Summary
rclone serve s3: --auth-proxy without --auth-key authenticates nobody - full SigV4 signature bypass
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):
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.
The application does not adequately verify the identity of a user, device, or process before granting access. Typical impact: unauthorized access to functions or data reserved for authenticated parties.
CVE-2026-88018 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.75.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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See it in your environmentNew to Kodem? Get a demo →Remediation advice
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:
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).
Frequently Asked Questions
- What is CVE-2026-88018? CVE-2026-88018 is a critical-severity improper authentication vulnerability in github.com/rclone/rclone (go), affecting versions < 1.75.1. It is fixed in 1.75.1. The application does not adequately verify the identity of a user, device, or process before granting access.
- How severe is CVE-2026-88018? CVE-2026-88018 has a CVSS score of 9.8 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/rclone/rclone are affected by CVE-2026-88018? github.com/rclone/rclone (go) versions < 1.75.1 is affected.
- Is there a fix for CVE-2026-88018? Yes. CVE-2026-88018 is fixed in 1.75.1. Upgrade to this version or later.
- Is CVE-2026-88018 exploitable, and should I be worried? Whether CVE-2026-88018 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-88018 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-88018? Upgrade
github.com/rclone/rcloneto 1.75.1 or later.