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EUVD-2026-318038

fkie_cve-2026-42882

oxyno-zeta/s3-proxy is an aws s3 proxy written in go. Prior to 5.0.0, s3-proxy contains an authentication bypass caused by inconsistent URL path interpretation between the authentication middleware and the bucket handler. The authentication middleware evaluates resource path patterns against the percent-encoded request URI (r.URL.RequestURI()), while the bucket handler constructs S3 object keys from the decoded path (r.URL.Path). This mismatch, combined with the glob library being invoked without a path separator (causing * to match across / boundaries), allows unauthenticated attackers to write to, read from, or delete objects in protected S3 namespaces. Exploitation is possible via three techniques: (1) using * patterns that match across path separators to reach protected routes via path traversal (e.g., /open/foo/drafts/../restricted/), (2) using percent-encoded slashes (%2F) to collapse multiple path segments into a single token at the auth layer while the decoded form resolves to a protected namespace at the storage layer, and (3) using dot-dot segments (../) under ** prefix patterns, where the raw path matches an open route while Go's URL parser resolves the traversal to a protected path before the bucket handler runs. An unauthenticated attacker with network access can perform unauthorized PUT, GET, or DELETE operations on objects in authentication-protected S3 namespaces. This vulnerability is fixed in 5.0.0.

GHSA-rfgq-wgg8-662p — S3-Proxy has Security Issues in its Resource Path Matching Implementation

Affected: Go: github.com/oxyno-zeta/s3-proxy

## Background

The original concern is functional: a resource pattern should treat a percent-encoded segment like some%2Fvalue as a single opaque token rather than splitting it into two path segments at the decoded /. Investigation into why %2F was being decoded and how routes matched against the result surfaced three related security issues, documented below.

Rather than landing a fix directly, the problem space warrants discussion first. Different fixes carry different compliance and compatibility tradeoffs, and every viable option is a breaking change in some form. Aligning on a direction before committing to an implementation is the safer path.

## Root cause: two different path representations

Go's `net/http` decodes percent-encoded characters when it parses an incoming URL: `%2F` becomes `/` in `r.URL.Path`, while the original encoded form is preserved in `r.URL.RawPath`. Two different parts of s3-proxy use different fields:

- The **auth middleware** calls `r.URL.RequestURI()`, which returns the encoded form (from `RawPath` when available). It sees `%2F` as literal characters, not as path separators. - The **bucket handler** reads `r.URL.Path` to build the S3 key. It sees the decoded form, where `%2F` has already become `/`.

All three issues stem from this mismatch, combined with how glob patterns are compiled. The examples below use PUT for concreteness, but the auth bypass applies to any HTTP method — a config that restricts GET or DELETE on a namespace is…