CVE-2026-88018

### Summary `rclone serve s3`'s handler chain, when `--auth-proxy` is configured, is (outermost first): `authPairMiddleware` -> `proxyAuthMiddleware` -> gofakes3's own SigV4-verifying handler. `authPairMiddleware` parses the accessKeyID straight out of the incoming request's own `Authorization` header (entirely client-controlled) and registers `{accessKey: ws.s3Secret}` into gofakes3's shared credential store via `AddAuthKeys`, for EVERY access key any client presents - not just ones previously known to the server. `ws.s3Secret` defaults to `""` whenever `--auth-key` is not set, which the `--auth-proxy` documentation (and the reference `bin/test_proxy.py`) presents as a complete, standalone authentication mechanism requiring no other flag - matching how it's used for `serve webdav`/`ftp`/`sftp`. gofakes3's SigV4 verification then checks the request's signature against exactly the secret `authPairMiddleware` just registered for that same client-chosen key. An empty string is a valid HMAC key, so a caller can trivially compute a correct SigV4 signature for ANY access key ID of their choosing using an empty secret, and verification passes. Crucially, the auth-proxy script never receives a real secret to verify against, for S3 specifically: `Server.auth()` calls `w.proxy.Call(md5(accessKeyID), accessKeyID, false, r.RemoteAddr)` - passing the access key ID itself as BOTH the hashed "user" and the raw "auth"/password fields. Contrast with `serve webdav`/`ftp`/`sftp`, whose proxy integration passes the client's actual typed password (see `bin/test_proxy.py`, which forwards it into a backing SFTP login for real verification). For S3, no independent secret is ever transmitted to the proxy script at all, so no script - however carefully written - can distinguish a legitimate holder of an access key ID from an attacker who merely picked the same string. Net effect: with `--auth-proxy` configured and `--auth-key` not also set (the configuration the feature is documented to support standalone), SigV4 signature verification authenticates nobody. ### Details Vulnerable code (before fix): ```go func authPairMiddleware(next http.Handler, ws *Server) http.Handler { return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { accessKey, _ := parseAccessKeyID(r) authPair := map[string]string{accessKey: ws.s3Secret} ws.faker.AddAuthKeys(authPair) next.ServeHTTP(w, r) }) } ``` ### PoC Built and signed a request by hand (via the vendored `github.com/aws/aws-sdk-go-v2/aws/signer/v4`) using a freshly-random access key ID never configured or returned by anything, with `SecretAccessKey: ""`, against a real `rclone serve s3 --auth-proxy <script>` instance with no `--auth-key` set: ``` status=200 <ListAllMyBucketsResult>...<Bucket><Name>mybucket</Name>... ``` A fully authenticated, successful bucket listing, with zero prior credential knowledge. ### Impact Any network-reachable, unauthenticated attacker who knows (or discovers) that a target is running `rclone serve s3 --auth-proxy` without `--auth-key` can choose an arbitrary access key ID, sign a request against an empty secret, and be treated as an authenticated user by the auth-proxy script - reaching whatever backend that script resolves the chosen identity to. No credentials, prior access, or user interaction of any kind are required. ### Fix Refuse to start `rclone serve s3` when `--auth-proxy` is set without `--auth-key`, rather than silently falling back to a signature check that authenticates nobody: ```go if proxyOpt.AuthProxy != "" && len(opt.AuthKey) == 0 { return nil, errors.New("serve s3: --auth-proxy requires --auth-key to also be set (SigV4 has no other way to verify a signature for a dynamically-proxied identity)") } ``` Note this is a minimal fix for the zero-knowledge bypass; once `--auth-key` is also set, every access key ID still shares that one static secret for signature-verification purposes (a caller who knows it can request any identity from the proxy script) - a narrower, pre-existing limitation flagged for awareness but not changed here, since a complete fix needs the auth-proxy wire protocol to carry a per-identity secret for S3 specifically (a larger design change).

rclone is a command-line program to sync files and directories to and from different cloud storage providers. Prior to 1.75.1, rclone serve s3 configured with --auth-proxy but without --auth-key allows authPairMiddleware to register any client-chosen accessKeyID with an empty ws.s3Secret. gofakes3 then verifies the request’s SigV4 signature against that same empty secret, while Server.auth passes the access key identifier as both the user and authentication value to the proxy without an independent per-identity secret. An unauthenticated network attacker can therefore choose an arbitrary access key, sign with an empty secret, and reach whatever backend the auth-proxy script resolves for that identity. This issue is fixed in version 1.75.1.

rclone serve s3: --auth-proxy without --auth-key authenticates nobody - full SigV4 signature bypass in github.com/rclone/rclone