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GHSA-6jwx-7vp4-9847 CVE-2026-40912 GO-2026-5180

Traefik has an StripPrefixRegex Middleware Authorization Bypass via Path/RawPath Desync

High Go
Severity
0.0 / 10
High · CVSS 4.0
Exploit chance (EPSS)
0.77%
Chance of exploitation in the next 30 days. Higher than 55.53% of scored vulnerabilities.
Known exploited
No
Not in CISA's catalog
Weakness
CWE-22 Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
CWE-706 Use of Incorrectly-Resolved Name or Reference
Fix
Available
2 packages have a fixed version

Description

Summary

There is a high severity authentication bypass vulnerability in Traefik's StripPrefixRegex middleware when used in combination with ForwardAuth, BasicAuth, or DigestAuth.

The middleware matches the regex against the decoded URL path but uses the resulting byte length to slice the percent-encoded raw path. When a dot (or multiple dots) appears in the prefix portion of the URL, the raw path after stripping becomes a dot-segment (e.g. /./admin/secret).

ForwardAuth receives this dot-segment path in X-Forwarded-Uri, which does not match the protected path patterns and therefore allows the request through. The backend then normalizes the dot-segment to the real path per RFC 3986 and serves the protected content

An unauthenticated attacker can exploit this against any backend that performs dot-segment normalization.

Patches

For more information

If there are any questions or comments about this advisory, please open an issue.

<details> <summary>Original Description</summary>

Summary

StripPrefixRegex uses the byte length of a decoded Path match to slice the encoded RawPath. When percent-encoded characters are in the prefix region, this produces a wrong RawPath. ForwardAuth then receives this wrong path in X-Forwarded-Uri, sees a path that doesn't match its protection rules, and approves the request. The backend serves protected content.

Details

pkg/middlewares/stripprefixregex/strip_prefix_regex.go, line 62:

req.URL.RawPath = ensureLeadingSlash(req.URL.RawPath[len(prefix):])

prefix comes from matching the regex against the decoded req.URL.Path (line 51). len(prefix) is then used to index into the encoded req.URL.RawPath. These lengths don't match when percent-encoding is present.

Example with regex ^/api:

  • GET /api%20/admin/secret
  • Decoded Path: /api /admin/secret -> prefix = /api (4 bytes)
  • Encoded RawPath: /api%20/admin/secret -> same region is 6 bytes
  • RawPath[4:] = %20/admin/secret -> after ensureLeadingSlash -> /%20/admin/secret
  • ForwardAuth sees X-Forwarded-Uri: /%20/admin/secret -> not /admin/* -> allows it
  • Backend serves the protected admin content

PoC

Requires Docker and Docker Compose. I have a setup that runs Traefik v3.6.11 with StripPrefixRegex + ForwardAuth + a backend. It sends a normal request (blocked, 403) and an encoded request (bypasses auth, 200, returns protected data). Can share the files here if useful.

Impact

Auth bypass. Any path protected by ForwardAuth, BasicAuth, or DigestAuth can be accessed without credentials when StripPrefixRegex is in the same middleware chain. The attacker only needs to add a percent-encoded character to the prefix portion of the URL.

Updated PoC (reporter follow-up)

After further testing, the confirmed working exploit uses %2e (percent-encoded dot) rather than %20. Dot-segment normalization (/./ -> /) is RFC 3986 standard behavior handled automatically by Express.js, Go's http.ServeMux, Spring Boot, and others — no custom configuration needed.

Chain:

GET /api%2e/admin/secret
-> StripPrefixRegex strips /api -> RawPath becomes /./admin/secret
-> ForwardAuth sees /./admin/secret -> does not match /admin/ -> allows
-> Express normalizes /./admin/secret -> /admin/secret -> serves protected content

Results (Traefik v3.6, unmodified Express.js express.static):

GET /api/admin/secret      -> 403 (blocked)
GET /api%2e/admin/secret   -> 200 (bypass — served protected content)
GET /api%20/admin/secret   -> 404 (space not normalized by backend)

Auth server logs:

X-Forwarded-Uri: '/admin/secret'    -> DENIED
X-Forwarded-Uri: '/./admin/secret'  -> ALLOWED

Reproduction:

docker compose up -d --build --wait
curl http://localhost:8080/api/admin/secret                       # -> 403
curl --path-as-is "http://localhost:8080/api%2e/admin/secret"     # -> 200

</details>

Affected versions

Go

PackageVulnerableFixed in
github.com/traefik/traefik
<= 1.7.34
 No fix yet
github.com/traefik/traefik
all versions
 No fix yet
github.com/traefik/traefik/v2
< 2.11.43
2.11.43
github.com/traefik/traefik/v3
>= 3.7.0-ea.1, < 3.7.0-rc.2
3.7.0-rc.2
github.com/traefik/traefik/v3
>= 3.0.0-beta1, < 3.6.14
3.6.14
github.com/traefik/traefik/v3
>= 3.0.0-beta1, < 3.6.14
>= 3.7.0-ea.1, < 3.7.0-rc.2
3.6.14
3.7.0-rc.2

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

github.com/traefik/traefik/v2 Go → 2.11.43

go
go get github.com/traefik/traefik/v2@v2.11.43 && go mod tidy

github.com/traefik/traefik/v3 Go → 3.7.0-rc.2

go
go get github.com/traefik/traefik/v3@v3.7.0-rc.2 && go mod tidy

Severity breakdown

CVSS 4.0 0.0

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

Attack vector
Network
Attack complexity
Low
Attack requirements
None
Privileges required
None
User interaction
None
Confidentiality (vulnerable system)
None
Integrity (vulnerable system)
None
Availability (vulnerable system)
None
Confidentiality (subsequent systems)
High
Integrity (subsequent systems)
Low
Availability (subsequent systems)
None

CVSS 3.1 8.2

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

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
Low
Availability
None

Weakness

CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')

The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.

How to prevent it
  • Implementation: 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. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright. When validating filenames, use stringent allowlists that limit the character set to be used. If feasible, only allow a single "." character in the filename to avoid weaknesses such as CWE-23, and exclude directory separators such as "/" to avoid CWE-36. Use a list of allowable file extensions, which will help to avoid CWE-434. Do not rely exclusively on a filtering mechanism that removes potentially dangerous characters. This is equivalent to a denylist, which may be incomplete (CWE-184). For example, filtering "/" is insufficient protection if the filesystem also supports the use of "\" as a directory separator. Another possible error could occur when the filtering is applied in a way that still produces dangerous data (CWE-182). For example, if "../" sequences are removed from the ".../...//" string in a sequential fashion, two instances of "../" would b
  • Architecture and Design: For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks entirely. Then, these modified values would be submitted to the server.
  • Implementation: 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). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked. Use a built-in path canonicalization function (such as realpath() in C) that produces the canonical version of the pathname, which effectively removes ".." sequences and symbolic links (CWE-23, CWE-59). This includes: realpath() in C getCanonicalPath() in Java GetFullPath() in ASP.NET realpath() or abs_path() in Perl realpath() in PHP
  • Architecture and Design: Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid [REF-1482].
  • Operation: Use an application firewall that can detect attacks against this weakness. It can be beneficial in cases in which the code cannot be fixed (because it is controlled by a third party), as an emergency prevention measure while more comprehensive software assurance measures are applied, or to provide defense in depth [REF-1481].
  • Architecture and Design, Operation: Run your code using the lowest privileges that are required to accomplish the necessary tasks [REF-76]. If possible, create isolated accounts with limited privileges that are only used for a single task. That way, a successful attack will not immediately give the attacker access to the rest of the software or its environment. For example, database applications rarely need to run as the database administrator, especially in day-to-day operations.
  • Architecture and Design: When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs. For example, ID 1 could map to "inbox.txt" and ID 2 could map to "profile.txt". Features such as the ESAPI AccessReferenceMap [REF-185] provide this capability.
  • Architecture and Design, Operation: Run the code in a "jail" or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which files can be accessed in a particular directory or which commands can be executed by the software. OS-level examples include the Unix chroot jail, AppArmor, and SELinux. In general, managed code may provide some protection. For example, java.io.FilePermission in the Java SecurityManager allows the software to specify restrictions on file operations. This may not be a feasible solution, and it only limits the impact to the operating system; the rest of the application may still be subject to compromise. Be careful to avoid CWE-243 and other weaknesses related to jails.
  • Architecture and Design, Operation: Store library, include, and utility files outside of the web document root, if possible. Otherwise, store them in a separate directory and use the web server's access control capabilities to prevent attackers from directly requesting them. One common practice is to define a fixed constant in each calling program, then check for the existence of the constant in the library/include file; if the constant does not exist, then the file was directly requested, and it can exit immediately. This significantly reduces the chance of an attacker being able to bypass any protection mechanisms that are in the base program but not in the include files. It will also reduce the attack surface.
  • Implementation: Ensure that error messages only contain minimal details that are useful to the intended audience and no one else. The messages need to strike the balance between being too cryptic (which can confuse users) or being too detailed (which may reveal more than intended). The messages should not reveal the methods that were used to determine the error. Attackers can use detailed information to refine or optimize their original attack, thereby increasing their chances of success. If errors must be captured in some detail, record them in log messages, but consider what could occur if the log messages can be viewed by attackers. Highly sensitive information such as passwords should never be saved to log files. Avoid inconsistent messaging that might accidentally tip off an attacker about internal state, such as whether a user account exists or not. In the context of path traversal, error messages which disclose path information can help attackers craft the appropriate attack strings to move through the file system hierarchy.
  • Operation, Implementation: When using PHP, configure the application so that it does not use register_globals. During implementation, develop the application so that it does not rely on this feature, but be wary of implementing a register_globals emulation that is subject to weaknesses such as CWE-95, CWE-621, and similar issues.

CWE-706: Use of Incorrectly-Resolved Name or Reference

The product uses a name or reference to access a resource, but the name/reference resolves to a resource that is outside of the intended control sphere.

References

Advisories

Packages

Other links

Timeline

  1. Published
  2. Last updated by the source
  3. Latest EPSS score
AdvisorySeverityWhy it's related
GO-2026-6558
Traefik: BasicAuth singleflight coalescing reintroduces an unauthenticated username-enumeration timing oracle
Medium Also affects github.com/traefik/traefik
GHSA-f52w-8j3h-j724
Traefik: Rootless HTTP/1 request-target routes as "/" but is forwarded verbatim, bypassing path-scoped routing, middleware guards and access logging
High Also affects github.com/traefik/traefik
GHSA-w4v4-9rw7-5326
Traefik: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') and Incorrect Authorization
Critical Also affects github.com/traefik/traefik
GHSA-qqjf-53cj-pwvv
Traefik HTTP/3 Backend NTLM Connection Reuse
Critical Also affects github.com/traefik/traefik
GO-2026-6467
Traefik entrypoint header-name sanitization bypassed via request trailers
High Also affects github.com/traefik/traefik
GHSA-rf44-j88r-hh8c
Traefik: ForwardAuth identity spoofing via dot-form header alias
High Also affects github.com/traefik/traefik
GHSA-7ghq-v6jf-g56c
Traefik: respondingTimeouts.readTimeout is not applied to HTTP/3, leaving slow-body uploads unbounded
Medium Also affects github.com/traefik/traefik
GHSA-62fc-8686-hfmq
Traefik: `allowCrossNamespace=false` bypass via `@kubernetescrd` TraefikService backendRef
Medium Also affects github.com/traefik/traefik
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