Caddy: Windows `file_server` path authorization bypass via encoded backslash
Date: 2026-06
Affected: go/github.com/caddyserver/caddy, go/github.com/caddyserver/caddy/v2
Attack vector: auth-bypass
Category: vulnerability-disclosure
Severity: High
CVE: CVE-2026-52844
CWE: CWE-22
CWE-284
CVSS: 7.5 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N)

### Summary On Windows, Caddy `path` matchers treat `/private\secret.txt` as outside `/private/*`, but `file_server` later resolves the same request path as `private\secret.txt` on disk. An unauthenticated remote client can request `/private%5csecret.txt` and bypass Caddy path-scoped auth/deny routes protecting `/private/*`. ### Details The mismatch is between two Caddy code paths: - `MatchPath.MatchWithError()` compares `r.URL.Path` using URL path semantics and does not normalize `\` to `/`: `modules/caddyhttp/matchers.go:429`, `:436`, `:490`, `:532`. - If the route matcher misses, Caddy skips that route: `modules/caddyhttp/routes.go:271`. - `file_server` then maps the same request path to a filesystem path with `SanitizedPathJoin(root, r.URL.Path)`: `modules/caddyhttp/fileserver/staticfiles.go:294`, `modules/caddyhttp/caddyhttp.go:257`, `:263`. - On Windows, Go filesystem path handling treats `\` as a separator, so the default filesystem opens the file under the protected directory: `internal/filesystems/os.go:18`. This is related to, but distinct from, `GHSA-4xrr-hq4w-6vf4 / CVE-2026-27585`. That advisory fixed backslash handling in the `file` matcher / `try_files` glob path. This report does not use `try_files` or the `file` matcher; it affects ordinary `path` route matchers in front of direct `file_server` serving and reproduces on current HEAD. ### PoC Tested on current HEAD `6c675e29f87cbe7326983ddb6d739175119d394c` with a Windows `caddy.exe` built from this re


Mitigations:

Architectural layers (MAESTRO):