Dutch NCSC Confirms Active Exploitation of Citrix NetScaler CVE-2025
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2025-5777 | Out-of-Bounds Read (Memory Overread) in Citrix NetScaler ADC and Gateway Citrix NetScaler ADC and NetScaler Gateway contain an out-of-bounds read (CWE-125) caused by insufficient input validation, which can cause the appliance to read beyond the intended memory buffer (a memory overread). The flaw is only triggerable when the appliance is configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, or RDP Proxy) or as an AAA virtual server, so attackers who can reach those services can potentially induce the overread and obtain sensitive memory contents. Such disclosure could aid follow-on compromise, for example by exposing session or authentication data, and CISA notes known ransomware use. Organizations running NetScaler ADC or NetScaler Gateway in the affected Gateway/AAA configurations are exposed. The vulnerability was added to CISA's Known Exploited Vulnerabilities catalog on 2025-07-10 with known ransomware use and an EPSS of 100% (100th percentile), indicating active exploitation, while no public PoC is known and a CVSS score has not yet been assigned. Do: Apply the fixed NetScaler ADC/Gateway builds per Citrix's security advisory (exact affected/fixed version ranges are not in the available data, so consult the bulletin); per CISA KEV, apply vendor mitigations, follow BOD 22-01 guidance for cloud services, or discontinue use if mitigations are unavailable. Inventory appliances for Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) or AAA virtual server configurations, since unconfigured/other deployments are not triggerable. After patching, terminate active and idle VPN sessions and hunt for anomalous access, given the known ransomware exploitation and the information-disclosure nature of the flaw. | 9.3 | 100% | KEV ransomware |
| massplausibly hundreds of thousands of installed/internet-exposed NetScaler ADC and Gateway appliances (public scans have historically shown on the order of… | |
| CVE-2025-6543 | Memory Buffer Overflow in Citrix NetScaler ADC and Gateway Exploited in the Wild Citrix NetScaler ADC and NetScaler Gateway appliances contain a memory buffer overflow (CWE-119) that can lead to unintended control flow and denial of service. The flaw is only reachable when the appliance is configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, or RDP Proxy) or as an AAA virtual server, and it is network-exploitable without authentication or user interaction, though attack complexity is rated high. A successful attacker could achieve unintended control flow — with the CVSS 4.0 vector rating impact high across confidentiality, integrity, and availability — or crash the appliance, disrupting VPN access and application delivery. Any organization running NetScaler ADC or NetScaler Gateway in an affected Gateway/AAA configuration is exposed, a population that public scan data places in the tens of thousands of internet-exposed devices. The vulnerability was added to CISA's KEV catalog on 2025-06-30, confirming exploitation in the wild, with EPSS at 10.1% and no public proof-of-concept known. Do: Apply the patched NetScaler release specified in Citrix's security bulletin for CVE-2025-6543 immediately, prioritizing appliances in Gateway or AAA configurations, per CISA KEV and BOD 22-01 requirements. Audit which virtual servers (VPN, ICA Proxy, CVPN, RDP Proxy, AAA) are in use and whether they are internet-exposed, and check appliances for signs of compromise before and after upgrading. | 9.2 | 10% | KEV |
| large≈50,000+ internet-exposed NetScaler ADC/Gateway devices (only Gateway/AAA configurations vulnerable) |
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Ravie LakshmananAug 12, 2025Vulnerability / Threat Intelligence
The Dutch National Cyber Security Centre (NCSC-NL) has warned of cyber attacks exploiting a recently disclosed critical security flaw impacting Citrix NetScaler ADC products to breach organizations in the country.
The NCSC-NL said it discovered the exploitation of CVE-2025-6543 targeting several critical organizations within the Netherlands, and that investigations are ongoing to determine the extent of the impact.
CVE-2025-6543 (CVSS score: 9.2) is a critical security vulnerability in NetScaler ADC that results in unintended control flow and denial-of-service (DoS) when the devices are configured as a Gateway (VPN virtual server, ICA Proxy, CVPN, RDP Proxy) OR AAA virtual server.
The vulnerability was first disclosed in late June 2025, with patches released in the following versions -
- NetScaler ADC and NetScaler Gateway 14.1 prior to 14.1-47.46
- NetScaler ADC and NetScaler Gateway 13.1 prior to 13.1-59.19
- NetScaler ADC 13.1-FIPS and NDcPP prior to 13.1-37.236-FIPS and NDcPP
As of June 30, 2025, CVE-2025-6543 has been added to the U.S. Cybersecurity and Infrastructure Security Agency's (CISA) Known Exploited Vulnerabilities (KEV) catalog. Another flaw in the same product (CVE-2025-5777, CVSS score: 9.3) was also placed on the list last month.
NCSC-NL described the activity as likely the work of a sophisticated threat actor, adding the vulnerability has been exploited as a zero-day since early May 2025 – almost two months before it was publicly disclosed – and the attackers took steps to erase traces in an effort to conceal the compromise. The exploitation was discovered on July 16, 2025.
"During the investigation, malicious web shells were found on Citrix devices," the agency said. "A web shell is a piece of rogue code that gives an attacker remote access to the system. The attacker can place a web shell by abusing a vulnerability."
To mitigate the risk arising from CVE-2025-6543, organizations are advised to apply the latest updates, and terminate permanent and active sessions by running the following commands -
- kill icaconnection -all
- kill pcoipConnection -all
- kill aaa session -all
- kill rdp connection -all
- clear lb persistentSessions
Organizations can also run a shell script made available by NCSC-NL to hunt for indicators of compromise associated with the exploitation of CVE-2025-6543.
"Files with a different .php extension in Citrix NetScaler system folders may be an indication of abuse," NCSC-NL said. "Check for newly created accounts on the NetScaler, and specifically for accounts with increased rights."
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