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Traefik: ForwardAuth identity spoofing via dot-form header alias

Moderate severity GitHub Reviewed Published Aug 27, 2026 in traefik/traefik

Package

gomod github.com/traefik/traefik/v2 (Go)

Affected versions

< 2.11.56

Patched versions

2.11.56
gomod github.com/traefik/traefik/v3 (Go)
>= 3.0.0, < 3.7.12
3.7.12

Description

Summary

There is a medium severity vulnerability in Traefik's handling of request headers whose name aliases another header name. Go canonicalizes header names on dashes only, so X-Auth-User, X_Auth_User and X.Auth.User are three distinct headers to Traefik, while backends that derive variable names from header names (CGI, WSGI, PHP, NGINX, and others) collapse all of them into the same variable. A client can therefore smuggle an alias of a header that Traefik manages past the middleware managing it — for example a dot-form X.Authenticated.User alongside the canonical X-Authenticated-User written by the ForwardAuth middleware — and have such a backend read the client-supplied value instead of the identity Traefik asserted. Any header Traefik sets is exposed, not only ForwardAuth's. This is an incomplete-fix sibling of GHSA-x677-9fxg-v5c5, which blocked only the underscore form.

The mitigation is the new aliasHeadersStrategy entry point option. It defaults to keep, which preserves the previous behavior for backwards compatibility, so it must be explicitly set to delete or reject to take effect.

Traefik v1.x, the v2 releases up to v2.11.55 and the v3 releases from v3.0.0 to v3.7.11 are affected. The unmaintained lines among them will not receive a patch of their own, and the remedy for their users is to upgrade to v2.11.56 or v3.7.12 and set aliasHeadersStrategy.

Patches

For more information

If you have any questions or comments about this advisory, please open an issue.

Original Description

Summary

Traefik's ForwardAuth middleware removes the configured canonical identity header before copying the value returned by the auth service. However, a client-supplied dot-form alias such as X.Authenticated.User survives both this replacement and underscoreHeadersStrategy: delete.

The tested PHP 8.2 built-in SAPI maps X-Authenticated-User and X.Authenticated.User to the same HTTP_X_AUTHENTICATED_USER server variable. In Traefik's tested HTTP/1 backend path, the client value is serialized last and overrides the identity asserted by ForwardAuth.

A client whom ForwardAuth permits as a lower-privilege identity can therefore be treated by the backend as another user or role.

Details

At v3.7.10, pkg/server/server_entrypoint_tcp.go:800-818 removes or rejects only names containing _. After successful authentication, pkg/middlewares/auth/forward.go:314-326 deletes and replaces only the canonical authResponseHeaders key. The dot alias remains in req.Header and the standard reverse proxy forwards both legal field names.

Go's HTTP/1 writer sorts header names lexically, placing X-Authenticated-User before X.Authenticated.User. PHP then collapses both into one $_SERVER key, so the attacker value deterministically wins.

This is an incomplete-fix sibling of GHSA-x677-9fxg-v5c5: the published underscore input is blocked by the new entry-point strategy, while the dot input bypasses that mitigation on the current stable release.

PoC

traefik-dot-forwardauth-poc.zip

run:

docker compose up -d
bash verify.sh
docker compose down

The decisive request is:

GET /probe HTTP/1.1
Host: 127.0.0.1:18080
X.Authenticated.User: admin
Connection: close

Expected backend identity: lab-user, as returned by ForwardAuth.

Observed on v3.7.10: admin.

The script also verifies that requests without the alias, with the canonical header, and with the already-fixed underscore alias all produce lab-user.

Impact

Applications that authorize requests using a ForwardAuth-provided identity header can receive an attacker-selected username or role instead. This was runtime-verified with PHP 8.2.27 and 8.2.33; impact on other normalization-prone backends is conditional. A lower-privilege permitted client may consequently impersonate another user or administrative role, affecting confidentiality and integrity.

This report does not claim bypass of a ForwardAuth denial: the auth service must first permit the request.

--- ### References - https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c - https://nvd.nist.gov/vuln/detail/CVE-2026-88011 - https://github.com/traefik/traefik/pull/13720 - https://github.com/traefik/traefik/commit/0331801c72329e0eaeb850e53ccce87c57fbecf8 - https://github.com/traefik/traefik/releases/tag/v2.11.56 - https://github.com/traefik/traefik/releases/tag/v3.7.12
@mmatur mmatur published to traefik/traefik Aug 27, 2026
Published by the National Vulnerability Database Sep 10, 2026
Published to the GitHub Advisory Database Sep 10, 2026
Reviewed Sep 10, 2026

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 Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity High
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:L/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(16th percentile)

Weaknesses

Authentication Bypass by Spoofing

This attack-focused weakness is caused by incorrectly implemented authentication schemes that are subject to spoofing attacks. Learn more on MITRE.

CVE ID

CVE-2026-88011

GHSA ID

GHSA-rf44-j88r-hh8c

Source code

Credits

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