the application logic. Since Mongoose is often used in security-sensitive embedded systems where patching cycles may be longer and manual intervention difficult, this vulnerability poses a persistent risk if left unaddressed.
To mitigate this issue, organizations must upgrade to version 7.22 of Mongoose or later, which corrects the protocol length validation logic and ensures that chunked encoding is handled appropriately in contexts where it conflicts with HTTP/1.0 proxy behavior. For environments unable to immediately update, implementing strict input validation at the network perimeter using a modern reverse proxy capable of normalizing requests before they reach the Mongoose server can provide an additional layer of defense. Security teams should also monitor for anomalous traffic patterns indicative of request smuggling attempts and ensure that all embedded devices are inventoried and prioritized for patching based on their exposure to untrusted networks. This vulnerability aligns with CWE-787 Improper Access Control in the context of protocol handling failures and maps to ATT&CK technique T1071 Application Layer Protocol, specifically HTTP/HTTPS traffic manipulation techniques used by adversaries to evade detection and execute unauthorized actions.