# CVE-2026-39858: Traefik: Forwarded alias spoofing top pre-auth decision bypass > Attackers can bypass authentication on protected routes by injecting spoofed trust context via unsanitized alias headers in Traefik. - **Severity:** High 7.8 (`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`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 30 Apr 2026 - **Updated:** 15 Jul 2026 - **Also known as:** CLEANSTART-2026-KQ85675, GHSA-5m6w-wvh7-57vm, GO-2026-5149, MINI-8f27-87m9-q3h6, MINI-cgx4-x4hc-hjjq, MINI-rrvw-f54g-chph ## What is CVE-2026-39858? CVE-2026-39858 is a high-severity authentication bypass vulnerability in Traefik Traefik, published on 30 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description Traefik is an HTTP reverse proxy and load balancer. Prior to versions 2.11.43, 3.6.14, and 3.7.0-rc.2, there is a high severity authentication bypass vulnerability in Traefik's ForwardAuth and snippet-based authentication middleware. Traefik's forwarded-header sanitization logic targets only canonical header names (e.g., X-Forwarded-Proto) and does not strip or normalize alias variants that use underscores instead of dashes (e.g., X_Forwarded_Proto). These unsanitized alias headers are forwarded intact to the authentication backend. 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. This issue has been patched in versions 2.11.43, 3.6.14, and 3.7.0-rc.2. Weakness: CWE-289 (Authentication Bypass by Alternate Name), CWE-306 (Missing Authentication for Critical Functionality), CWE-290 (Authentication Bypass by Spoofing) ## Which versions are affected by CVE-2026-39858? 11 open source packages ship an affected version, including github.com/traefik/traefik/v3, github.com/traefik/traefik and github.com/traefik/traefik/v2. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `github.com/traefik/traefik/v3` | Go | From 3.0.0-beta1 | 3.6.14, 3.7.0-rc.2 | | `github.com/traefik/traefik/v3` | Go | From 3.0.0-beta1 | 3.6.14 | | `github.com/traefik/traefik/v3` | Go | From 3.0.0-beta1 | 3.6.14 | | `github.com/traefik/traefik/v3` | Go | From 3.7.0-ea.1 | 3.7.0-rc.2 | | `github.com/traefik/traefik/v3` | Go | From 3.7.0-ea.1 | 3.7.0-rc.2 | | `github.com/traefik/traefik` | Go | All versions | None | | `github.com/traefik/traefik` | Go | All versions to 1.7.34 | None | | `github.com/traefik/traefik` | Go | All versions to 1.7.34 | None | | `github.com/traefik/traefik/v2` | Go | All versions | 2.11.43 | | `github.com/traefik/traefik/v2` | Go | All versions | 2.11.43 | | `github.com/traefik/traefik/v2` | Go | All versions | 2.11.43 | Products: Traefik Traefik. ## How do I fix CVE-2026-39858? Upgrade github.com/traefik/traefik/v3 to a fixed version for your release line: 3.6.14 or 3.7.0-rc.2. ## Is CVE-2026-39858 being exploited? There is no known exploitation of CVE-2026-39858 in the wild and no public exploit code. ## Other vulnerabilities in Traefik Traefik - [CVE-2024-45410](https://insights.sbomify.com/CVE-2024-45410/): HTTP client can remove the X-Forwarded headers in Traefik (critical 9.8) - [CVE-2026-88877](https://insights.sbomify.com/CVE-2026-88877/): Traefik v3.7.0 Authentication Bypass via from-to-www-redirect (critical 9.3) - [CVE-2026-85595](https://insights.sbomify.com/CVE-2026-85595/): Traefik before v2.11.55 and v3.0.0 through v3.7.10 Authentication Bypass via digestAuth (critical 9.3) - [CVE-2026-88007](https://insights.sbomify.com/CVE-2026-88007/): Traefik HTTP/3 Backend NTLM Connection Reuse (critical 9.1) - [CVE-2026-88009](https://insights.sbomify.com/CVE-2026-88009/): Traefik: Rootless HTTP/1 request-target routes as "/" but is forwarded verbatim, bypassing path-scoped routing, middleware guards and access logging (high 8.8) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-39858 and fix it if it is affected. What it is: CVE-2026-39858 is a high-severity authentication bypass vulnerability in Traefik Traefik, published on 30 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2026-39858 in the wild and no public exploit code. Affected packages and fixed versions: - github.com/traefik/traefik/v3 (Go): affected from 3.0.0-beta1; fixed in 3.6.14, 3.7.0-rc.2 - github.com/traefik/traefik/v3 (Go): affected from 3.0.0-beta1; fixed in 3.6.14 - github.com/traefik/traefik/v3 (Go): affected from 3.0.0-beta1; fixed in 3.6.14 - github.com/traefik/traefik/v3 (Go): affected from 3.7.0-ea.1; fixed in 3.7.0-rc.2 - github.com/traefik/traefik/v3 (Go): affected from 3.7.0-ea.1; fixed in 3.7.0-rc.2 - github.com/traefik/traefik (Go): affected all versions; no fix listed - github.com/traefik/traefik (Go): affected all versions up to 1.7.34; no fix listed - github.com/traefik/traefik (Go): affected all versions up to 1.7.34; no fix listed - github.com/traefik/traefik/v2 (Go): affected all versions; fixed in 2.11.43 - github.com/traefik/traefik/v2 (Go): affected all versions; fixed in 2.11.43 - github.com/traefik/traefik/v2 (Go): affected all versions; fixed in 2.11.43 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-39858/ (Markdown: https://insights.sbomify.com/CVE-2026-39858/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-39858 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-39858 - OSV: https://osv.dev/vulnerability/CVE-2026-39858 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-39858/ Summaries are generated and may contain mistakes; the official description above is the record.