Threat actors are actively exploiting a critical vulnerability to access TeamCity On-Premises servers, the Australian Cyber Security Centre (ACSC) has warned.
The flaw, CVE 2026-63077, can allow unauthenticated attackers with HTTP(S) access to a TeamCity server to bypass authentication checks and execute arbitrary operating system commands. It affects all TeamCity On-Premises versions.
The ACSC said it does not have evidence to indicate that a specific industry or sector is being targeted, but all Australian organizations that utilize the TeamCity On-Premises server are at risk of compromise.
The agency urged TeamCity customers to urgently review networks for use of vulnerable versions of the TeamCity On-Premises server and apply patches if necessary.
It also advised organizations to consider whether they need to have their TeamCity interface exposed to the internet.
TeamCity is a Continuous Integration and Continuous Deployment (CI/CD) server used by thousands of organizations across the world. It automates the processes of building, testing, and deploying software on a single system.
TeamCity Flaw a Popular Target for Attackers
The vulnerability, which has a critical CVSS score of 9.8, was first disclosed by TeamCity’s owner, software development giant JetBrains, in July 2026 when patches were issued.
CVE 2026-63077 was added to the US Cybersecurity and Infrastructure Agency (CISA)’s Known Exploited Vulnerabilities (KEV) Catalog on August 5, due to evidence of active exploitation.
“This type of vulnerability is a frequent attack vector for malicious cyber actors and poses significant risks to the federal enterprise,” CISA warned.
Two days later, JetBrains issued a follow-up advisory on CVE 2026-63077 as it had received reports of active exploitation, as well as attempted exploitation, targeting unpatched TeamCity servers.
The firm said customers who have not yet updated to TeamCity 2025.11.7 or 2026.1.3, or installed the security patch plugin, should do so immediately.
In 2024, it was reported that two vulnerabilities affecting TeamCity On-Premises software were being extensively exploited by attackers. The most severe of these flaws allowed for a complete compromise of a vulnerable TeamCity server by a remote unauthenticated attacker.
Another critical vulnerability disclosed in 2023 affecting the software was found to have been targeted by Russian and North Korean nation-state actors.