{"id":"CVE-2026-121440","kind":"vulnerability","aliases":["CVE-2026-121440"],"related":[],"title":"Wasmtime: forwarded trailers bypass embedder header filtering in WASIp3","description":"Wasmtime 46.0.0 before 48.0.7 and 49.0.0 before 49.0.4 fails to check forwarded trailers against the WasiHttpHooks::is_forbidden_header hook in wasi-http p3. WASIp3 guests can forward a request or response body so its trailers skip embedder-customized forbidden header filtering, undermining header restrictions the embedder intended.","summary":"Wasmtime's WASI HTTP p3 implementation does not run forwarded trailers through the embedder's forbidden-header hook. A WASIp3 guest can therefore forward trailers that the host meant to block, undermining header restrictions.","severity":"medium","cvss":6.3,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["runtime","wasi-http","wasip3","wasmtime","webassembly"],"not_affected_if":["default-header-hooks","no-wasip3","trusted-guests-only"],"how_to_check":"- Check the Wasmtime version with `wasmtime --version` or in the project's Cargo.lock for `wasmtime`.\n- Look for use of the wasi-http p3 feature and a custom `WasiHttpHooks` implementation that defines `is_forbidden_header`.","fix":"- Wasmtime `48.0.7` for the 48.x line\n- Wasmtime `49.0.4` for the 49.x line","status":"open","published_at":"2026-11-03T15:17:35+00:00","first_seen_at":"2026-11-03T15:22:00+00:00","triaged_at":"2026-11-03T15:23:18+00:00","updated_at":"2026-11-03T15:23:18+00:00","category":"runtime","confidence":0.6,"triage":{"at":"2026-11-03T15:23:18Z","model":"claude-haiku-5-5","output":{"fix":"- Wasmtime `48.0.7` for the 48.x line\n- Wasmtime `49.0.4` for the 49.x line","reach":"unknown","title":"Wasmtime: forwarded trailers bypass embedder header filtering in WASIp3","impact":"low","summary":"Wasmtime's WASI HTTP p3 implementation does not run forwarded trailers through the embedder's forbidden-header hook. A WASIp3 guest can therefore forward trailers that the host meant to block, undermining header restrictions.","audience":"businesses","category":"runtime","nicknames":[],"who_cares":["wasmtime","wasi-http","wasip3","webassembly"],"confidence":0.6,"fix_status":"available","needs_auth":"unknown","worst_case":"Guest bypasses intended header restrictions set by the embedder","analyst_note":"","how_to_check":"- Check the Wasmtime version with `wasmtime --version` or in the project's Cargo.lock for `wasmtime`.\n- Look for use of the wasi-http p3 feature and a custom `WasiHttpHooks` implementation that defines `is_forbidden_header`.","plain_action":"","needs_analyst":false,"not_affected_if":["no-wasip3","default-header-hooks","trusted-guests-only"],"who_is_affected":"Only embedders running Wasmtime 46.0.0 to before 48.0.7, or 49.0.0 to before 49.0.4, that serve WASIp3 guests through wasi-http and rely on a custom WasiHttpHooks::is_forbidden_header filter to restrict headers. Applications that do not run untrusted WASIp3 guests or do not customize forbidden headers are unlikely to be meaningfully exposed.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"unknown"},"cost_usd":0.000741,"input_tokens":4210,"output_tokens":640,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-121440':12A,15A '-2026':11A,14A '46.0.0':59C '48.0.7':61C '49.0.0':63C '49.0.4':65C 'block':54B 'bodi':91C 'bypass':4A 'check':68C 'custom':98C 'cve':10A,13A 'embedd':5A,36B,97C,106C 'embedder-custom':96C 'fail':66C 'filter':7A,101C 'forbidden':39B,75C,99C 'forbidden-head':38B 'forward':2A,32B,47B,69C,86C 'guest':44B,84C 'header':6A,40B,56B,76C,100C,103C 'hook':41B,77C 'host':51B 'http':19B,26B,81C 'implement':28B 'intend':107C 'meant':52B 'p3':27B,82C 'request':88C 'respons':90C 'restrict':57B,104C 'run':31B 'runtim':16B 'skip':95C 'therefor':46B 'trailer':3A,33B,48B,70C,94C 'undermin':55B,102C 'wasi':18B,25B,80C 'wasi-http':17B,79C 'wasihttphook':73C 'wasip3':9A,20B,43B,83C 'wasmtim':1A,21B,23B,58C 'webassembl':22B","nicknames":[],"mitigations":[],"who_affected":"Only embedders running Wasmtime 46.0.0 to before 48.0.7, or 49.0.0 to before 49.0.4, that serve WASIp3 guests through wasi-http and rely on a custom WasiHttpHooks::is_forbidden_header filter to restrict headers. Applications that do not run untrusted WASIp3 guests or do not customize forbidden headers are unlikely to be meaningfully exposed.","tier":"C","verified":{},"analyst_revisit_at":null,"fix_status":"available","audience":"businesses","plain_action":null,"pkeys":[],"ptokens":[],"labels":["medium","runtime","wasihttp","wasip3","wasmtime","webassembly"],"first_public_at":"2026-10-19T19:20:21+00:00","kev":null,"cvss_vector":"CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","cvss_by":"disclosure@vulncheck.com","poc_count":0,"sources":[{"source":"nvd","external_id":"CVE-2026-121440","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-121440","severity":null,"published_at":"2026-11-03T15:17:35+00:00","modified_at":"2026-11-03T15:17:35+00:00","first_seen_at":"2026-11-03T15:22:00+00:00","fetched_at":"2026-11-03T15:22:00+00:00","note":null}],"documents":[{"id":58407,"url":"https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-jxcf-8qw5-5c33","title":"Vendor advisory: github.com","kind":"vendor","source":"nvd","publisher":"vendor","text":null,"published_at":"2026-10-19T19:20:21+00:00","relation":"advisory","confidence":null,"note":"Vendor advisory: github.com"},{"id":58408,"url":"https://www.vulncheck.com/advisories/wasmtime-46.0.0-before-48.0.7-and-49.0.0-before-49.0.4-forwarded-trailers-header-filter-bypass","title":"Third-party advisory: www.vulncheck.com","kind":"advisory","source":"nvd","publisher":"research","text":null,"published_at":null,"relation":"advisory","confidence":null,"note":"Third-party advisory: www.vulncheck.com"}],"notice":"Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai"}