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CVE-2026-52844 – Authorization Bypass – Caddy Web Server prior to 2.11.4 (Windows)

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Summary

CVE-2026-52844 is a high-severity authorization bypass vulnerability in the Caddy web server affecting Windows deployments running versions prior to 2.11.4. The flaw arises from an inconsistency between Caddy’s path matchers and its file_server module in how URL-encoded backslash characters (%5c) are handled, allowing an unauthenticated remote attacker to access files in path-protected directories that should be blocked by configured access controls. The CVSS v3.1 base score is 7.5 (High).

Technical details

  • Root cause: Caddy’s path matchers evaluate request URLs without normalizing %5c (URL-encoded backslash) to a forward slash. When a path like /private%5csecret.txt is evaluated against a route protecting /private/*, the matcher does not recognize it as being inside the protected subtree and allows the request to pass. The file_server module subsequently resolves the same path to disk, where Windows treats the backslash as a directory separator — effectively reading privatesecret.txt from the protected directory.
  • Trigger conditions: The target Caddy instance must be running on Windows and must use path-scoped access controls (e.g., basic_auth or deny directives) in front of file_server for a protected path prefix such as /private/*.
  • Attack vector: An unauthenticated remote attacker sends an HTTP request with a URL-encoded backslash in the path (e.g., GET /private%5csecret.txt), bypassing the configured route authorization without credentials.
  • Impact: Confidentiality impact is HIGH. An attacker can read arbitrary files within protected directory trees — including credentials, configuration files, backups, or other sensitive data served by file_server — without any authentication.

Affected software

  • Caddy (caddyserver/caddy) — all versions prior to 2.11.4 running on Windows
  • Caddy deployments on Linux and macOS are not affected

Severity

  • CVSS v3.1 Base Score: 7.5 (High)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
  • CWE-22 (Improper Limitation of a Pathname to a Restricted Directory – Path Traversal)
  • CWE-284 (Improper Access Control)

Mitigation and recommended actions

  • Immediate: Upgrade to Caddy v2.11.4 or later, which normalizes Windows backslashes in the path matcher before route evaluation, ensuring consistent behavior between the matcher and file_server.
  • If immediate patching is not possible: No vendor-documented workaround exists. As a temporary network-level mitigation, consider restricting access to affected Caddy instances to trusted IP ranges and auditing which paths are protected by file_server with path-scoped access controls on Windows hosts.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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