CVE-2025-51691 is a Cross-Site Scripting (XSS) vulnerability identified in the MarkTwo application, specifically in the commit e3a1d3f90cce4ea9c26efcbbf3a1cbfb9dcdb298 from May 2025. The vulnerability arises because the application fails to properly sanitize user-supplied Markdown content before rendering it in the editor interface. This improper sanitization allows a remote attacker to inject crafted script inputs that execute arbitrary client-side code within the context of the victim's browser session. Exploitation of this vulnerability can lead to session hijacking, credential theft, or other malicious actions performed on behalf of the victim user. The vulnerability is triggered remotely without requiring authentication or user interaction beyond viewing or interacting with the maliciously crafted Markdown content. No CVSS score has been assigned yet, and no known exploits are reported in the wild at the time of publication. The vulnerability affects unspecified versions of MarkTwo, and no patches or mitigation links have been published yet. Given the nature of the vulnerability, it targets the client-side security boundary and leverages the trust users place in the MarkTwo editor interface to execute malicious scripts.

For European organizations using MarkTwo, this vulnerability poses a significant risk to the confidentiality and integrity of user sessions and data. Successful exploitation could allow attackers to steal session cookies, leading to unauthorized access to sensitive information or internal systems. Credential theft could facilitate further lateral movement within an organization’s network or enable phishing campaigns leveraging compromised accounts. The arbitrary code execution in the browser context could also be used to deploy additional malware or manipulate user interactions, potentially disrupting business operations. Organizations in sectors with high regulatory requirements for data protection, such as finance, healthcare, and government, could face compliance violations and reputational damage if exploited. Moreover, since the vulnerability is client-side and can be triggered remotely, it increases the attack surface, especially for organizations with remote or hybrid workforces relying on web-based collaboration tools like MarkTwo.

To mitigate this vulnerability, organizations should first monitor for official patches or updates from the MarkTwo development team and apply them promptly once available. In the absence of patches, organizations can implement Content Security Policy (CSP) headers to restrict the execution of unauthorized scripts within the MarkTwo application context. Additionally, web application firewalls (WAFs) can be configured to detect and block suspicious Markdown inputs containing script tags or event handlers. User education is important to raise awareness about the risks of interacting with untrusted Markdown content. Network segmentation and limiting access to the MarkTwo editor interface to trusted users can reduce exposure. Security teams should also conduct regular security assessments and penetration testing focusing on client-side injection vulnerabilities. Finally, monitoring user sessions for anomalous activity can help detect exploitation attempts early.