TeamCity CVE-2026-63077: Agent Protocol Is the Vector

Two days after JetBrains published the advisory for CVE-2026-63077, Rapid7 has released an extended technical report that names the specific component carrying the flaw: the TeamCity agent polling protocol.

The vulnerability class is deserialization of untrusted data. According to Rapid7’s ETR, an unauthenticated attacker with HTTP(S) access to a TeamCity server can send a crafted request via the agent polling protocol, bypass authentication checks entirely, and execute operating system commands as the TeamCity service account.

No credentials required. Just network access to the port.

Why the agent polling protocol matters

TeamCity agents poll the server continuously over HTTP(S) to pick up build jobs. That communication channel is, by design, reachable from anywhere build agents operate — which in a lot of environments means it crosses network boundaries you don’t fully control. If you’ve been thinking about this CVE in terms of “internet-exposed vs. not,” the honest answer is more granular: anything that can reach the TeamCity port is a potential attacker.

For defenders evaluating interim controls while patching is scheduled: restricting access to the TeamCity port to known build agent CIDRs narrows the attack surface. It’s containment, not a fix, and it doesn’t replace the upgrade.

Current status

No active exploitation confirmed as of this writing. CVE-2026-63077 has not been added to the CISA Known Exploited Vulnerabilities catalog. The CVSS score is 9.8 critical — unauthenticated, remote, no interaction required.

That combination — deserialization, unauthenticated, widely-deployed CI infrastructure — doesn’t stay unexecuted indefinitely. The window for patching before exploitation shows up in the wild is real and finite.

What to do

1. Update now: 2025.11.7 (LTS branch) or 2026.1.3 (current branch). Both are available on the JetBrains download page.
2. If patching is delayed: block the TeamCity port from everything except known build agent CIDRs. Understand that’s a temporary measure.
3. TeamCity Cloud: already mitigated. No action required.

Full attack surface analysis is in Rapid7’s ETR. JetBrains’ advisory remains the authoritative remediation reference.

Patch this first. Then check what else is on your CI infrastructure.