Bug 2531542 (CVE-2026-88018)

Summary: CVE-2026-88018 rclone: rclone serve s3: --auth-proxy without --auth-key authenticates nobody - full SigV4 signature bypass
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
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Priority: urgent    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in rclone. When the `rclone serve s3` command is configured with `--auth-proxy` but without `--auth-key`, an unauthenticated network attacker can bypass the SigV4 signature verification process. This allows the attacker to use an arbitrary access key and sign requests with an empty secret, leading to unauthorized access to the backend storage resolved by the authentication proxy script. This vulnerability results in a complete authentication bypass.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Bug Depends On: 2537399, 2537401    
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Description OSIDB Bzimport 2026-09-10 16:14:06 UTC 
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.