CVE-2026-27586 — Vulnerability in github.com/caddyserver/caddy/v2...

### Summary Two swallowed errors in `ClientAuthentication.provision()` cause mTLS client certificate authentication to silently fail open when a CA certificate file is missing, unreadable, or malformed. The server starts without error but accepts any client certificate signed by any system-trusted CA, completely bypassing the intended private CA trust boundary. ### Details In `modules/caddytls/connpolicy.go`, the `provision()` method has two `return nil` statements that should be `return err`: **Bug #1 — line 787:** ```go ders, err := convertPEMFilesToDER(fpath) if err != nil { return nil // BUG: should be "return err" } ``` **Bug #2 — line 800:** ```go err := caPool.Provision(ctx) if err != nil { return nil // BUG: should be "return err" } ``` Compare with line 811 which correctly returns the error: ```go caRaw, err := ctx.LoadModule(clientauth, "CARaw") if err != nil { return err // CORRECT } ``` When the error is swallowed on line 787, the chain is: 1. `TrustedCACerts` remains empty (no DER data appended from the file) 2. The `len(clientauth.TrustedCACerts) > 0` guard on line 794 is false — skipped 3. `clientauth.CARaw` is nil — line 806 returns nil 4. `clientauth.ca` remains nil — no CA pool was created 5. `provision()` returns nil — caller thinks provisioning succeeded Then in `ConfigureTLSConfig()`: 6. `Active()` returns true because `TrustedCACertPEMFiles` is non-empty 7. Default mode is set to `RequireAndVerifyClientCert` (line 860) 8. But `clientauth.ca` is nil, so `cfg.ClientCAs` is never set (line 867 skipped) 9. Go's `crypto/tls` with `RequireAndVerifyClientCert` + nil `ClientCAs` verifies client certs against the **system root pool** instead of the intended CA The fix is changing `return nil` to `return err` on lines 787 and 800. ### PoC 1. Configure Caddy with mTLS pointing to a nonexistent CA file: ``` { "apps": { "http": { "servers": { "srv0": { "listen": [":443"], "tls_connection_policies": [{ "client_authentication": { "trusted_ca_certs_pem_files": ["/nonexistent/ca.pem"] } }] } } } } } ``` 2. Start Caddy — it starts without any error or warning. 3. Connect with any client certificate (even self-signed): ```bash openssl s_client -connect localhost:443 -cert client.pem -key client-key.pem ``` 4. The TLS handshake succeeds despite the certificate not being signed by the intended CA. A full Go test that proves the bug end-to-end (including a successful TLS handshake with a random self-signed client cert) is here: https://gist.github.com/moscowchill/9566c79c76c0b64c57f8bd0716f97c48 Test output: ``` === RUN TestSwallowedErrorMTLSFailOpen BUG CONFIRMED: provision() swallowed the error from a nonexistent CA file. tls.Config has RequireAndVerifyClientCert but ClientCAs is nil. CRITICAL: TLS handshake succeeded with a self-signed client cert! The server accepted a client certificate NOT signed by the intended CA. --- PASS: TestSwallowedErrorMTLSFailOpen (0.03s) ``` ### Impact Any deployment using `trusted_ca_cert_file` or `trusted_ca_certs_pem_files` for mTLS will silently degrade to accepting any system-trusted client certificate if the CA file becomes unavailable. This can happen due to a typo in the path, file rotation, corruption, or permission changes. The server gives no indication that mTLS is misconfigured.