GHSA-MJQX-C6F6-7RC2
Vulnerability from github – Published: 2026-07-09 23:19 – Updated: 2026-07-09 23:19Summary
Mint'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.
Details
'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.
A 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.
PoC
- 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>. - Connect a Mint HTTP/1 client to the server and issue a request.
- 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.
Impact
Response-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.
Resources
- Introduction commit: https://github.com/elixir-mint/mint/commit/65e0e86d799a6d3b08e4372fccdd9747535e0dd6
- Patch commit: https://github.com/elixir-mint/mint/commit/47e48027480228e4e32a0b4df39db497b4804921
{
"affected": [
{
"package": {
"ecosystem": "Hex",
"name": "mint"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.9.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-49753"
],
"database_specific": {
"cwe_ids": [
"CWE-444"
],
"github_reviewed": true,
"github_reviewed_at": "2026-07-09T23:19:16Z",
"nvd_published_at": "2026-06-02T16:16:44Z",
"severity": "MODERATE"
},
"details": "### Summary\n\nMint\u0027s 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\u0027s response stream.\n\n### Details\n\n`\u0027Elixir.Mint.HTTP1.Parse\u0027: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 \u003e= 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\u003csmuggled bytes\u003e`.\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\u0027s HTTP/1 client parser. Anyone using Mint (directly or via Finch, Tesla\u0027s 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",
"id": "GHSA-mjqx-c6f6-7rc2",
"modified": "2026-07-09T23:19:17Z",
"published": "2026-07-09T23:19:16Z",
"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"
}
],
"schema_version": "1.4.0",
"severity": [
{
"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",
"type": "CVSS_V4"
}
],
"summary": "mint: Content-Length header accepts non-RFC \"+\" sign prefix"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
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- Not confirmed: The user expressed doubt about the validity of the vulnerability.
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