TeamCity’s agent polling trust model is the break. An attacker with only HTTP(S) access can slip past authentication and run OS commands as the TeamCity server process, so the real exposure is the control plane, stored credentials, and server state, not just the login page.
JetBrains assigned CVE-2026-63077 a CVSS score of 9.8 and patched TeamCity On-Premises in 2025.11.7 and 2026.1.3. It also shipped a security patch plugin for versions 2017.1+ for teams that cannot upgrade yet; TeamCity Cloud was already updated.
A successful compromise can expose TeamCity data, configurations, and credentials, or let an attacker alter server state. Internet-facing CI/CD operators should treat the agent channel as part of the trust boundary, not a benign backend path.