CVE-2026-27585 is a vulnerability classified under CWE-20 (Improper Input Validation) affecting the Caddy web server platform versions prior to 2.11.1. Caddy is an extensible server platform that uses TLS by default and is popular for its ease of use and security features. The vulnerability stems from the path sanitization routine within Caddy's file matcher component, which fails to properly sanitize backslash characters ('\'). This improper input validation allows attackers to bypass path-related security protections, potentially enabling unauthorized access to files or directories outside the intended scope. The flaw is particularly relevant in environments where Caddy is configured to serve files or directories with strict path access controls. Exploitation is possible remotely without authentication or user interaction, as indicated by the CVSS vector (AV:N/AC:L/AT:P/PR:N/UI:N). The vulnerability impacts confidentiality and integrity by enabling unauthorized file access or manipulation. The issue was addressed and fixed in Caddy version 2.11.1 by improving the sanitization logic to correctly handle backslashes, thereby preventing bypasses. No known exploits have been reported in the wild to date, but the vulnerability's presence in widely used versions of Caddy necessitates prompt remediation. Organizations relying on Caddy for web serving or reverse proxy functions should assess their exposure and upgrade accordingly.

The vulnerability can lead to unauthorized access to files or directories that should be protected by path restrictions, potentially exposing sensitive data or allowing attackers to manipulate file contents. This compromises confidentiality and integrity of the affected systems. Since Caddy is often used as a web server or reverse proxy, exploitation could facilitate further attacks such as information disclosure, privilege escalation, or lateral movement within a network. The ease of exploitation (no authentication or user interaction required) and network accessibility increase the risk. However, the impact is somewhat limited by the need for specific Caddy configurations and environment setups that expose the vulnerable file matcher functionality. Organizations with sensitive data hosted on Caddy servers or those relying on strict path-based access controls are at higher risk. The vulnerability does not directly affect availability but could indirectly cause service disruptions if exploited to alter critical files.

1. Upgrade all Caddy server instances to version 2.11.1 or later, where the vulnerability is fixed. 2. Review and harden file serving configurations to minimize exposure of sensitive directories and files. 3. Implement additional input validation and filtering at the application or reverse proxy level to detect and block suspicious path characters such as backslashes. 4. Monitor server logs for unusual access patterns or attempts to exploit path traversal or bypass protections. 5. Employ file integrity monitoring to detect unauthorized changes to critical files. 6. Restrict network access to Caddy servers to trusted sources where possible, reducing the attack surface. 7. Conduct regular security assessments and penetration testing focused on path traversal and input validation weaknesses. 8. Educate administrators about the importance of timely patching and secure configuration management for web servers.