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Response snapshot Nov 4, 2026, 04:16 UTC

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VulnerabilityCVE-2026-33433

Traefik Vulnerable to BasicAuth/DigestAuth Identity Spoofing via Non-Canonical headerField

There is a potential vulnerability in Traefik's Basic and Digest authentication middlewares when headerField is configured with a non-canonical HTTP header name.

Published Updated

At a glance

Exploited
Not known
Severity
5.1 Medium (CVSS 4.0)
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: github.com/traefik/traefik/v2: 2.11.42; github.com/traefik/traefik/v3: 3.6.12; github.com/traefik/traefik/v3: 3.7.0-ea.3. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-qr99-7898-vr7c (opens a new tab)

What happened

Summary

There is a potential vulnerability in Traefik's Basic and Digest authentication middlewares when headerField is configured with a non-canonical HTTP header name.

An authenticated attacker with valid credentials can inject the canonical version of the configured header to impersonate any identity to the backend. Because Traefik writes the authenticated username using a non-canonical map key, it creates a separate header entry rather than overwriting the attacker's canonical one — causing most backend frameworks to read the attacker-controlled value instead.

Patches

Continue reading

- - -

For more information

If there are any questions or comments about this advisory, please [open an issue](https://github.com/traefik/traefik/issues).

---

Original Description

Summary

When headerField is configured with a non-canonical HTTP header name (e.g., x-auth-user instead of X-Auth-User), an authenticated attacker can inject a canonical version of that header to impersonate any identity to the backend. The backend receives two header entries — the attacker-injected canonical one is read first, overriding Traefik's non-canonical write.

Tested on Traefik v3.6.10.

Details

At pkg/middlewares/auth/basic_auth.go:92, the authenticated username is written using direct map assignment:

req.Header[b.headerField] = []string{user}

Go's http.Header map is keyed by canonical names (e.g., X-Auth-User). Direct assignment with a non-canonical key (x-auth-user) creates a separate map entry from any canonical-key entry already present. The attacker's X-Auth-User: superadmin occupies the canonical slot and is never overwritten by Traefik's non-canonical write.

The same bug exists in pkg/middlewares/auth/digest_auth.go:100. Notably, forward.go:254 correctly uses http.CanonicalHeaderKey(), showing the fix pattern already exists in the codebase.

PoC

Traefik config (YAML, Docker labels, or REST API):

middlewares:
 auth:
 basicAuth:
 users: ["admin:$2y$05$..."]
 headerField: "x-auth-user"

Normal request (baseline):

curl -u admin:admin http://traefik/secure/test
# Backend receives: x-auth-user: admin
# Identity = admin ✓

Attack request:

curl -u admin:admin -H "X-Auth-User: superadmin" http://traefik/secure/test
# Backend receives BOTH headers:
# X-Auth-User: superadmin ← attacker-injected (canonical key, read first by most frameworks)
# x-auth-user: admin ← Traefik-set (non-canonical, ignored by most frameworks)
# Identity seen by backend = superadmin ✗

Control test — when headerField uses canonical casing (X-Auth-User), the attack fails. Traefik's write correctly overwrites the attacker's header.

This is realistic because YAML conventions favor lowercase keys, Traefik docs don't warn about canonicalization, and the pattern of backends trusting the headerField header is recommended in Traefik's own documentation.

Fix suggestion:

// basic_auth.go:92 and digest_auth.go:100 — change:
req.Header[b.headerField] = []string{user}
// to:
req.Header.Set(b.headerField, user)

Also strip any incoming headerField header before the auth check with req.Header.Del(b.headerField).

Impact

An authenticated attacker with valid credentials (even low-privilege) can impersonate any other user identity to backend services. If backends use the headerField header for authorization decisions (which is the intended use case per Traefik docs), this enables privilege escalation — e.g., a regular user impersonating an admin.

The attack requires the operator to configure headerField with a non-canonical header name, which is the natural thing to do in YAML and is not warned against in documentation.

Affected package ranges reported by GitHub: github.com/traefik/traefik/v2: < 2.11.42 github.com/traefik/traefik/v3: >= 3.0.0-beta1, < 3.6.11 github.com/traefik/traefik/v3: >= 3.7.0-ea.1, < 3.7.0-ea.3

Scores and how they were measured

CVSS base severity

5.1

Version 4.0As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

Not available

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

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Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-qr99-7898-vr7c (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-qr99-7898-vr7c, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, November 4). Traefik Vulnerable to BasicAuth/DigestAuth Identity Spoofing via Non-Canonical headerField (CVE-2026-33433). https://kevwire.com/record/CVE-2026-33433

MLA

"Traefik Vulnerable to BasicAuth/DigestAuth Identity Spoofing via Non-Canonical headerField (CVE-2026-33433)." KEVWire, 4 Nov. 2026, kevwire.com/record/CVE-2026-33433.

Chicago

KEVWire. "Traefik Vulnerable to BasicAuth/DigestAuth Identity Spoofing via Non-Canonical headerField (CVE-2026-33433)." Last modified November 4, 2026. https://kevwire.com/record/CVE-2026-33433.

Evidence summary (plain text)

Traefik Vulnerable to BasicAuth/DigestAuth Identity Spoofing via Non-Canonical headerField (Mar 27, 2026).
There is a potential vulnerability in Traefik's Basic and Digest authentication middlewares when headerField is configured with a non-canonical HTTP header name.

An authenticated attacker with valid credentials can inject the canonical version of the configured header to impersonate any identity to the backend. Because Traefik writes the authenticated username using a non-canonical map key, it creates a separate header entry rather than overwriting the attacker's canonical one — causing most backend frameworks to read the attacker-controlled value instead.
CVSS 5.1 (version 4.0; evidence as of Nov 4, 2026; https://github.com/advisories/GHSA-qr99-7898-vr7c).
Sources:
GitHub-reviewed advisory GHSA-qr99-7898-vr7c: https://github.com/advisories/GHSA-qr99-7898-vr7c
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: CVE-2026-33433.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Nov 4, 2026, 04:15 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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