Pakistan’s national cyber-incident response team, the Pakistan Computer Emergency Response Team (PKCERT), has issued an urgent security advisory regarding a significant vulnerability in Microsoft Windows Server Update Services (WSUS), a crucial software employed by many organizations for managing updates across their networks.
WSUS serves as the backbone for large entities, including government agencies and major corporations, to manage, distribute, and install crucial updates for Windows servers. This vulnerability arises from the unsafe deserialization of the WSUS Authorisation Cookie, which permits attackers to send a maliciously modified cookie to the server, tricking it into executing their code instead of rejecting the flawed input.
The exploit leads to remote code execution (RCE) capabilities on compromised systems. An intruder can remotely run their malicious software or commands on the vulnerable server from any location, potentially resulting in a complete takeover of the server. Notably, an attacker exploiting this vulnerability requires no authentication, such as a username or password, making the threat particularly severe. PKCERT has indicated that the flaw is being actively exploited in live attacks.
The process of serialization involves a web application converting complex data—like session information—into a streamlined format for easier transmission and storage. The risk arises when an application improperly trusts the deserialized data, leading to what is termed as "unsafe deserialization." If an attacker alters that data and the server does not validate it before deserialization, malicious code can be injected into the server.
In this situation, the WSUS Authorisation Cookie, which is used by WSUS to verify users connecting to the server and their permissions, does not undergo adequate validation before deserialization. This vulnerability poses a serious risk as a compromised WSUS server could push infected updates to numerous connected machines, spreading malware or ransomware across networks, stealing sensitive information, and potentially allowing attackers full control over all connected devices.
PKCERT has rated this vulnerability a critical threat, assigning it a score of 9.8 on the Common Vulnerability Scoring System. Organizations running outdated Windows systems, especially those accessible on the public internet, are at risk.
To counter this vulnerability, PKCERT has recommended several corrective measures, including the deployment of Microsoft’s October 2025 out-of-band patch, temporarily disabling affected Internet ports, and enhancing server security measures to prevent WSUS servers from being publicly accessible.
Organizations are urged to remain vigilant against suspicious cyber activity and to monitor for unauthorized server access to safeguard their systems effectively.
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