CVE-2026-73252 in Mongoose
Summary
by MITRE • 10/15/2026
Mongoose is an embedded web server and network library. Prior to 7.23, the built-in TLS stack selected by MG_TLS_BUILTIN in src/tls_builtin.c accepts a TLS 1.3 record whose decrypted size is exactly the 16-byte authentication tag with no plaintext or ContentType byte. In mg_tls_recv_record(), the msgsz length check permits that record and the recv_len calculation underflows to SIZE_MAX, so a later handshake step treats stale buffer contents as a trusted message with an effectively unbounded length. An unauthenticated malicious TLS peer can trigger a heap out-of-bounds read before certificate or Finished validation, causing a remote denial of service. This issue is fixed in version 7.23.
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Analysis
by VulDB Data Team • 10/15/2026
Mongoose serves as a widely utilized embedded web server and network library designed for resource-constrained environments such as IoT devices and microcontrollers. The security integrity of these systems often relies heavily on the robustness of their underlying TLS implementations, particularly when handling encrypted communications over untrusted networks. In versions prior to 7.23, the built-in TLS stack configured via MG_TLS_BUILTIN contains a critical logic flaw within its record processing mechanism that undermines the confidentiality and availability guarantees provided by Transport Layer Security protocols. This vulnerability specifically targets the parsing of TLS records during the handshake phase, where proper validation of message lengths is essential for preventing memory corruption or unauthorized data access.
The technical root cause lies in the function mg_tls_recv_record() located within src/tls_builtin.c. When processing a TLS 1.3 record, the implementation fails to adequately validate the decrypted payload size against expected structural constraints. Specifically, it accepts records where the decrypted content consists solely of the sixteen-byte authentication tag with no accompanying plaintext data or ContentType byte. Under normal protocol operation, such a structure is invalid because every TLS record must contain at least a type and length field before any encrypted application data. However, due to an insufficient length check on msgsz, this malformed record passes initial validation gates. Consequently, the subsequent calculation of recv_len suffers from integer underflow, resulting in a value that wraps around to SIZE_MAX, which represents the maximum possible size for a size_t variable on most systems.
This arithmetic error has severe operational consequences because it causes the application to treat stale or uninitialized buffer contents as valid data with an effectively unbounded length. During the TLS handshake process, before certificate verification and Finished message validation occur, the server attempts to read from this corrupted memory region. This leads directly to a heap out-of-bounds read operation. An attacker who can establish a connection without authentication is able to trigger this condition by sending specially crafted malicious TLS records. The immediate impact of this flaw is a remote denial of service, as the application will likely crash or terminate abnormally when attempting to access memory outside its allocated bounds. This compromises the availability of services running on embedded devices that rely on Mongoose for secure communication.
From a classification perspective, this vulnerability aligns with CWE-190 Integer Overflow or Wraparound and CWE-20 Improper Input Validation. The failure to correctly validate input lengths allows an attacker to manipulate internal state variables in ways that bypass security checks. In the context of the MITRE ATT&CK framework, this behavior is consistent with techniques used for reconnaissance or denial-of-service attacks against networked services, specifically leveraging protocol-level ambiguities to destabilize target systems before deeper exploitation attempts can be made. The lack of authentication requirement makes this particularly dangerous in environments where devices are exposed directly to public networks without intermediate firewalls or access controls.
To mitigate this risk, organizations utilizing Mongoose must upgrade immediately to version 7.23 or later, which includes the necessary patches to enforce strict length validation and prevent integer underflow during record processing. For systems that cannot be updated instantly due to operational constraints, network-level mitigations should be implemented. These include deploying web application firewalls or intrusion prevention systems capable of inspecting TLS handshake traffic for anomalous record structures. Additionally, implementing rate limiting on new connections can help mitigate the impact by reducing the frequency with which an attacker can attempt to trigger this vulnerability. Regular security audits and static analysis tools configured to detect integer overflow patterns should also be employed during development cycles to prevent similar logical errors in future releases of embedded network libraries.