Summary
mint: Content-Length header accepts non-RFC "+" sign prefix
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.
Resources
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.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Frequently Asked Questions
- What is CVE-2026-49753? CVE-2026-49753 is a medium-severity security vulnerability in mint (erlang), affecting versions < 1.9.0. It is fixed in 1.9.0.
- Which versions of mint are affected by CVE-2026-49753? mint (erlang) versions < 1.9.0 is affected.
- Is there a fix for CVE-2026-49753? Yes. CVE-2026-49753 is fixed in 1.9.0. Upgrade to this version or later.
- Is CVE-2026-49753 exploitable, and should I be worried? Whether CVE-2026-49753 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-49753 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-49753? Upgrade
mintto 1.9.0 or later.