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HTTP/1 chunk-size desync in Mint via Integer.parse/2 sign tolerance

Moderate
whatyouhide published GHSA-x3x7-96vm-6h2w Jul 16, 2026

Package

erlang mint (Erlang)

Affected versions

>= 0.1.0 and < 1.9.3

Patched versions

1.9.3

Description

Summary

Mint's HTTP/1 chunked-body decoder parses chunk-size lines with Integer.parse(data, 16), which accepts a leading + or - in violation of RFC 7230. On a pooled keep-alive connection that traverses an RFC-strict intermediary, a malicious origin can send sign-prefixed chunk sizes that Mint accepts but the intermediary rejects, producing a response-smuggling primitive against subsequent requests on the connection.

Details

Mint.HTTP1.decode_body/5 in lib/mint/http1.ex dispatches on Integer.parse(data, 16) when reading the next chunk-size line. Because Integer.parse/2 treats a leading +/- as an optional sign:

  • +5 is accepted as a five-byte chunk.
  • +0 and -0 are accepted as the terminating zero-length chunk and end the body early.
  • Only -5 and other negative non-zero values fall through to :invalid_chunk_size.

A strict RFC 7230 parser rejects any sign in chunk-size, so Mint and an intermediary in the response path disagree on chunk boundaries. On a pooled keep-alive connection the origin can emit trailing bytes that the intermediary treats as body but Mint reads as the next response, poisoning the response queue for other in-flight requests on the shared socket.

The fix introduces Mint.HTTP1.Parse.chunk_size/1 in lib/mint/http1/parse.ex, a strict HEXDIG-only parser that rejects any sign or non-hex prefix, and switches decode_body/5 to use it.

PoC

  1. Point a Mint HTTP/1 client at an attacker-controlled origin through a strict HTTP/1 intermediary that reuses keep-alive connections.
  2. Have the origin respond with Transfer-Encoding: chunked and a first chunk-size line of +0\r\n\r\n, followed by a crafted second "response" starting with HTTP/1.1 200 OK\r\n....
  3. The intermediary rejects +0 and keeps reading; Mint treats +0 as the last chunk and parses the crafted bytes as the next response on the pooled connection.

Impact

An attacker who controls or influences an HTTP/1 origin that a Mint client talks to through an RFC-strict pooled intermediary can inject responses that Mint attributes to unrelated in-flight requests, corrupting response bodies, headers, or status codes seen by the application.

References

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality Low
Integrity Low
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N

CVE ID

CVE-2026-59249

Weaknesses

Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

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. Learn more on MITRE.

Credits