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CVE-2026-11404 Cesanta CVE debrief

CVE-2026-11404 is a high-severity vulnerability in Cesanta Mongoose, a popular embedded web server. The vulnerability exists in the built-in TLS server function mg_tls_server_recv_hello(), which does not properly validate the session_id_len byte from a TLS ClientHello. This allows a remote, unauthenticated attacker to send a crafted ClientHello with an oversized session id length, causing the server to read past the receive buffer and crash. This vulnerability affects any HTTPS, MQTTS, or WSS service built on MG_TLS_BUILTIN. Organizations should prioritize patching this vulnerability to prevent potential crashes and disruptions.

Vendor
Cesanta
Product
Mongoose
CVSS
HIGH 8.7
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-09
Original CVE updated
2026-08-29
Advisory published
2026-07-09
Advisory updated
2026-08-29

Who should care

Organizations using Cesanta Mongoose in their products or services should prioritize patching this vulnerability to prevent potential crashes and disruptions. This vulnerability has a high CVSS score of 8.7, indicating a significant risk to affected systems. Operators, platform administrators, vulnerability management teams, and security teams should be aware of the vulnerability and take necessary actions.

Technical summary

The vulnerability is caused by the lack of validation of the session_id_len byte from a TLS ClientHello in the mg_tls_server_recv_hello() function. This allows an attacker to send a crafted ClientHello with an oversized session id length, causing the server to read past the receive buffer and crash. The vulnerability can be exploited by a remote, unauthenticated attacker. Affected systems include any HTTPS, MQTTS, or WSS service built on MG_TLS_BUILTIN.

Defensive priority

High

Recommended defensive actions

  • Apply the patch: Upgrade to Cesanta Mongoose version 7.22 or later.
  • Inventory and prioritize: Identify and prioritize systems using Cesanta Mongoose for patching.
  • Verify patch application: Ensure the patch has been successfully applied and the system is no longer vulnerable.
  • Review compensating controls for exposed systems while remediation is scheduled and verified.
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review.
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented.
  • Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up.

Evidence notes

The CVE record was published on 2026-07-09T16:16:34.640Z and was last modified on 2026-07-10T18:46:32.250Z. The NVD entry is currently Deferred. There is limited information available about the vulnerability, and defenders should verify the affected scope and severity with the vendor. The vulnerability exists in the built-in TLS server function mg_tls_server_recv_hello().

Sources and references

Verified primary and authoritative sources

  • CVE-2026-11404 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-11404

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-11404 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-11404

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.