CVE-2026-29045 is a vulnerability classified under CWE-177 (Improper Handling of URL Encoding) affecting the honojs web application framework prior to version 4.12.4. The root cause is an inconsistency in how URL encoding is handled between the router and the serveStatic middleware. Specifically, the router uses decodeURI, which does not decode encoded slashes (%2F), while serveStatic uses decodeURIComponent, which does decode these characters. This mismatch allows an attacker to craft URLs with encoded slashes that bypass route-based middleware protections (e.g., app.use('/admin/*', ...)) intended to restrict access to certain static resources. When such a crafted URL is processed, the router does not recognize the encoded slash as a path separator, thus allowing the request to pass middleware checks, but serveStatic decodes it and serves the protected file. This leads to unauthorized access to static resources that should be protected. The vulnerability is remotely exploitable without authentication or user interaction, making it a critical risk for confidentiality breaches. The issue was addressed and patched in hono version 4.12.4 by ensuring consistent decoding behavior between routing and static file serving components.

The primary impact of CVE-2026-29045 is unauthorized disclosure of sensitive static resources that are intended to be protected by route-based middleware. This can lead to leakage of confidential information such as configuration files, private documents, or other sensitive assets hosted on the web server. Since the vulnerability does not affect data integrity or availability, the main concern is confidentiality compromise. Exploitation requires no authentication or user interaction and can be performed remotely, increasing the risk of widespread attacks. Organizations relying on honojs for web applications, especially those serving sensitive static content behind middleware protections, face significant risk of data exposure. This can result in regulatory compliance violations, reputational damage, and potential downstream attacks leveraging leaked information.

The definitive mitigation is to upgrade all honojs instances to version 4.12.4 or later, where the inconsistent URL decoding issue is fixed. Until upgrade is possible, organizations should implement strict input validation and URL normalization to reject or sanitize requests containing encoded slashes (%2F) or other suspicious encodings that could bypass middleware protections. Additionally, consider deploying Web Application Firewalls (WAFs) with custom rules to detect and block attempts to exploit this decoding inconsistency. Review and audit route-based middleware configurations to ensure they do not rely solely on path matching that can be circumvented by encoding tricks. Logging and monitoring access to sensitive static resources should be enhanced to detect anomalous access patterns. Finally, educate developers and security teams about the risks of inconsistent URL decoding and encourage secure coding practices in middleware and routing logic.