# Critical TeamCity Authentication Bypass Lets Unauthenticated Attackers Execute OS Commands
## The Threat
JetBrains has disclosed a critical vulnerability in TeamCity On-Premises that allows an unauthenticated attacker to execute arbitrary operating system commands on any exposed server. The flaw, CVE-2026-63077, carries a CVSS score of 9.8 — about as high as it gets — and the attack path requires nothing more than HTTP or HTTPS access to the target host. No credentials, no social engineering, no insider foothold.
The mechanism is the TeamCity agent polling protocol, the communication channel build agents use to check in with the server for queued jobs. An attacker can abuse that protocol to sidestep authentication checks entirely, smuggling OS commands in through a path the server treats as trusted internal traffic. Once that works, the attacker runs commands as the TeamCity server process itself.
What that means in practice depends on how tightly the TeamCity installation is locked down — and in a lot of shops, the answer is "not very." TeamCity routinely stores VCS credentials, deployment keys, API tokens, and signing certificates. Compromising the process means reading or modifying server-side configurations, exfiltrating everything the build system has access to, and potentially pivoting into connected infrastructure. This is CI/CD compromise, which historically cascades fast.
## Severity and Impact
| Field | Detail |
|---|---|
| CVE | CVE-2026-63077 |
| CVSS Score | 9.8 (Critical) |
| Attack Vector | Network |
| Attack Complexity | Low |
| Authentication Required | None |
| User Interaction | None |
| Privileges Required | None |
| CWE | CWE-306 (Missing Authentication for Critical Function) |
| Impact | Arbitrary OS command execution as TeamCity server process |
No exploitation in the wild has been confirmed as of the advisory date, but the CVSS vector and attack description — unauthenticated, low complexity, network-accessible — place this firmly in the category of vulnerabilities that get weaponized within days of public disclosure.
## Affected Products
JetBrains TeamCity On-Premises
Not affected:
## Mitigations
Primary: Update immediately
If you cannot update right now:
Network-level controls (apply regardless of patch status):
Post-patch hygiene:
The vulnerability was discovered by Antoni Tremblay and reported to JetBrains on July 10, 2026.
## References
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## HackWire Analysis
TeamCity has been here before. CVE-2024-27198 was another authentication bypass in early 2024 — CVSS 9.8, unauthenticated, network-reachable — and it was exploited in the wild within roughly 24 hours of the PoC dropping. Nation-state actors and ransomware crews both made a run at it. The attack profile of CVE-2026-63077 is nearly identical: no auth, low complexity, network access, critical score. If the 2024 incident is any guide, organizations have a very short window.
What makes CI/CD compromise particularly damaging is the blast radius. A TeamCity server isn't just a box — it's a key that opens most of the other boxes. Build pipelines routinely carry cloud provider credentials, container registry tokens, SSH keys for production deployments, and code signing certificates. An attacker who lands on that server can poison artifacts quietly, exfiltrate secrets without touching the applications those secrets protect, or use the build network as a pivot point into otherwise segmented environments. None of those actions necessarily set off alerts, because CI/CD systems are supposed to pull keys and push binaries.
The advice to put TeamCity behind a VPN is correct but undersells the urgency. A lot of organizations deployed TeamCity internet-facing during the remote-work buildout and never walked it back. If you're in that category, treat this as a stop-everything situation. Patch first, rotate secrets second, then do the network hardening. In that order — because if you're already compromised, hardening the perimeter after the fact just locks an attacker inside.
— HackWire Editorial
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