How severe is CVE-2026-41263?

No CVSS score has been assigned to CVE-2026-41263 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-41263?

CVE-2026-41263 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-2026-41263?

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-2026-41263 is resolved across your whole dependency graph.

How do I detect CVE-2026-41263 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-2026-41263 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2026-41263 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-41263 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-2026-41263?

CVE-2026-41263 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 31th 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-2026-41263?

CVE-2026-41263 is classified as CWE-208 (CWE-208). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-41263 published, and has it been updated?

CVE-2026-41263 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-2026-41263 — traefik

CVE-2026-41263 is a CWE-208 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: BasicAuth middleware: timing side-channel vulnerability

Also known asGHSA-6x2q-h3cr-8j2hGO-2026-5195

Published

Apr 30, 2026

Updated

Oct 28, 2026

Affected

4 pkgs

Patched

3 / 4

Exploits

None indexed

Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

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-2026-41263.

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs31th percentile — riskier than 31% 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 timing side-channel vulnerability in Traefik's BasicAuth middleware that allows an attacker to enumerate valid usernames through response-time differences.

The variable intended to hold a constant-time fallback secret always resolves to an empty string, causing the constant-time comparison to short-circuit in microseconds rather than performing a full bcrypt evaluation. This restores the original timing oracle and makes it possible to distinguish existing users from non-existing ones by measuring authentication response times.

Patches

For more information

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

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

BasicAuth Timing Regression: CVE-2026-32595 Fix Is a No-Op Due to Map Key/Value Confusion

TL;DR

The patch for CVE-2026-32595 is a no-op. Line 49 of basic_auth.go has a map key/value confusion that makes notFoundSecret always "". The "constant time" fallback calls goauth.CheckSecret(password, ""), which fast-fails in ~1us instead of running bcrypt (~60ms).

Evidence (HEAD `786f7192e`, 2026-04-09)

Black-box PoC against live traefik binary on port 28080:

bucket	n	median	min
existing user (wrong pw)	240	62.85 ms	57.54 ms
nonexistent user (wrong pw)	400	0.48 ms	0.35 ms

Median ratio: 130.4x. Classification: 8/8 correct.

Go in-tree test: goauth.CheckSecret direct ratio 12,746x.

Root cause (4-step trace)

basic_auth.go:49: users[slices.Collect(maps.Values(users))[0]] -- looks up a hash as a username key, returns "".
basic_auth.go:119-120: calls goauth.CheckSecret(password, "").
go-http-auth/basic.go:87: empty string matches no prefix, falls to default compareMD5HashAndPassword.
basic.go:107-109: bytes.SplitN("", "$", 4) returns length 1, function returns instantly.

Files

poc/exploit.py -- black-box Python timing oracle
poc/basic_auth_timing_regression_test.go -- Go in-tree test
poc/traefik.yml + poc/dynamic.yml -- traefik config
poc/live_http_poc_output_head.txt -- verbatim PoC output on HEAD

Koda Reef

</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-2026-41263 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

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 HatModerate

Workaround published by Red Hat

To mitigate this issue, restrict network access to Traefik instances that utilize the BasicAuth middleware. Implement firewall rules or network access controls to limit connections to trusted networks or clients only. This reduces the exposure to remote attackers attempting username enumeration. Always ensure that any changes to network configurations are thoroughly tested to prevent service disruption. A service reload or restart may be required for changes to take effect.

Source: Red Hat security advisory for CVE-2026-41263 (CC BY 4.0)

Frequently Asked Questions

## Summary There is a timing side-channel vulnerability in Traefik's BasicAuth middleware that allows an attacker to enumerate valid usernames through response-time differences. The variable intended to hold a constant-time fallback secret always resolves to an empty string, causing the constant-time comparison to short-circuit in microseconds rather than performing a full bcrypt evaluation. This restores the original timing oracle and makes it possible to distinguish existing users from non-existing ones by measuring authentication response times. ## Patches - https://github.com/traefik/t

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