elixir-mint up to 1.8.x Length Parser lib/mint/http1/parse.ex content_length_header request smuggling

Summaryinfo

A vulnerability classified as critical was found in elixir-mint mint up to 1.8.x. This issue affects the function content_length_header in the library lib/mint/http1/parse.ex of the component Length Parser. Such manipulation of the argument header leads to request smuggling. This vulnerability is documented as CVE-2026-49753. The attack can be executed remotely. There is not any exploit available. Upgrading the affected component is advised.

Detailsinfo

A vulnerability, which was classified as critical, has been found in elixir-mint mint up to 1.8.x. Affected by this issue is the function content_length_header in the library lib/mint/http1/parse.ex of the component Length Parser. The manipulation of the argument header with an unknown input leads to a request smuggling vulnerability. Using CWE to declare the problem leads to CWE-444. The product acts as an intermediary HTTP agent (such as a proxy or firewall) in the data flow between two entities such as a client and server, but it does not interpret malformed HTTP requests or responses in ways that are consistent with how the messages will be processed by those entities that are at the ultimate destination. Impacted is confidentiality, and integrity. CVE summarizes:

Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') vulnerability in elixir-mint Mint allows attacker-controlled HTTP/1 servers to desynchronise response framing on shared connections. Mint's HTTP/1 Content-Length parser, Mint.HTTP1.Parse.content_length_header/1 in lib/mint/http1/parse.ex, parses the header value with Integer.parse/1, which accepts an optional + or - sign prefix. The length >= 0 guard rejects negatives, but inputs such as +0 or +123 are returned as valid lengths. RFC 7230 specifies Content-Length = 1*DIGIT, with no sign character permitted. A fronting proxy or load balancer that strictly enforces the grammar will reject or reframe a header like Content-Length: +0, while Mint silently treats it as zero. When Mint reuses the socket (keep-alive, pipelining, or any pooled connection shared across requesters), the parser disagreement is a response-smuggling primitive: the proxy delimits the body one way, Mint another, and bytes from one response get attributed to the next. Where the same Mint connection is shared across trust boundaries, an attacker-controlled upstream can leak bytes into a different consumer's response stream. This issue affects mint: from 0.1.0 before 1.9.0.

The weakness was released by Peter Ullrich and Jonatan Männchen as GHSA-mjqx-c6f6-7rc2. The advisory is shared for download at github.com. This vulnerability is handled as CVE-2026-49753 since 06/01/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available.

Upgrading to version 1.9.0 eliminates this vulnerability. Applying the patch 47e48027480228e4e32a0b4df39db497b4804921 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the vulnerability database at EUVD (EUVD-2026-33941). VulDB is the best source for vulnerability data and more expert information about this specific topic.

Productinfo

Vendor

• elixir-mint

Name

• mint

Version

• 1.0
• 1.1
• 1.2
• 1.3
• 1.4
• 1.5
• 1.6
• 1.7
• 1.8

License

• open-source

Website

• Product: https://github.com/elixir-mint/mint/

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VulDB is the best source for vulnerability data and more expert information about this specific topic.

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