{"id":"CVE-2026-49753","aliases":["EEF-CVE-2026-49753","GHSA-mjqx-c6f6-7rc2"],"url":"https://o3.security/vulnerability/CVE-2026-49753","summary":"HTTP response smuggling in Mint HTTP/1 client via lenient Content-Length parsing","details":"### Summary\n\nMint's HTTP/1 client accepts `Content-Length` header values with a leading `+` sign (e.g. `+0`, `+123`), which RFC 7230 forbids (`Content-Length = 1*DIGIT`). On a connection shared with a strict fronting proxy or load balancer, this parser disagreement is a response-smuggling primitive: the proxy frames the body one way, Mint frames it another, and bytes meant for one response leak into the next consumer's response stream.\n\n### Details\n\n`'Elixir.Mint.HTTP1.Parse':content_length_header/1` in `lib/mint/http1/parse.ex` parses the header value with `Integer.parse/1`. By design, `Integer.parse/1` accepts an optional `+` or `-` sign prefix. The `length >= 0` guard rules out negatives, but inputs such as `\"+0\"`, `\"+123\"`, or `\"+1\"` pass through and are returned as valid lengths.\n\nA strict proxy or load balancer rejects or reframes `Content-Length: +0\\r\\n`, while Mint silently treats it as `0`. When Mint reuses the socket (keep-alive, pipelining, or any pooled connection) and the connection is shared with a proxy that frames the same bytes differently, trailing bytes the proxy attributes to response N are attributed by Mint to response N+1. Across trust boundaries (shared pools, multi-tenant fronting) this enables response smuggling.\n\n### PoC\n\n1. Stand up a raw TCP server that returns `HTTP/1.1 200 OK\\r\\nContent-Length: +0\\r\\nConnection: keep-alive\\r\\n\\r\\n<smuggled bytes>`.\n2. Connect a Mint HTTP/1 client to the server and issue a request.\n3. Observe that Mint reports the response as status 200 with `Content-Length: \"+0\"` and an empty body, leaving the smuggled bytes sitting in the socket buffer for the next response.\n\n### Impact\n\nResponse-smuggling / request-response desync primitive in Mint's HTTP/1 client parser. Anyone using Mint (directly or via Finch, Tesla's Mint adapter, Req, etc.) to talk through a shared or pooled connection where a fronting proxy enforces RFC 7230 strictly while Mint does not is exposed. The attacker is the response producer (a malicious or compromised upstream, or anything that can inject bytes into a shared origin response); exploitation into a cross-request data leak additionally requires the deployment to share a Mint connection across trust boundaries.\n\n## Resources\n\n* Introduction commit: https://github.com/elixir-mint/mint/commit/65e0e86d799a6d3b08e4372fccdd9747535e0dd6\n* Patch commit: https://github.com/elixir-mint/mint/commit/47e48027480228e4e32a0b4df39db497b4804921","published":"2026-06-02T14:15:17.078Z","modified":"2026-10-21T02:58:23.290896305Z","cvss":null,"epss":{"score":0.00301,"percentile":0.22876,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Hex","name":"mint","fixedVersion":"1.9.0"}],"fix":{"url":"https://github.com/elixir-mint/mint/commit/47e48027480228e4e32a0b4df39db497b4804921","label":"elixir-mint/mint@47e4802"},"references":[{"type":"WEB","url":"https://cna.erlef.org/cves/CVE-2026-49753.html"},{"type":"WEB","url":"https://github.com"},{"type":"WEB","url":"https://osv.dev/vulnerability/EEF-CVE-2026-49753"},{"type":"WEB","url":"https://repo.hex.pm"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/49xxx/CVE-2026-49753.json"},{"type":"ADVISORY","url":"https://github.com/elixir-mint/mint/security/advisories/GHSA-mjqx-c6f6-7rc2"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-49753"},{"type":"FIX","url":"https://github.com/elixir-mint/mint/commit/47e48027480228e4e32a0b4df39db497b4804921"},{"type":"PACKAGE","url":"https://github.com/elixir-mint/mint"},{"type":"PACKAGE","url":"https://github.com/elixir-mint/mint.git"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:58:23.290896305Z"}}