A newly patched security flaw impacting on-premise versions of JetBrains TeamCity has come under active exploitation in the wild, according to the U.S. Cybersecurity and Infrastructure Security Agency (CISA).
The vulnerability in question is CVE-2026-63077 (CVSS score: 9.8), a case of deserialization of untrusted data that could allow an unauthenticated attacker with access to a TeamCity server to bypass authentication checks and execute arbitrary operating system commands with the privileges of the TeamCity server process.
"JetBrains TeamCity contains a deserialization of untrusted data vulnerability that could allow unauthenticated remote code execution via the agent polling protocol," CISA said.
According to JetBrains, the vulnerability can be exploited by an unauthenticated attacker via the TeamCity agent polling protocol to sidestep authentication checks and execute arbitrary operating system commands.
The exact impact varies depending on the privileges granted to the TeamCity server process. A successful attack can expose TeamCity data, configurations, and stored credentials, modify server state, and potentially compromise the integrity of build artifacts and downstream CI/CD pipelines, per JetBrains.
It's currently not known how the vulnerability is being exploited in the wild, the identity of the threat actors behind the attacks, and the scale of such efforts. JetBrains has yet to update its advisory to confirm active exploitation.
In light of the latest development, users running on-premise versions are recommended to apply the updates as soon as possible. Per Binding Operational Directive (BOD) 26-04, Federal Civilian Executive Branch (FCEB) agencies are required to prioritize patching high-risk vulnerabilities listed in the Known Exploited Vulnerabilities (KEV) catalog.
The deadline by which federal agencies must apply software patches or mitigations for CVE-2026-63077 is August 8, 2026.
Update
In a follow-up analysis published on August 7, 2026, Rapid7 noted that a TeamCity vulnerable to the flaw creates a permissive XStream allowlist, which limits the Java classes that can be deserialized when servicing unauthenticated agent requests.
"However, this allowlist incorrectly adds TeamCity protocol classes without removing XStream's existing default permissions," security researcher Stephen Fewer said. "This introduces an unsafe deserialization issue. A patched TeamCity server remediates this by adding NoTypePermission.NONE before the TeamCity allowlist, which removes the default permissions and makes the allowlist exclusive."
The cybersecurity company has also released a proof-of-concept (PoC) Python script for CVE-2026-63077 that organizations can use to validate their detection and remediation posture. The PoC works by writing a .JSPWS file, a customized JavaServer Pages (JSP) extension, to execute an arbitrary command and then deleting the file from disk.
(The story was updated after publication on August 10, 2026, to include additional details of the flaw published by Rapid7.)
