CVE-2026-25529 is a cross-site scripting (XSS) vulnerability identified in Postal, an open source SMTP server widely used for managing email delivery. The vulnerability exists in Postal versions earlier than 3.3.5 and arises from improper neutralization of input during web page generation (CWE-79). Specifically, the API's send/raw method allows unescaped data to be injected into the admin interface's HTML. This flaw permits an attacker with access to the API to inject arbitrary HTML or JavaScript code into the admin UI. Because the injected scripts run in the context of the admin interface, they can manipulate the page content misleadingly or execute unauthorized JavaScript, potentially leading to session hijacking, credential theft, or unauthorized actions within the admin panel. The vulnerability requires low privileges (API access) but no user interaction, making it easier to exploit remotely. The flaw does not affect availability but severely impacts confidentiality and integrity. The issue was publicly disclosed on March 12, 2026, with a CVSS v3.1 base score of 8.1, indicating high severity. The vulnerability has been fixed in Postal version 3.3.5 and later. No known exploits have been reported in the wild yet, but the ease of exploitation and high impact make it a critical patching priority for affected organizations.

The vulnerability allows attackers to execute arbitrary JavaScript in the context of the Postal admin interface, compromising the confidentiality and integrity of the system. Attackers could steal admin credentials, hijack sessions, alter email configurations, or manipulate the admin UI to mislead administrators. This could lead to unauthorized email sending, interception of sensitive email content, or further lateral movement within the organization’s infrastructure. Since Postal is often used in email infrastructure, exploitation could disrupt trust in email communications or enable phishing campaigns originating from compromised servers. The lack of required user interaction and the low privilege needed to exploit the vulnerability increase the risk of automated or targeted attacks. Organizations relying on Postal for email delivery and management face significant risks until patched, especially those with exposed or poorly secured API endpoints.

1. Upgrade Postal to version 3.3.5 or later immediately to apply the official fix. 2. Restrict API access to trusted and authenticated users only, employing strong authentication and network segmentation to limit exposure. 3. Implement Web Application Firewall (WAF) rules to detect and block suspicious HTML or JavaScript payloads targeting the send/raw API endpoint. 4. Conduct regular security audits and penetration testing focused on web interface input validation and API security. 5. Monitor logs for unusual API usage patterns or unexpected HTML content injections. 6. Educate administrators on recognizing signs of UI manipulation or unexpected behavior in the admin interface. 7. If upgrading immediately is not possible, consider temporarily disabling or restricting the send/raw API method to reduce attack surface. 8. Employ Content Security Policy (CSP) headers to limit the execution of unauthorized scripts in the admin interface.