{"id":"CVE-2026-55367","title":"h2o: heap exhaustion using capsules","summary":"h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 4212ce114506a073505fd389db16ad2ba4e582a0, a client could send an incomplete CONNECT-UDP Capsule with an excessively large declared length, causing H2O to…","severity":"high","cvss":7.5,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H","cvssSource":"cna","cwe":["CWE-770"],"vendor":"h2o","product":"h2o","affected":["h2o < 4212ce114506a073505fd389db16ad2ba4e582a0"],"ssvc":{"exploitation":"none","automatable":"yes","technicalImpact":"partial","timestamp":"2026-11-05T19:59:16.316316Z"},"published":"2026-11-04","updated":"2026-11-05","sourceUpdated":"2026-11-05T20:01:20.480Z","source":"CVEORG","sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-55367","references":[{"url":"https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c","label":"https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c"},{"url":"https://github.com/h2o/h2o/pull/3754","label":"https://github.com/h2o/h2o/pull/3754"},{"url":"https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0","label":"https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0"}],"tags":["cve.org"],"epss":0.00339,"epssPercentile":0.27484,"ingestedAt":"2026-11-05T22:38:31.690Z","slug":"CVE-2026-55367","body":"## Overview\n\nh2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 4212ce114506a073505fd389db16ad2ba4e582a0, a client could send an incomplete CONNECT-UDP Capsule with an excessively large declared length, causing H2O to keep buffering received bytes without a per-Capsule limit while waiting for the remainder. Deployments that enabled proxy.connect-udp and accepted CONNECT-UDP requests could exhaust server memory and lose availability. This vulnerability is fixed with commit 4212ce114506a073505fd389db16ad2ba4e582a0.\n\n## Affected\n\n- `h2o < 4212ce114506a073505fd389db16ad2ba4e582a0`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"twilight","depthScore":41,"depthScoreParts":{"impact":41.3,"likelihood":0.1,"exploitation":0,"ransomware":0},"changes":[]}