CVE-2026-35051VulnerabilityPublished 24 Apr 2026Updated 6 May 2026

github.com/traefik/traefik/v3 · V3: Traefik's ForwardAuth trustForwardHeader=false allows spoofed X-Forwarded-Prefix to bypass authentication

Affects github.com/traefik/traefik/v3Source GCVE Database

Not known to be exploited · CVSS 10.0 Critical : upgrade to 3.7.0-rc.2 as a priority.

Data as of 27 Oct 2026, 00:07 UTC

CVSS 3.1 base score

10.0Critical

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

Patch: Available – fixed in 3.7.0-rc.2, 3.6.14, 2.11.43

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Changed
Confidentiality: High
Integrity: High
Availability: None

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, 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: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
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 impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 24 Apr 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/v3 to 3.7.0-rc.2; github.com/traefik/traefik/v3 to 3.6.14; github.com/traefik/traefik/v2 to 2.11.43.

Affected software

Product	Affected	Fixed in
Gogithub.com/traefik/traefik/v3	Affected >=3.7.0-ea.1 <3.7.0-rc.2	Fixed in 3.7.0-rc.2
Gogithub.com/traefik/traefik/v3	Affected >=3.0.0-beta1 <3.6.14	Fixed in 3.6.14
Gogithub.com/traefik/traefik/v2	Affected <2.11.43	Fixed in 2.11.43
Gogithub.com/traefik/traefik	Affected <=1.7.34	Fixed in–

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 authentication bypass vulnerability in Traefik's `ForwardAuth` middleware when `trustForwardHeader=false` is configured and Traefik is deployed behind a trusted upstream proxy. While `X-Forwarded-*` headers (such as `X-Forwarded-For`, `X-Forwarded-Host`, and `X-Forwarded-Proto`) from trusted context are correctly rebuilt, it does not strip or rebuild `X-Forwarded-Prefix`, leaving any attacker-supplied value intact in the subrequest forwarded to the authentication service. When the authentication service makes authorization decisions based on `X-Forwarded-Prefix`, an external attacker can spoof a trusted prefix value and gain unauthorized access to protected backend routes. ## Patches - https://github.com/traefik/traefik/releases/tag/v2.11.43 - https://github.com/traefik/traefik/releases/tag/v3.6.14 - https://github.com/traefik/traefik/releases/tag/v3.7.0-rc.2 ## For more information If there are any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues). <details> <summary>Original Description</summary> ### Summary `ForwardAuth` with `trustForwardHeader=false` still forwards an attacker-controlled `X-Forwarded-Prefix` header to the authentication service when Traefik is deployed behind a trusted upstream proxy. If the auth service relies on `X-Forwarded-Prefix` for authorization or routing decisions, an external attacker can bypass access controls and reach protected backend routes. This was validated this against Traefik `v3.6.12` using the official Docker image and a minimal local Docker setup. A direct request to Traefik is correctly rejected, but the same request succeeds when sent through a trusted reverse proxy, which shows the issue is in the `ForwardAuth` subrequest handling rather than general ingress header stripping. ### Details The vulnerable behavior comes from the way Traefik builds the subrequest sent to the forward-auth server. In [`pkg/middlewares/auth/forward.go`](pkg/middlewares/auth/forward.go), `writeHeader` first copies all incoming request headers into the auth subrequest: ```go func writeHeader(req, forwardReq *http.Request, trustForwardHeader bool, allowedHeaders []string) { utils.CopyHeaders(forwardReq.Header, req.Header) ... forwardReq.Header = filterForwardRequestHeaders(forwardReq.Header, allowedHeaders) ``` It then selectively rebuilds only a subset of forwarded headers when `trustForwardHeader=false`, for example: - `X-Forwarded-For` - `X-Forwarded-Method` - `X-Forwarded-Proto` - `X-Forwarded-Port` - `X-Forwarded-Host` - `X-Forwarded-Uri` However, it does **not** remove or rebuild `X-Forwarded-Prefix`, so an attacker-supplied value remains in the auth request even when forwarded headers are supposed to be untrusted. This becomes security-relevant when `StripPrefix` is used before `ForwardAuth`. In [`pkg/middlewares/stripprefix/strip_prefix.go`](pkg/middlewares/stripprefix/strip_prefix.go), Traefik appends the stripped prefix using `Header.Add`: ```go func (s *stripPrefix) serveRequest(rw http.ResponseWriter, req *http.Request, prefix string) { req.Header.Add(ForwardedPrefixHeader, prefix) ``` If the attacker already sent `X-Forwarded-Prefix: /admin`, and `StripPrefix` later adds `/forbidden`, the auth service receives both values in this order: 1. `/admin` (attacker-controlled) 2. `/forbidden` (Traefik-generated) An auth service that uses the first `X-Forwarded-Prefix` value can therefore be tricked into authorizing a protected route. Why this appears unintended: - The docs say `trustForwardHeader` means "Trust all X-Forwarded-* headers" and defaults to `false`. - The migration notes say `X-Forwarded-Prefix` is handled like other `X-Forwarded-*` headers and removed from untrusted sources. - The direct-to-Traefik test case behaves consistently with that expectation and returns `403`. - Only the auth subrequest path still honors the spoofed `X-Forwarded-Prefix`. Relevant source/documentation locations: - `pkg/middlewares/auth/forward.go` lines 393-459 - `pkg/middlewares/stripprefix/strip_prefix.go` lines 65-68 - `pkg/middlewares/forwardedheaders/forwarded_header.go` lines 15-43 - `docs/content/reference/routing-configuration/http/middlewares/forwardauth.md` lines 59-62 and 130-140 - `docs/content/migrate/v3.md` lines 192-196 This was only tested and validated with `X-Forwarded-Prefix`. By source review, other forwarded headers that are copied but not rebuilt in `writeHeader` may deserve separate review, but I am not claiming impact for them here. ### PoC The following uses the official `traefik:v3.6.12` Docker image and a mounted `traefik.toml`, matching the documented deployment style. 1. Create `traefik.toml`: ```toml [entryPoints] [entryPoints.web] address = ":80" [entryPoints.web.forwardedHeaders] trustedIPs = ["172.31.79.0/24"] [providers] [providers.file] filename = "/etc/traefik/dynamic.toml" watch = false [log]

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).

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What to do

Affected software

Description

WeaknessesCWE-345

Technical details

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