CVE-2026-29086 identifies a vulnerability in the honojs hono web application framework, specifically in versions prior to 4.12.4. The vulnerability arises from the setCookie() utility function, which is responsible for constructing the Set-Cookie HTTP header used to manage cookies in web browsers. The issue is due to insufficient validation of special characters—specifically semicolons (;), carriage returns (\r), and newline characters (\n)—in the domain and path options passed to setCookie(). Since cookie attributes in HTTP headers are delimited by semicolons, an attacker who can supply untrusted input to these fields could inject additional cookie attributes. This injection could manipulate cookie behavior, potentially leading to session fixation, cookie poisoning, or other attacks that compromise confidentiality and integrity of user sessions. The vulnerability does not affect availability and does not require authentication but does require user interaction, such as visiting a maliciously crafted web page. The vulnerability is classified under CWE-1113 (Inappropriate Comment Style), which here relates to improper input validation in comment or delimiter handling within cookie attributes. The issue was publicly disclosed and patched in hono version 4.12.4. No known exploits have been reported in the wild to date, but the vulnerability's medium CVSS score of 5.4 reflects moderate risk due to ease of exploitation and potential impact on confidentiality and integrity.

The primary impact of this vulnerability is on the confidentiality and integrity of web session cookies. By injecting additional cookie attributes, an attacker could manipulate cookie behavior, potentially bypassing security controls such as HttpOnly or Secure flags, or altering cookie scope and expiration. This can lead to session hijacking, fixation, or cross-site scripting (XSS) attack facilitation. For organizations, this could result in unauthorized access to user accounts, data leakage, and erosion of user trust. While the vulnerability does not directly affect system availability, the indirect consequences of compromised sessions could disrupt business operations and lead to regulatory compliance issues, especially in sectors handling sensitive personal or financial data. Since honojs is a framework supporting any JavaScript runtime, the scope includes a broad range of web applications globally, increasing the potential attack surface. Organizations relying on vulnerable versions may face targeted attacks exploiting this flaw, particularly in environments where user input is not properly sanitized before being passed to setCookie().

The most effective mitigation is to upgrade the honojs hono framework to version 4.12.4 or later, where the vulnerability has been patched. Organizations should audit their codebases and dependencies to identify usage of the setCookie() utility and ensure that untrusted input is never directly passed to cookie domain or path attributes without proper sanitization. Implement strict input validation and encoding on all user-supplied data used in cookie construction. Employ Content Security Policy (CSP) headers to reduce the impact of potential injection attacks. Monitor web application logs for suspicious cookie header manipulations or anomalies. Additionally, consider implementing web application firewalls (WAFs) with rules to detect and block malformed Set-Cookie headers containing unexpected delimiters or control characters. Educate developers on secure cookie handling practices and the risks of improper input validation in HTTP headers. Finally, conduct regular security testing, including fuzzing and penetration testing, to detect similar injection vulnerabilities in web application components.