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CVE-2026-11404 – Denial of Service (Out-of-Bounds Read) – Cesanta Mongoose before 7.22

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Summary

CVE-2026-11404 is a high-severity out-of-bounds read vulnerability (CWE-125) in Cesanta Mongoose, a widely-deployed embedded C networking library used in IoT devices, industrial controllers, microcontrollers, and embedded appliances. The flaw resides in the built-in TLS server function mg_tls_server_recv_hello(), where an attacker-controlled session_id_len byte from a TLS ClientHello is used as a buffer index without bounds validation. A single crafted packet sent by a remote, unauthenticated attacker is sufficient to crash any HTTPS, MQTTS, or WSS service built on Mongoose’s built-in TLS implementation, scoring CVSS 4.0 8.7 (HIGH).

Technical details

  • Root cause: mg_tls_server_recv_hello() reads the session_id_len field directly from an incoming TLS ClientHello and uses it as a buffer index without validating it against the actual length of received data, allowing an attacker to control the read offset into the receive buffer.
  • Trigger condition: A single crafted TLS ClientHello packet containing an oversized session_id_len value, requiring no prior session, credentials, or user interaction.
  • Attack vector: Fully remote (network-accessible), unauthenticated, no user interaction required. Only services configured to use Mongoose’s built-in TLS implementation (MG_TLS_BUILTIN) are affected; deployments offloading TLS to an external library are not.
  • Impact: The out-of-bounds read causes the process to read past the receive buffer, crashing any HTTPS, MQTTS, or WSS service running on the affected Mongoose instance. Impact is exclusively availability (no confidentiality or integrity impact is established). In embedded and IoT/ICS environments — Mongoose’s primary deployment context — a crash may require physical device intervention to recover, with no automatic failover.

Affected software

  • Cesanta Mongoose all versions before 7.22

Severity

  • CVSS 4.0: 8.7 (HIGH) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
  • CVSS 3.1: 7.5 (HIGH) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Mitigation and recommended actions

  • Immediate: Upgrade to Cesanta Mongoose 7.22 or later (released June 24, 2026), which contains the confirmed fix for the mg_tls_server_recv_hello session_id_len out-of-bounds read.
  • If immediate patching is not feasible: Restrict network-level access to TLS-enabled ports (HTTPS, MQTTS, WSS) exposed by Mongoose-based services using firewall rules or network segmentation, limiting reachability to trusted hosts only.
  • Vendors embedding Mongoose as a component in their products should verify whether their shipped firmware or software versions include an affected build of Mongoose and issue updates accordingly.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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