Introduction
JetBrains recently patched a flaw in TeamCity On Premises that lets authenticated users expose internal server API endpoints to unauthorized parties. It has a CVSS score of 8.2. For organizations that run their CI/CD pipelines on TeamCity, this means a low-privilege account, or even a guest account, can be used to bypass authentication on critical API surfaces.
TeamCity is JetBrains' Continuous Integration and Deployment server. It automates building, testing, and releasing software, and enterprises and development teams around the world use it as the central orchestration point for software delivery. CI/CD servers hold privileged access to source code, deployment environments, and infrastructure credentials, so a vulnerability in one of these platforms carries significant supply-chain risk.
Technical Information
CVE-2026-44413 falls under CWE-306: Missing Authentication for Critical Function. Certain internal TeamCity server API endpoints do not properly enforce authentication. When an authenticated user, even one with minimal privileges, performs actions on the server, parts of the internal API become accessible to unauthenticated parties under specific network conditions.
Prerequisites for Exploitation
Exploitation requires both of these conditions at the same time:
-
An authenticated session: The attacker needs access to a TeamCity account. A standard user account works, and so does the guest user account if guest access is enabled on the server. Guest access is a common configuration in development environments where teams want to give people read access to build results without requiring individual logins.
-
A permissive network environment: At least one of these must be true:
- The firewall permits inbound connections on ports other than the standard HTTP or HTTPS ports used by TeamCity.
- Build agents run on the same host as the TeamCity server.
Attack Flow
Based on the available disclosures, exploitation works like this:
- The attacker authenticates to the TeamCity server with any valid account, including a guest account if that feature is enabled.
- From that session, the attacker triggers actions that expose parts of the internal server API.
- The exposed endpoints lack proper authentication checks (the CWE-306 root cause), so unauthenticated users can reach them.
- Where non-standard ports are open or build agents share the server host, the exposed API is reachable over the network, which allows unauthorized access to server functionality.
Risk Assessment by Deployment Scenario
Severity depends heavily on how TeamCity is deployed:
| Deployment Scenario | Exploitation Preconditions | Relative Risk Level |
|---|---|---|
| Internet-facing server | Public access enabled with guest account active | High |
| Internal server with custom ports | Authenticated network access and non-standard inbound ports permitted | Medium |
| Co-hosted build agents | Authenticated user access and agents running on the server host | Medium |
JetBrains has not publicly disclosed which API endpoints become exposed. The current disclosures also include no public proof-of-concept code and no detailed CVSS vector string. Organizations should assume that any exposed API endpoint could lead to sensitive data disclosure or pipeline manipulation.
TeamCity Cloud environments are not affected and require no action.
Patch Information
JetBrains made two fixes for CVE-2026-44413 available on May 11, 2026: a new release and a standalone security patch plugin.
Primary Fix: TeamCity 2026.1
The full remediation ships in TeamCity On Premises version 2026.1. The NVD description confirms the vulnerability existed "before 2026.1" and also before the maintenance release 2025.11.5, which indicates that JetBrains back-ported the fix to the previous release branch as well. Because the weakness is CWE-306, the patch adds authentication enforcement to the previously exposed internal server API. Unauthenticated parties can no longer reach those endpoints, even when an authenticated user's actions would otherwise have exposed them.
Administrators can get the fixed version from the JetBrains download page or through TeamCity's built-in automatic update mechanism.
Back-port Fix: Security Patch Plugin for TeamCity 2017.1+
For environments that cannot upgrade to 2026.1 right away, JetBrains released a standalone security patch plugin at the same time. It works with every TeamCity On Premises version from 2017.1 onward and is available as a direct download:
https://download.jetbrains.com/teamcity/plugins/internal/fix_CVE_2026_44413.zip
The plugin fixes CVE-2026-44413 and nothing else. There are two ways to get it:
- Manual download and install: Download the ZIP above and install it through the TeamCity web UI (Administration, then Plugins).
- Automatic notification (TeamCity 2024.03+): These instances download available security patches automatically and list them under Administration | Updates, then Available security updates, where administrators can review and apply them.
The install behavior depends on the server version. Installations running TeamCity 2017.1 through 2018.1 need a full server restart after the plugin is installed. From TeamCity 2018.2 onward, the plugin can be enabled at runtime with no downtime.
The JetBrains advisory calls the plugin a targeted stop-gap. Upgrading to 2026.1 is still the recommended path, because the latest release includes the full set of security hardening.
Interim Network Controls
If immediate patching is not feasible, organizations should put these controls in place:
- Temporarily restrict external access to servers that are publicly accessible over the internet.
- Require connections through a VPN, or add another security layer that prevents unauthorized access to the login screen or REST API.
- Allow inbound network access only on required ports.
- Run TeamCity servers on dedicated hosts, separate from build agents.
Affected Systems and Versions
The vulnerability affects:
- JetBrains TeamCity On Premises: All versions before 2026.1
- JetBrains TeamCity On Premises: All versions in the 2025.11.x branch before 2025.11.5
In practice, every TeamCity On Premises installation from the earliest versions through 2025.11.4 is vulnerable.
Not affected: TeamCity Cloud instances are not affected and require no action.
Vulnerable configurations:
- Servers with guest access enabled
- Servers where firewalls permit inbound connections on non-standard ports (beyond the configured HTTP/HTTPS ports)
- Deployments where build agents run on the same host as the TeamCity server
Vendor Security History
TeamCity has drawn security scrutiny before. In March and April 2024, CISA added two TeamCity vulnerabilities, CVE-2024-27198 and CVE-2024-27199, to its Known Exploited Vulnerabilities (KEV) catalog after confirmed active exploitation in the wild. Given that history of threat actor interest in TeamCity infrastructure, prompt patching of CVE-2026-44413 matters more than usual.
Security researcher Martin Orem reported the vulnerability privately to JetBrains on April 30, 2026. JetBrains responded quickly and released patches and the dedicated security plugin about 11 days after the initial report.
