A critical vulnerability in Cisco AnyConnect VPN software has emerged as a significant threat to enterprise network security, affecting organizations that rely on Meraki MX and Z Series devices for remote access connectivity. The security flaw, designated as CVE-2025-20271, carries a CVSS score of 8.6, reflecting its high severity and potential impact on business operations across multiple sectors.
The vulnerability originates from improper initialization of variables during SSL VPN session setup, allowing attackers to send specially crafted HTTPS requests that exploit the configuration weakness. Unauthenticated attackers can remotely crash the VPN service by targeting devices where client certificate authentication is implemented, severing all active connections instantly and forcing users to re-authenticate after service restart.
Attackers exploit SSL VPN initialization flaws through malicious HTTPS requests, instantly crashing services and disconnecting all authenticated users.
Affected hardware includes numerous Cisco Meraki MX models, spanning from the MX64 to the MX450, in addition to the virtual MX platform.
Z Series Teleworker devices, including Z3, Z3C, Z4, and Z4C models, face similar exposure when running vulnerable firmware versions. Devices require Meraki MX firmware 16.2 or later with AnyConnect VPN activated, while MX64 and MX65 models particularly need firmware 17.6 or higher for VPN functionality.
The attack vector involves malformed network requests that target SSL VPN session establishment, causing immediate denial-of-service until automatic service recovery occurs.
Although the vulnerability does not allow for persistent code execution or data compromise, the availability impact proves critical for distributed workforces requiring continuous remote access. All active VPN sessions terminate upon exploitation, disrupting business continuity and preventing staff connectivity during attack periods.
Organizations face heightened operational risks when sustained attacks prevent both new and recovering VPN sessions from establishing connections. The vulnerability’s capability to be triggered without authentication in certain configurations significantly expands the potential attack surface. Enterprise security teams must implement continuous monitoring to detect exploitation attempts and prepare for potential service disruptions.
Cisco has acknowledged the security issue and released advisories detailing affected versions, though MX400 and MX600 models remain vulnerable because of their end-of-life status. Cisco currently provides no workarounds to mitigate this vulnerability while organizations await firmware updates. Repeated exploitation can create persistent service disruptions, making this vulnerability particularly concerning for enterprises relying heavily on secure remote access infrastructure.
