CVE-2026-29085 is a vulnerability classified under CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component) affecting the honojs web application framework before version 4.12.4. The issue lies in the streamSSE() function of the Streaming Helper, which handles Server-Sent Events (SSE). Specifically, the event, id, and retry fields are not properly validated against carriage return (\r) or newline (\n) characters. Since the SSE protocol uses line breaks as delimiters between fields, an attacker can inject additional SSE fields within the same event frame by including these special characters in untrusted input. This injection can manipulate the event stream, potentially causing downstream components or clients to process malicious or altered data. The vulnerability can be exploited remotely without authentication or user interaction, increasing its risk profile. The flaw was patched in hono version 4.12.4 by adding proper validation to these fields to reject or sanitize carriage return and newline characters. No known exploits have been reported in the wild, but the vulnerability’s CVSS score of 6.5 reflects a medium severity due to its potential impact on confidentiality and integrity without affecting availability.

The primary impact of CVE-2026-29085 is on the integrity and confidentiality of data transmitted via Server-Sent Events in applications using the vulnerable honojs versions. An attacker who can supply untrusted input to the event, id, or retry fields can inject additional SSE fields, potentially manipulating event streams to deliver malicious payloads or disrupt expected client behavior. This can lead to unauthorized data disclosure or corruption of real-time event data, undermining trust in the application’s communication channel. Since SSE is often used for real-time notifications, updates, or streaming data, exploitation could affect user experience and application correctness. The vulnerability does not impact availability directly but could be leveraged as part of a broader attack chain. Organizations relying on honojs for critical real-time web services may face reputational damage and operational risks if exploited.

To mitigate CVE-2026-29085, organizations should upgrade all honojs deployments to version 4.12.4 or later, where the vulnerability is patched. Until upgrades are applied, developers should implement strict input validation and sanitization on all data passed into the event, id, and retry fields of the streamSSE() function, explicitly disallowing carriage return (\r) and newline (\n) characters. Employing a whitelist approach for allowed characters in these fields can reduce injection risk. Additionally, monitoring and logging SSE traffic for anomalous or unexpected field values can help detect exploitation attempts. Security teams should review application code to ensure no other SSE-related injection vectors exist and consider applying web application firewalls (WAFs) with custom rules to block suspicious SSE payloads. Regularly auditing dependencies and staying current with honojs releases will reduce exposure to similar vulnerabilities.