Microsoft has disclosed a significant vulnerability in its Windows Remote Desktop Gateway (RD Gateway) that could allow attackers to exploit a race condition, resulting in a denial-of-service (DoS) attack.
The flaw, identified as CVE-2025-21225, was addressed in the company’s January 2025 Patch Tuesday update.
A race condition vulnerability occurs when the behavior of a system depends on the timing or sequence of events in concurrent operations, and attackers exploit this lack of synchronization.
In the context of CVE-2025-21225, the Windows Remote Desktop Gateway (RD Gateway) denial-of-service vulnerability, the race condition arises during the processing of network requests by the RD Gateway service.
Windows Remote Desktop Gateway Vulnerability
The vulnerability arises from a type confusion issue, classified under CWE-843: Access of Resource Using Incompatible Type. This flaw allows attackers to exploit the RD Gateway component, which is bound to the network stack, making it remotely exploitable over the internet. By successfully triggering the race condition, attackers can disrupt the availability of the RD Gateway service.
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While existing connections remain unaffected, new connections can be blocked, potentially rendering the service unusable after repeated exploitation.
This type of denial-of-service attack poses a serious risk to organizations relying on RD Gateway for secure remote access. Although the vulnerability does not enable data theft or remote code execution, the impact on system availability is significant.
The vulnerability affects multiple versions of Windows Server, including:
- Windows Server 2016 (Core and Standard installations)
- Windows Server 2019 (Core and Standard installations)
- Windows Server 2022 (Core and Standard installations)
- Windows Server 2025 (Core and Standard installations)
Each affected version has received a security update with unique identifiers. For instance:
- Windows Server 2019: Update KB5050008 (Build 10.0.17763.6775)
- Windows Server 2022: Update KB5049983 (Build 10.0.20348.3091)
- Windows Server 2025: Update KB5050009 (Build 10.0.26100.2894)
Exploiting this vulnerability requires an attacker to win a race condition—a challenging but not impossible task for skilled threat actors. The flaw has been rated as “Important” due to its potential to disrupt critical services but does not currently have a publicly available exploit code.
As of January 15, 2025, no reports or evidence are indicating that CVE-2025-21225, the Windows Remote Desktop Gateway (RD Gateway) denial-of-service vulnerability, has been actively exploited in the wild. Additionally, no proof-of-concept (PoC) exploits or public exploit tools for this vulnerability have been disclosed.
Mitigation and Recommendations
Microsoft has released patches as part of its January 2025 security updates to address this vulnerability. Organizations are strongly advised to apply these updates immediately to mitigate the risk of exploitation.
In addition:
- Ensure robust network monitoring to detect unusual activity targeting RD Gateway services.
- Limit exposure of RD Gateway to only trusted networks through firewall rules.
- Consider implementing additional layers of security, such as VPNs or multi-factor authentication, for remote access.
The January 2025 Patch Tuesday update addressed 159 vulnerabilities across Microsoft’s ecosystem, including eight zero-day flaws and several critical remote code execution vulnerabilities.
While CVE-2025-21225 is not classified as critical, its potential impact on service availability highlights the importance of proactive patch management and system hardening.
As cyber threats continue to evolve, organizations must remain vigilant in applying security updates and monitoring their systems for signs of compromise.
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