JetBrains has patched a critical flaw in TeamCity, the on-premises server many software teams use to build, test and ship their code. The bug lets an attacker who has never logged in run operating system commands on that server, which in practice means reach into source code, stored credentials and the pipeline that pushes software into production.
The issue is tracked as CVE-2026-63077 and carries a CVSS severity score of 9.8 out of 10. JetBrains published its advisory on July 27, 2026, and Rapid7 followed with an Emergent Threat Response analysis in the original report.
What is affected
Every version of TeamCity On-Premises is vulnerable, according to JetBrains. TeamCity Cloud customers do not need to take any action.
The flaw sits in the agent polling protocol, the channel TeamCity build agents use to check in with the server for work. It is a deserialization of untrusted data issue, meaning the server accepts attacker supplied data and rebuilds it into live objects before checking whether it should. That is enough to slip past the authentication checks and reach command execution.
How bad the outcome is depends on the privileges the TeamCity server process runs with. JetBrains said in its own write-up that attackers who exploit the bug can read stored credentials and compromise the integrity of the CI/CD pipeline. Those credentials are usually the crown jewels of a build environment: cloud keys, code signing secrets, package registry tokens and repository access.
Is it being exploited
JetBrains said it was not aware of active exploitation at the time of disclosure. That is a snapshot rather than a guarantee. An internet facing build server with an unauthenticated remote code execution bug is exactly the kind of target that attracts opportunistic scanning quickly, and TeamCity authentication flaws have drawn fast attacker interest in previous years.
What you should do
Update TeamCity On-Premises now, either through the update workflow in the TeamCity UI or by installing a fixed build: TeamCity 2025.11.7 or TeamCity 2026.1.3.
Organizations that cannot upgrade immediately can apply the JetBrains security patch plugin, which works on TeamCity 2017.1 and later. JetBrains notes the plugin addresses only this vulnerability and does not carry other security updates, so treat it as a stopgap rather than a substitute for upgrading.
Rapid7 also recommends restricting network access to TeamCity servers so that only the users and systems that genuinely need them can connect. Build servers rarely need to face the open internet, and putting one behind a VPN or an access proxy removes most of the exposure on its own.
If an unpatched instance was reachable from the internet, treat it as potentially compromised rather than merely vulnerable. Review build agent connections, look for build configurations or scripts nobody remembers adding, and rotate the credentials the server holds if there is any doubt. It is the same lesson as recent CI/CD attacks, including the GitHub Actions flaw abused to plant npm backdoors and the growing habit of ransomware crews partnering with supply chain intrusion specialists. The build system is a production system, and attackers already treat it that way.
This briefing is provided by IntelFusions for informational and defensive purposes only. It is based on sources assessed to be reliable at the time of writing, and analytic judgments carry the confidence levels indicated. Indicators of compromise are defanged; re-arm them only in controlled environments. IntelFusions is not affiliated with the organizations named and makes no warranty as to completeness or accuracy.