Government security agencies in the UK, US, Canada, Australia and New Zealand have urged Cisco customers to take immediate action to patch a critical zero-day bug in their SD-WAN kit that has been exploited since 2023. CVE-2026-20127 is an authentication bypass vulnerability in the peering authentication in Cisco Catalyst SD-WAN Controller (formerly SD-WAN vSmart) and Cisco Catalyst SD-WAN Manager (formerly SD-WAN vManage). According to Cisco, it could allow an unauthenticated, remote attacker to bypass authentication and obtain administrative privileges on an affected system. It has a maximum CVSS score of 10.0. “This vulnerability exists because the peering authentication mechanism in an affected system is not working properly. An attacker could exploit this vulnerability by sending crafted requests to an affected system,” the Cisco advisory noted. “A successful exploit could allow the attacker to log in to an affected Cisco Catalyst SD-WAN Controller as an internal, high-privileged, non -root user account. Using this account, the attacker could access NETCONF, which would then allow the attacker to manipulate network configuration for the SD-WAN fabric.” Read more on zero-day exploits: Zero-Day Exploits Surge, Nearly 30% of Flaws Attacked Before Disclosure. According to a detailed “Threat Hunt” guide issued by several Five Eyes security agencies, the sophisticated threat actors likely downgraded target systems to an older version, in order to exploit legacy local privilege escalation vulnerability CVE-2022-20775. They then restored back to the original software version, in order to gain root access. Customers are urged to patch both the legacy 2022 bug and the new zero-day vulnerability for which Cisco released a fix yesterday. The US Cybersecurity and Infrastructure Security Agency (CISA) issued an emergency directive requiring all federal agencies to find and patch the vulnerabilities by 5pm ET on February 27, 2026. “Based on collaboration with international partners and CISA’s forensic analysis, the ease with which these vulnerabilities can be exploited demands immediate action from all federal agencies,” said CISA acting director, Madhu Gottumukkala. “We urge all entities to implement the measures outlined in this emergency directive without delay.” Immediate Actions for Securing Vulnerable SD‑WAN Deployment The Cisco Catalyst SD-WAN product streamlines and secures connectivity between branch offices, datacenters and the cloud. According to the UK’s National Cyber Security Centre (NCSC), threat actors “are compromising SD-WANs to add a malicious rogue peer and then conduct a range of follow-on actions to achieve root access and maintain persistent access to the SD-WAN.” By adding a malicious device in this way, they could silently penetrate a target’s network infrastructure. The NCSC urged affected organizations to work through the following steps, in order: Perform threat hunting for evidence of compromise detailed in the Hunt Guide If compromise looks likely, collect artefacts from the device and report it to the NCSC (for UK organizations) Update to the latest version of Cisco Catalyst SD-WAN Manager and Cisco Catalyst SD-WAN Controller as detailed in their respective advisories Apply the Cisco Catalyst SD-WAN Hardening Guide Perform continuous threat hunting activities The hardening guide lists steps covering network perimeter controls, SD-WAN manager access, control and data plane security, session timeout and logging.