CVE-2026-88879: traefik — Fix

Description

Traefik is an HTTP reverse proxy and load balancer. In Traefik v1.x, v2.x through v2.11.55, and v3.0.0 through v3.7.11, header names are canonicalized only on dashes, so X-Auth-User, X_Auth_User and X.Auth.User are treated as three distinct headers by Traefik, while backends that derive variable names from header names (CGI, WSGI, PHP, NGINX and others) collapse them into a single variable. A client can therefore smuggle a dot-form alias of a header that Traefik manages past the middleware managing it — for example supplying X.Authenticated.User alongside the canonical X-Authenticated-User written by the ForwardAuth middleware — causing such a backend to read the client-supplied value instead of the identity Traefik asserted. In the tested configuration (PHP 8.2 built-in SAPI over an HTTP/1 backend path), Go's lexical header ordering makes the attacker-supplied value win deterministically, so a client that ForwardAuth admits as a low-privilege identity can be treated by the backend as a different user or role. Any header Traefik sets is affected, not only ForwardAuth's. This is an incomplete fix for GHSA-x677-9fxg-v5c5, which blocked only the underscore form. Fixed in v2.11.56 and v3.7.12, which add the aliasHeadersStrategy entry-point option; because it defaults to 'keep' for backwards compatibility, it must be explicitly set to 'delete' or 'reject' for the fix to take effect. Unmaintained release lines will not receive a patch.

Is your site exposed to CVE-2026-88879?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

8.2

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:N

EPSS — Exploit Prediction

0.0021

Probability of exploitation

0.11%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-290 CWE-290

Affected Products

Vendor	Product	Versions
traefik	traefik	1.0 — 2.11.56
traefik	traefik	3.0.0 — 3.7.12

References

Advisories & Patches

https://github.com/traefik/traefik/security/advisories/GHSA-rf44-j88r-hh8c

Other References

https://www.vulncheck.com/advisories/traefik-before-2.11.56-identity-spoofing-via-header-alias

Frequently Asked Questions

What is CVE-2026-88879? +

Traefik is an HTTP reverse proxy and load balancer. In Traefik v1.x, v2.x through v2.11.55, and v3.0.0 through v3.7.11, header names are canonicalized only on dashes, so X-Auth-User, X_Auth_User and X.Auth.User are treated as three distinct headers by Traefik, while backends that derive variable names from header names (CGI, WSGI, PHP, NGINX and others) collapse them into a single variable. A client can therefore smuggle a dot-form alias of a header that Traefik manages past the middleware managing it — for example supplying X.Authenticated.User alongside the canonical X-Authenticated-User written by the ForwardAuth middleware — causing such a backend to read the client-supplied value instead of the identity Traefik asserted. In the tested configuration (PHP 8.2 built-in SAPI over an HTTP/1 backend path), Go's lexical header ordering makes the attacker-supplied value win deterministically, so a client that ForwardAuth admits as a low-privilege identity can be treated by the backend as a different user or role. Any header Traefik sets is affected, not only ForwardAuth's. This is an incomplete fix for GHSA-x677-9fxg-v5c5, which blocked only the underscore form. Fixed in v2.11.56 and v3.7.12, which add the aliasHeadersStrategy entry-point option; because it defaults to 'keep' for backwards compatibility, it must be explicitly set to 'delete' or 'reject' for the fix to take effect. Unmaintained release lines will not receive a patch. It has a CVSS v3.1 base score of 8.2 (HIGH).

How severe is CVE-2026-88879? +

CVE-2026-88879 has a CVSS v3.1 score of 8.2 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0021, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-88879? +

CVE-2026-88879 affects products from traefik, specifically: traefik. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-88879? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-6213

A vulnerability in Remote Spark SparkView before build 1122 allows an attacker to bypasses the local connection check and achieve …

CVE-2026-34025

The Wertheim SafeController Software, AssemblyVersion 6.15.8328.28014, contains an IP restriction bypass vulnerability in the login process. The application restricts user …

CVE-2025-22271

The application or its infrastructure allows for IP address spoofing by providing its own value in the "X-Forwarded-For" header. Thus, …

CVE-2025-31122

scratch-coding-hut.github.io is the website for Coding Hut. In 1.0-beta3 and earlier, the login link can be used to login to …

CVE-2025-46345

Auth0 Account Link Extension is an extension aimed to help link accounts easily. Versions 2.3.4 to 2.6.6 do not verify …

CVE-2025-34053

An authentication bypass vulnerability exists in AVTECH IP camera, DVR, and NVR devices’ streamd web server. The strstr() function is …

CVE-2026-88879