CVE-2025-5777: Citrix NetScaler Gateway Memory Overread Vulnerability

On July 22, 2025, the National Vulnerability Database (NVD) updated details for CVE-2025-5777, a critical memory overread vulnerability in Citrix NetScaler ADC and Gateway appliances, confirming active exploitation and adding it to CISA's Known Exploited Vulnerabilities (KEV) catalog. Dubbed "CitrixBleed 2" due to similarities with the 2023 CitrixBleed flaw (CVE-2023-4966), this issue arises from insufficient input validation in HTTP POST requests to authentication endpoints, allowing unauthenticated attackers to disclose sensitive memory contents. Affecting configurations as Gateway (VPN, ICA Proxy, CVPN, RDP Proxy) or AAA virtual servers, the vulnerability has a CVSS v4.0 score of 9.3 and enables session hijacking, credential theft, or further compromises. With proof-of-concept exploits publicly available and exploitation detected since mid-June 2025, urgent patching is essential. Below, we explore the flaw, attack methods, impacts, and mitigation strategies.

The Vulnerability: Insufficient Input Validation in HTTP Handling

CVE-2025-5777 is an out-of-bounds read vulnerability caused by improper handling of user-supplied input in the NetScaler's authentication endpoints. When configured as a Gateway or AAA virtual server, the appliance fails to validate inputs adequately, leading to disclosure of uninitialized memory regions containing sensitive data like session tokens, passwords, or cryptographic keys. The flaw affects multiple versions of NetScaler ADC and Gateway, though exact ranges are detailed in Citrix's advisory CTX693420.Classified under CWE-125 (Out-of-bounds Read) by Citrix and CWE-908 (Use of Uninitialized Resource) by NIST, the vulnerability was published on June 17, 2025, with NVD updates on July 14 and exploitation details confirmed by July 22. Citrix assigned a CVSS v4.0 vector of AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:L/SI:L/SA:L, scoring 9.3, while earlier assessments rated it as medium (6.5) under CVSS v3.1—though the higher impact on confidentiality, integrity, and availability elevates its severity. Proof-of-concept exploits, including scanners and memory disclosure tools, are available on GitHub, facilitating rapid replication by attackers.

Attack Vectors and Impacts

The primary attack vector is unauthenticated remote access via crafted HTTP POST requests to the NetScaler's login or authentication pages. Attackers can send malformed data to trigger the overread, leaking memory contents without credentials. Exploitation has been observed since mid-June 2025, with sensors like GreyNoise and ReliaQuest detecting activity, and CISA mandating federal agencies to patch by July 11, 2025.Impacts include:

• Sensitive Data Exposure: Leaked memory may reveal session cookies, enabling attackers to hijack active sessions and bypass authentication.
• Privilege Escalation and Persistence: Stolen credentials could allow deeper network access, lateral movement, or persistent backdoors.
• System Instability: Overreads might cause crashes or DoS, disrupting VPN, proxy, or authentication services.

This vulnerability affects organizations relying on NetScaler for secure remote access, potentially leading to data breaches or operational disruptions if unpatched.

Why Update ASAP?

As a pre-authentication flaw with public exploits and confirmed in-the-wild activity, CVE-2025-5777 poses an immediate threat. CISA's KEV inclusion underscores its urgency, with agencies given just one day to patch due to active exploitation risks. Delaying updates could expose sensitive data, enabling attackers to compromise networks undetected, as seen in similar past Citrix vulnerabilities. Citrix has released patches via CTX693420, and applying them is critical to prevent session hijacking or further exploits.

Tips for Security Hygiene

Beyond patching, enhance NetScaler security with these practices:

• Apply Patches Immediately: Upgrade to fixed versions per Citrix advisory CTX693420; test in staging to avoid post-patch login issues.
• Restrict Exposure: Avoid exposing management interfaces to the internet; use VPNs or firewalls for access.
• Monitor for Anomalies: Enable logging and use tools like intrusion detection systems (IDS) to watch for unusual HTTP traffic or memory leaks.
• Enable Enhanced Protection: Configure multi-factor authentication (MFA) and least-privilege access for virtual servers.
• Scan for Vulnerabilities: Use scanners like those in PoCs to test your deployment; follow CISA guidance for KEV entries.

By prioritizing these steps, organizations can mitigate CVE-2025-5777 and similar threats. For the latest details, check NVD, Citrix advisories, or vulnerability databases like Vulmon.