The National Cyber Security Centre (NCSC) in the Netherlands has issued an urgent update on a series of sophisticated cyberattacks exploiting a zero-day vulnerability in Citrix NetScaler systems, identified as CVE-2025-6543.
This flaw, affecting Citrix NetScaler Application Delivery Controller (ADC) and Gateway products, has enabled threat actors to compromise multiple critical organizations since at least early May 2025.
According to the NCSC’s ongoing investigation, the attackers employed advanced techniques, including active trace erasure to obscure their activities, making forensic analysis particularly challenging.
Target Critical Dutch Infrastructure
The vulnerability was publicly disclosed and patched by Citrix on June 25, 2025, but exploitation predated this, classifying it as a zero-day.
Despite the patch, the NCSC emphasizes that updating systems alone is insufficient to mitigate risks, as attackers may retain persistent access through mechanisms like malicious web shells, which provide remote control over compromised devices.
Citrix NetScaler serves as a critical network appliance for load balancing, secure remote access, and application delivery, often facilitating remote work by enabling employees to connect to corporate intranets and cloud environments.
The exploited vulnerability, CVE-2025-6543, allows unauthorized remote code execution, potentially leading to the deployment of web shells that grant attackers persistent backdoor access.
The NCSC’s research has uncovered evidence of these web shells on affected systems, with attackers deliberately wiping logs and other indicators to evade detection.
This has resulted in significant uncertainty regarding the full scope of compromises, including which organizations remain infiltrated and the extent of data exfiltration or further lateral movement within networks.
In addition to CVE-2025-6543, related vulnerabilities CVE-2025-5349 and CVE-2025-5777 have been identified in vulnerable Citrix deployments across the Netherlands and internationally, though not all exposed systems have been confirmed as exploited.
The NCSC has proactively reached out to potentially affected parties and urges all organizations using Citrix products to conduct thorough internal investigations, even if patches have been applied.
Ongoing Investigations
The timeline of events underscores the protracted nature of this campaign: initial exploitations traced back to May 2025, with detections escalating through June and July, and continued monitoring into August.
According to the report, The NCSC’s collaborative efforts with incident response teams, affected organizations, and security partners have yielded new indicators of compromise (IOCs), which are being shared to aid in identifying infections.
However, the agency cautions that the attacker’s trace-erasure tactics mean some aspects of the incident such as the complete list of victims, ongoing actor activity, and total impact may never be fully resolved.
This uncertainty highlights the challenges in attributing the attacks, though the methods suggest involvement of one or more highly capable threat actors, possibly state-sponsored or advanced persistent threats (APTs) focused on espionage or disruption.
To bolster defenses, the NCSC strongly recommends adopting a defense-in-depth strategy, incorporating layered security controls such as network segmentation, multi-factor authentication, continuous monitoring for anomalous behavior, and regular forensic audits.
Organizations discovering IOCs related to this campaign should perform detailed compromise assessments and contact the NCSC’s CERT team for assistance.
This incident serves as a stark reminder of the evolving threat landscape, where zero-day exploits in widely used infrastructure like Citrix NetScaler can lead to widespread breaches.
By prioritizing resilience measures, organizations can better withstand not only this specific threat but also future vulnerabilities in similar remote access and application delivery systems.
The NCSC continues its investigations, emphasizing information sharing to enhance collective cybersecurity posture amid these persistent risks.
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