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CVE-2026-65637 – HTTP/2 SNI Validation Bypass – Apache Tomcat 9.0.115–9.0.120, 10.1.53–10.1.57, 11.0

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Summary

CVE-2026-65637 is an Improper Input Validation vulnerability in Apache Tomcat’s HTTP/2 request handling, stemming from an incomplete fix for the earlier CVE-2026-32990. The flaw allows an attacker to bypass strict SNI (Server Name Indication) validation via HTTP/2 requests that omit an authority, undermining host-based security controls in multi-host TLS deployments. NVD/MITRE record this as a CRITICAL severity issue with a CVSS v3.1 base score of 9.8.

Technical details

  • Root cause: The original patch for CVE-2026-32990 did not account for all HTTP/2 edge cases; requests without an authority component can circumvent strict SNI validation checks.
  • Trigger condition: Sending crafted HTTP/2 requests that omit the authority field to a Tomcat instance with strict SNI validation enabled.
  • Attack vector: Network-based, no authentication or user interaction required, low attack complexity.
  • Impact: Potential access to unintended virtual hosts, bypass of host-based access controls, and possible certificate validation bypass in multi-host/virtual-hosting configurations — affecting confidentiality, integrity, and availability.

Affected software

  • Apache Tomcat 11.0.20 through 11.0.24
  • Apache Tomcat 10.1.53 through 10.1.57
  • Apache Tomcat 9.0.115 through 9.0.120

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade to Apache Tomcat 11.0.25, 10.1.58, or 9.0.121, each of which contains the complete fix for this vulnerability.
  • If immediate patching is not possible: Review virtual host / SNI configurations for reliance on strict SNI validation, and restrict or monitor HTTP/2 traffic at the network layer (e.g., WAF or reverse proxy filtering of malformed/no-authority HTTP/2 requests) until the upgrade can be applied.

References

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