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Request smuggling via first-wins Content-Length parsing in inets httpd

High
Whaileee published GHSA-w4jc-9wpv-pqh7 Mar 13, 2026

Package

OTP

Affected versions

>= 17.0

Patched versions

28.4.1, 27.3.4.9, 26.2.5.18
inets (OTP)
>= 5.10
9.1.0.5, 9.3.2.3, 9.6.1

Description

Impact

The Erlang/OTP httpd server contains an HTTP request smuggling vulnerability when processing requests with duplicate Content-Length headers. When multiple Content-Length headers with different values are present, httpd uses the first value without validating that all values are identical, violating RFC 9112 Section 6.3.

This vulnerability enables HTTP request smuggling attacks when httpd is deployed behind a reverse proxy, load balancer, or CDN that uses a different Content-Length resolution strategy (e.g., "last-wins" instead of "first-wins"). Successful exploitation can lead to:

  • Authentication bypass: Smuggled requests can bypass proxy-layer authentication and access protected backend resources
  • Cache poisoning: Attackers can poison caches with malicious content by desynchronizing request/response boundaries
  • Request hijacking: Smuggled requests can be prepended to legitimate user requests on persistent connections, enabling unauthorized actions

You are affected if:

  • You deploy httpd behind a reverse proxy, load balancer, or CDN
  • The frontend proxy uses a different Content-Length handling strategy than httpd
  • HTTP persistent connections (keep-alive) are enabled

You are NOT affected if:

  • You run httpd as a standalone server without a frontend proxy
  • Your frontend proxy rejects requests with duplicate Content-Length headers
  • Your frontend proxy uses the same first-wins strategy as httpd

RFC Violation:
RFC 9112 Section 6.3, Point 5 requires servers to reject requests with multiple Content-Length headers having different values with a 400 Bad Request response.

CWE Classification:

  • CWE-444: Inconsistent Interpretation of HTTP Requests (HTTP Request Smuggling)

CAPEC Classification:

  • CAPEC-33: HTTP Request Smuggling

Workarounds

  1. Configure frontend proxy to reject duplicate Content-Length headers (recommended)

    • Most modern proxies support strict header validation modes

  2. Disable HTTP keep-alive on httpd

    • Add {keep_alive, false} to httpd configuration
    • This prevents request smuggling by closing connections after each request
    • Note: This impacts performance for clients making multiple requests

  3. Deploy Web Application Firewall (WAF)

    • Configure WAF rules to reject requests with multiple Content-Length headers

Affected/Unaffected Versions

A version larger than or equal to one of the listed patched versions is unaffected; otherwise, a version that satisfies an expression listed under affected versions is affected, and if it does not, it is unaffected.

The documentation of the new OTP version scheme describes how versions should be compared. Note that versions used prior to OTP 17.0, when the new OTP version scheme was introduced, are never listed since it is not well defined how to compare those versions.

In the case of this vulnerability, versions prior to OTP 17.0 are likely also affected.

Credits

Thanks to Luigino Camastra (LuiginoC) for finding and responsibly disclosing this vulnerability to the Erlang/OTP project.

Severity

High

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 High
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity High
Availability Low

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:H/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:L

CVE ID

CVE-2026-23941

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