CVE-2026-49753: mint Content-Length Header Accepts Non-RFC "+" Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-mjqx-c6f6-7rc2",
  "modified" : "2026-07-09T23:19:17Z",
  "published" : "2026-07-09T23:19:16Z",
  "aliases" : [ "CVE-2026-49753" ],
  "summary" : "mint: Content-Length header accepts non-RFC \"+\" sign prefix",
  "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",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Hex",
      "name" : "mint"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.9.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "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" : "WEB",
    "url" : "https://github.com/elixir-mint/mint/commit/47e48027480228e4e32a0b4df39db497b4804921"
  }, {
    "type" : "WEB",
    "url" : "https://cna.erlef.org/cves/CVE-2026-49753.html"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/elixir-mint/mint"
  }, {
    "type" : "WEB",
    "url" : "https://osv.dev/vulnerability/EEF-CVE-2026-49753"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-444" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-09T23:19:16Z",
    "nvd_published_at" : "2026-06-02T16:16:44Z"
  }
}