{"id":"CVE-2026-121441","kind":"vulnerability","aliases":["CVE-2026-121441"],"related":[],"title":"Wasmtime: wasi-http lets guests send conflicting content-length headers","description":"Wasmtime before 36.0.19, 37.0.0 before 48.0.7, and 49.0.0 before 49.0.4 contains a request smuggling vulnerability in wasi-http that allows guests to set multiple conflicting content-length headers. Malicious guests can make the host send HTTP requests with conflicting content-length values to smuggle requests through servers or intermediaries that do not reject them.","summary":"Wasmtime's wasi-http layer accepts multiple conflicting content-length headers from a guest module. An attacker running code inside a guest can make the host send requests that servers or proxies may read differently, enabling request smuggling.","severity":"medium","cvss":6.3,"epss":null,"exploited":false,"exploited_at":null,"products":[],"ecosystems":[],"tags":["plugin-hosts","runtime","wasi-http","wasmtime","webassembly"],"not_affected_if":["no-wasi-http","trusted-guests-only","versions-patched"],"how_to_check":"- Check the Wasmtime version in use with `wasmtime --version` or the wasmtime crate version in Cargo.lock\n- Look for wasi-http usage (outgoing HTTP from guest modules) in your embedding code","fix":"- Wasmtime `36.0.19` for the 36 line\n- Wasmtime `48.0.7` for the 48 line\n- Wasmtime `49.0.4` for the 49 line","status":"open","published_at":"2026-11-03T15:17:35+00:00","first_seen_at":"2026-11-03T15:22:00+00:00","triaged_at":"2026-11-03T15:23:15+00:00","updated_at":"2026-11-03T15:23:15+00:00","category":"runtime","confidence":0.6,"triage":{"at":"2026-11-03T15:23:15Z","model":"claude-haiku-5-5","output":{"fix":"- Wasmtime `36.0.19` for the 36 line\n- Wasmtime `48.0.7` for the 48 line\n- Wasmtime `49.0.4` for the 49 line","reach":"network","title":"Wasmtime: wasi-http lets guests send conflicting content-length headers","impact":"medium","summary":"Wasmtime's wasi-http layer accepts multiple conflicting content-length headers from a guest module. An attacker running code inside a guest can make the host send requests that servers or proxies may read differently, enabling request smuggling.","audience":"businesses","category":"runtime","nicknames":[],"who_cares":["wasmtime","wasi-http","webassembly","plugin-hosts"],"confidence":0.6,"fix_status":"available","needs_auth":"unknown","worst_case":"Requests smuggled through intermediaries reach backends unexpectedly","analyst_note":"","how_to_check":"- Check the Wasmtime version in use with `wasmtime --version` or the wasmtime crate version in Cargo.lock\n- Look for wasi-http usage (outgoing HTTP from guest modules) in your embedding code","plain_action":"","needs_analyst":false,"not_affected_if":["no-wasi-http","trusted-guests-only","versions-patched"],"who_is_affected":"Applications that embed Wasmtime and run untrusted or semi-trusted guest modules that make outgoing HTTP requests through wasi-http. Exposure depends on whether guests can set headers and whether a downstream server or proxy accepts conflicting content-length values. Deployments that run only trusted guests or do not use wasi-http are less exposed.","user_interaction":"no","needs_special_setup":"yes","severity_if_unknown":"medium"},"cost_usd":0.000725,"input_tokens":4219,"output_tokens":607,"severity_before":"medium"},"analyst_status":null,"analyst_note":null,"search":"'-121441':15A,18A '-2026':14A,17A '36.0.19':70C '37.0.0':71C '48.0.7':73C '49.0.0':75C '49.0.4':77C 'accept':34B 'allow':88C 'attack':46B 'code':48B 'conflict':8A,36B,93C,108C 'contain':78C 'content':10A,38B,95C,110C 'content-length':9A,37B,94C,109C 'cve':13A,16A 'differ':64B 'enabl':65B 'guest':6A,43B,51B,89C,99C 'header':12A,40B,97C 'host':21B,55B,103C 'http':4A,25B,32B,86C,105C 'insid':49B 'intermediari':119C 'layer':33B 'length':11A,39B,96C,111C 'let':5A 'make':53B,101C 'malici':98C 'may':62B 'modul':44B 'multipl':35B,92C 'plugin':20B 'plugin-host':19B 'proxi':61B 'read':63B 'reject':123C 'request':57B,66B,80C,106C,115C 'run':47B 'runtim':22B 'send':7A,56B,104C 'server':59B,117C 'set':91C 'smuggl':67B,81C,114C 'valu':112C 'vulner':82C 'wasi':3A,24B,31B,85C 'wasi-http':2A,23B,30B,84C 'wasmtim':1A,26B,28B,68C 'webassembl':27B","nicknames":[],"mitigations":[],"who_affected":"Applications that embed Wasmtime and run untrusted or semi-trusted guest modules that make outgoing HTTP requests through wasi-http. Exposure depends on whether guests can set headers and whether a downstream server or proxy accepts conflicting content-length values. Deployments that run only trusted guests or do not use wasi-http are less exposed.","tier":"C","verified":{},"analyst_revisit_at":null,"fix_status":"available","audience":"businesses","plain_action":null,"pkeys":[],"ptokens":[],"labels":["medium","pluginhosts","runtime","wasihttp","wasmtime","webassembly"],"first_public_at":"2026-10-19T19:20:02+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-2026-121441","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-121441","severity":null,"published_at":"2026-11-03T15:17:35+00:00","modified_at":"2026-11-03T15:17:36+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":58409,"url":"https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-r8xh-cgf7-392j","title":"Vendor advisory: github.com","kind":"vendor","source":"nvd","publisher":"vendor","text":null,"published_at":"2026-10-19T19:20:02+00:00","relation":"advisory","confidence":null,"note":"Vendor advisory: github.com"},{"id":58410,"url":"https://www.vulncheck.com/advisories/wasmtime-before-36.0.19-37.0.0-before-48.0.7-and-49.0.0-before-49.0.4-wasi-http-request-smuggling","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"}