CVE-2026-39991 in Notes Station
Summary
by MITRE • 10/15/2026
A cross-site scripting (XSS) vulnerability has been reported to affect Notes Station 3. If a remote attacker gains a user account, they can then exploit the vulnerability to inject malicious code.
We have already fixed the vulnerability in the following version: Notes Station 3 3.9.12 and later
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Analysis
by VulDB Data Team • 10/15/2026
The identified security flaw resides within Synology's Notes Station application, specifically affecting versions prior to 3.9.12. This vulnerability is classified as a cross-site scripting issue, which fundamentally compromises the integrity of web-based interactions by allowing malicious scripts to execute in the context of a victim's browser session. The root cause typically involves insufficient sanitization or validation of user-supplied input within the application's interface, permitting an attacker to inject executable code that bypasses standard security controls such as the same-origin policy. This type of flaw is formally categorized under CWE-79, which addresses Improper Neutralization of Input During Web Page Generation, a critical category in web application security assessments due to its prevalence and potential for severe impact.
The operational prerequisites for exploiting this vulnerability require that an attacker first obtains valid credentials or access to a user account within the Notes Station environment. This distinguishes it from purely remote unauthenticated attacks, placing it higher on the attack complexity scale but significantly lowering the barrier for exploitation once initial access is achieved. Once authenticated, the adversary can craft specific requests containing malicious JavaScript payloads targeting areas where input data is reflected back into the HTML output without proper encoding. When another user views the compromised content or interacts with the affected feature, their browser executes the injected script as if it originated from the trusted Notes Station domain. This capability aligns with ATT&CK technique T1059, specifically sub-technique 007 for JavaScript execution, which is frequently leveraged in phishing and web-based attack campaigns to steal session cookies, capture keystrokes, or perform actions on behalf of the victim without their knowledge.
The impact of a successful exploitation extends beyond simple defacement or alert boxes. Attackers can utilize this vector to conduct cross-site request forgery attacks by leveraging the victim's active authentication tokens to modify data within the application. They may also exfiltrate sensitive information stored in local storage or cookies, leading to potential account takeover and unauthorized access to private notes and documents hosted on the NAS device. In enterprise environments where Notes Station is integrated with other services such as Active Directory or cloud sync solutions, this breach can serve as a pivot point for lateral movement if session tokens are reused across different internal applications. The confidentiality of user data is severely compromised, and the integrity of the application's state management systems is undermined by unauthorized script execution.
To mitigate this risk, administrators must ensure that all instances of Notes Station 3 running on Synology NAS devices are updated to version 3.9.12 or later, where the vulnerability has been resolved through improved input validation and output encoding mechanisms. Beyond immediate patching, organizations should enforce strict access controls by limiting administrative privileges to essential personnel only and implementing multi-factor authentication for all user accounts accessing the application. Network-level segmentation can further reduce exposure by restricting access to management interfaces from untrusted networks. Regular security audits and vulnerability scans should be conducted to identify any lingering misconfigurations or outdated components that could serve as entry points for similar exploitation attempts, ensuring a defense-in-depth strategy against web-based threats.