{"id":"CVE-2026-55368","kind":"vulnerability","aliases":["CVE-2026-55368"],"related":[],"title":"H2O HTTP server: access-log formatter overflow can crash the server","description":"h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 48b7ae12107fd2b118e4641da49599e9aac8edd8, H2O's access-log formatter could write past its log-line buffer when an affected custom access-log format was configured. The allocation errors affected %{error}x, %q when the query string was empty, and numeric elements whose temporary terminating NUL reached the end of the buffer. A matching request could crash the server, while the effect of the one-byte NUL overwrite beyond a crash was not established. This vulnerability is fixed with commit 48b7ae12107fd2b118e4641da49599e9aac8edd8.","summary":"H2O's access-log formatter can write past its log-line buffer when a custom access-log format uses %{error}x, %q with an empty query string, or numeric elements. A request that matches the affected format can crash the server, and the sources do not establish any further effect from the one-byte NUL overwrite.","severity":"high","cvss":8.1,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["h2o","runtime","self-hosted"],"not_affected_if":["default-access-log","no-custom-log-format"],"how_to_check":"- Check whether the H2O config sets a custom `access-log` format containing `%{error}x`, `%q`, or numeric elements.\n- Check the installed H2O version against the fixed commit `48b7ae12107fd2b118e4641da49599e9aac8edd8`.","fix":"- Move to a build that includes commit `48b7ae12107fd2b118e4641da49599e9aac8edd8`; no fixed release version is stated in the sources.","status":"open","published_at":"2026-11-04T22:18:06+00:00","first_seen_at":"2026-11-04T23:08:49+00:00","triaged_at":"2026-11-04T23:09:48+00:00","updated_at":"2026-11-04T23:09:48+00:00","category":"runtime","confidence":0.6,"triage":{"at":"2026-11-04T23:09:48Z","model":"claude-haiku-5-5","output":{"fix":"- Move to a build that includes commit `48b7ae12107fd2b118e4641da49599e9aac8edd8`; no fixed release version is stated in the sources.","reach":"internet","title":"H2O HTTP server: access-log formatter overflow can crash the server","impact":"medium","summary":"H2O's access-log formatter can write past its log-line buffer when a custom access-log format uses %{error}x, %q with an empty query string, or numeric elements. A request that matches the affected format can crash the server, and the sources do not establish any further effect from the one-byte NUL overwrite.","audience":"businesses","category":"runtime","nicknames":[],"who_cares":["h2o","self-hosted"],"confidence":0.6,"fix_status":"available","needs_auth":"no","worst_case":"server crashes on a matching request; further memory effects unestablished","analyst_note":"Confirm the fixed release version and whether the one-byte NUL overwrite has any exploitable effect beyond a crash, using the linked advisory and commit.","how_to_check":"- Check whether the H2O config sets a custom `access-log` format containing `%{error}x`, `%q`, or numeric elements.\n- Check the installed H2O version against the fixed commit `48b7ae12107fd2b118e4641da49599e9aac8edd8`.","plain_action":"","needs_analyst":true,"not_affected_if":["default-access-log","no-custom-log-format"],"who_is_affected":"Only servers running H2O with a custom access-log format that includes %{error}x, %q, or numeric elements are affected, since the default format is not described as vulnerable. A matching request can crash the server. Operators using the default log format or no access log can skip it.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"high"},"cost_usd":0.000746,"input_tokens":4385,"output_tokens":616,"severity_before":"high"},"analyst_status":null,"analyst_note":null,"search":"'-2026':14A,17A '-55368':15A,18A '48b7ae12107fd2b118e4641da49599e9aac8edd8':99C,179C 'access':5A,27B,42B,103C,119C 'access-log':4A,26B,41B,102C,118C 'affect':62B,116C,127C 'alloc':125C 'beyond':167C 'buffer':37B,113C,149C 'byte':81B,164C 'commit':98C,178C 'configur':123C 'could':106C,153C 'crash':10A,65B,154C,169C 'custom':40B,117C 'cve':13A,16A 'effect':76B,159C 'element':56B,139C 'empti':51B,136C 'end':146C 'error':46B,126C,128C 'establish':73B,172C 'fix':176C 'format':44B,63B,121C 'formatt':7A,29B,105C 'h2o':1A,19B,24B,84C,100C 'host':23B 'http':2A,87C 'http/1.x':92C 'http/2':93C 'http/3':95C 'line':36B,112C 'log':6A,28B,35B,43B,104C,111C,120C 'log-lin':34B,110C 'match':60B,151C 'nul':82B,143C,165C 'numer':55B,138C 'one':80B,163C 'one-byt':79B,162C 'overflow':8A 'overwrit':83B,166C 'past':32B,108C 'prior':96C 'q':48B,130C 'queri':52B,133C 'reach':144C 'request':58B,152C 'runtim':20B 'self':22B 'self-host':21B 'server':3A,12A,67B,88C,156C 'sourc':70B 'string':53B,134C 'support':90C 'temporari':141C 'termin':142C 'use':45B 'vulner':174C 'whose':140C 'write':31B,107C 'x':47B,129C","nicknames":[],"mitigations":[],"who_affected":"Only servers running H2O with a custom access-log format that includes %{error}x, %q, or numeric elements are affected, since the default format is not described as vulnerable. A matching request can crash the server. Operators using the default log format or no access log can skip it.","tier":"B","verified":{},"analyst_revisit_at":null,"fix_status":"available","audience":"businesses","plain_action":null,"pkeys":[],"ptokens":[],"labels":["h2o","high","runtime","selfhosted"],"campaign_type":null,"first_public_at":"2026-10-23T08:26:14+00:00","kev":null,"cvss_vector":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H","cvss_by":"security-advisories@github.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-55368","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-55368","severity":null,"published_at":"2026-11-04T22:18:06+00:00","modified_at":"2026-11-04T22:18:06+00:00","first_seen_at":"2026-11-04T23:08:49+00:00","fetched_at":"2026-11-04T23:08:49+00:00","note":null}],"documents":[{"id":71163,"url":"https://github.com/h2o/h2o/commit/48b7ae12107fd2b118e4641da49599e9aac8edd8","title":"Patch: github.com","kind":"other","source":"nvd","publisher":"vendor","text":null,"published_at":null,"relation":"fix","confidence":null,"note":"Patch: github.com"},{"id":71164,"url":"https://github.com/h2o/h2o/security/advisories/GHSA-m998-r5ww-rqjf","title":"Vendor advisory: github.com","kind":"vendor","source":"nvd","publisher":"vendor","text":null,"published_at":"2026-10-23T08:26:14+00:00","relation":"advisory","confidence":null,"note":"Vendor advisory: github.com"}],"exploit_sources":[],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}