CVE-2026-54763VulnerabilityPublished 6 Aug 2026

github.com/traefik/traefik/v2 · Traefik: Incomplete fix for CVE-2026-33433 + CVE-2026-39858 cross-cohort: headerField underscore-variant identity

Affects github.com/traefik/traefik/v2Source GCVE Database

Mentioned vulnerabilities

Not known to be exploited · CVSS 7.8 High : upgrade to 2.11.51 soon.

Data as of 26 Oct 2026, 17:16 UTC

CVSS 4.0 base score

7.8High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.2% chance of exploitation in 30 days – higher than 14% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 2.11.51, 3.6.22, 3.7.6

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: None
Vulnerable system availability: None
Subsequent system confidentiality: High
Subsequent system integrity: High
Subsequent system availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: High: There is a total loss of confidentiality, resulting in all resources within the Subsequent System being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
Subsequent system integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the Subsequent System. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the Subsequent System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 6 Aug 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade github.com/traefik/traefik/v2 to 2.11.51; github.com/traefik/traefik/v3 to 3.6.22; github.com/traefik/traefik/v3 to 3.7.6.

Affected software

Product	Affected	Fixed in
Gogithub.com/traefik/traefik/v2	Affected <2.11.51	Fixed in 2.11.51
Gogithub.com/traefik/traefik/v3	Affected <3.6.22	Fixed in 3.6.22
Gogithub.com/traefik/traefik/v3	Affected >=3.7.0 <3.7.6	Fixed in 3.7.6

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

## Summary There is a high severity vulnerability in Traefik's BasicAuth, DigestAuth, and ForwardAuth middlewares. The fix for CVE-2026-33433 stripped canonical-cased spoofed identity headers (e.g. `X-Auth-User`) before writing Traefik's own value, but did not account for underscore-variant header names (e.g. `X_Auth_User`), which many backends normalize identically to the dashed form. An attacker able to reach a protected route could inject an underscore-variant header that survives Traefik's stripping and reaches the backend alongside — or, on the unauthenticated ForwardAuth `authResponseHeaders` path, instead of — the value Traefik intended to set, spoofing identity or authorization context. This is fixed by setting the new `allowHeadersWithUnderscores: false` entry point option, which strips all headers with underscores in their names before routing. ## Patches - https://github.com/traefik/traefik/releases/tag/v2.11.51 - https://github.com/traefik/traefik/releases/tag/v3.6.22 - https://github.com/traefik/traefik/releases/tag/v3.7.6 ## For more information If you have any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues). <details> <summary>Original Description</summary> # Incomplete fix for CVE-2026-33433 + CVE-2026-39858 cross-cohort: `headerField` underscore-variant identity spoofing in BasicAuth / DigestAuth / ForwardAuth ## Summary The fix for CVE-2026-33433 (GHSA-qr99-7898-vr7c, "BasicAuth/DigestAuth Identity Spoofing via Non-Canonical headerField", patched in v2.11.42 / v3.6.12 / v3.7.0-ea.3) added `req.Header.Del(headerField)` before the literal-key writeback in `pkg/middlewares/auth/basic_auth.go` and `pkg/middlewares/auth/digest_auth.go`. Go's `Header.Del` calls `textproto.CanonicalMIMEHeaderKey` which canonicalizes ASCII CASE and treats `-` as a word separator — so the fix correctly strips canonical-cased attacker headers (`X-Auth-User`, `x-auth-user`, `X-AUTH-USER`, etc.). However, `textproto.CanonicalMIMEHeaderKey` does **NOT** treat `_` as a separator. Attacker-supplied **underscore-variant** headers such as `X_Auth_User` survive `Header.Del("X-Auth-User")` intact and are forwarded to the backend alongside Traefik's own writeback. Many common backends (CGI/WSGI per RFC 3875, PHP `$_SERVER`, nginx with `underscores_in_headers on`, Tomcat / Java EE servlet containers, ASGI/WSGI frameworks) normalize `_` ↔ `-` equivalently or expose both forms to application code that may read the attacker's value. This is the **direct cross-cohort sibling** of the threat model the maintainer accepted in **CVE-2026-39858** (GHSA-5m6w-wvh7-57vm, "Forwarded alias spoofing pre-auth decision bypass"), which fixed the underscore-variant of the X-Forwarded-* family via `isManagedXHeader` in `pkg/middlewares/forwardedheaders/forwarded_header.go`. The CVE-2026-39858 advisory body states verbatim: > "When the backend normalizes underscore and dash header forms equivalently, an attacker can inject spoofed trust context — such as a trusted scheme or host — through the alias headers and bypass authentication on protected routes without valid credentials." The same threat model applies to the operator-configurable `headerField` (BasicAuth, DigestAuth) and `authResponseHeaders` (ForwardAuth, ingress-nginx snippet provider), but the underscore-handling primitive (`isManagedXHeader`) was not extended to those middlewares. I verified the bypass end-to-end on `traefik:v3.6.14` (the latest patched release containing both fixes) using a default-recommended canonical `headerField: "X-Auth-User"` config and reproduced the bypass with a single `curl -H "X_Auth_User: superadmin" ...` request alongside valid BasicAuth credentials. The defect is present in four code paths at HEAD `eec68dce064f843b4317c4393aaea81b6dea31d6`: 1. `pkg/middlewares/auth/basic_auth.go:101-105` — BasicAuth `headerField` 2. `pkg/middlewares/auth/digest_auth.go:99-103` — DigestAuth `headerField` 3. `pkg/middlewares/auth/forward.go:304-310` — ForwardAuth `authResponseHeaders` per-name writeback 4. `pkg/middlewares/ingressnginx/snippet/snippet.go:480-486` — Ingress-NGINX snippet `authResponseHeaders` per-name writeback The ForwardAuth instance (#3) is particularly notable: the attacker does NOT need credentials. The `authResponseHeaders` mechanism is intended to copy identity headers from the trusted auth server only; the underscore-variant bypass lets an unauthenticated attacker pre-inject the same identity header before any auth happens. The fast proxy at `pkg/proxy/fast/proxy.go:139` explicitly calls `DisableNormalizing()` on the outgoing fasthttp request, guaranteeing that the underscore-variant header reaches the backend wire verbatim. The standard `httputil.ReverseProxy` path at `pkg/proxy/httputil/proxy.go:55` likewise copies `req.Header` keys as-is during the wire write. ## Affected versions - `traefik` v3.6.x ≤ 3.6.14, v3.7.x ≤ 3.7.0-rc.2, v2.11.x ≤ 2.11.43, and all earlier vers

CWE-178Improper Handling of Case Sensitivity (opens in a new tab)
Base weakness
The product does not properly account for differences in case sensitivity when accessing or determining the properties of a resource, leading to inconsistent results.
Architecture and Design · Input Validation
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
Implementation · Input Validation
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174).
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-290Authentication Bypass by Spoofing (opens in a new tab)
Base weaknessOWASP Top 10 A07:2025: Authentication Failures (opens in a new tab)
This attack-focused weakness is caused by incorrectly implemented authentication schemes that are subject to spoofing attacks.
OWASP Cheat Sheets
REST Security (opens in a new tab)
JSON Web Token for Java (opens in a new tab)
OWASP Authentication (opens in a new tab)
OWASP ASVS 4.0.3
V3.5.1 (opens in a new tab)
V2.8.5 (opens in a new tab)
V2.8.4 (opens in a new tab)
Attack patterns
Creating a Rogue Certification Authority Certificate (opens in a new tab)
Web Services API Signature Forgery Leveraging Hash Function Extension Weakness (opens in a new tab)
Signature Spoof (opens in a new tab)
Signature Spoofing by Misrepresentation (opens in a new tab)
Session Credential Falsification through Prediction (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-345Insufficient Verification of Data Authenticity (opens in a new tab)
Class weaknessOWASP Top 10 A08:2025: Software or Data Integrity Failures (opens in a new tab)
The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
OWASP Cheat Sheets
XML Security (opens in a new tab)
REST Security (opens in a new tab)
JSON Web Token for Java (opens in a new tab)
REST Assessment (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.2 (opens in a new tab)
V3.5.3 (opens in a new tab)
V13.2.6 (opens in a new tab)
Attack patterns
JSON Hijacking (aka JavaScript Hijacking) (opens in a new tab)
Cache Poisoning (opens in a new tab)
DNS Cache Poisoning (opens in a new tab)
Spoofing of UDDI/ebXML Messages (opens in a new tab)
Application API Message Manipulation via Man-in-the-Middle (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

WeaknessesCWE-178, CWE-290, CWE-345

Technical details

References

Community reviews

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