JetBrains issued a warning on March 4, 2024 about two serious vulnerabilities in TeamCity server. The flaws can be used by a remote, unauthenticated attacker with HTTP(S) access to a TeamCity on-premises server to bypass authentication checks and gain administrative control of the TeamCity server.

TeamCity is a build management and continuous integration and deployment server from JetBrains that allows developers to commit code changes into a shared repository several times a day. Each commit is followed by an automated build to ensure that the new changes integrate well into the existing code base and as such can be used to detect problems early.

Compromising a TeamCity server allows an attacker full control over all TeamCity projects, builds, agents and artifacts. Which, depending on the use-case of your projects, could make for a suitable attack vector leading to a supply chain attack.

The two vulnerabilities are CVE-2024-27198, an authentication bypass vulnerability with a CVSS score of 9.8, and CVE-2024-27199, a path traversal issue with a CVSS score of 7.3. The Cybersecurity and Infrastructure Security Agency (CISA) has added CVE-2024-27198 to its Known Exploited Vulnerabilities Catalog, based on evidence of active exploitation. This means that Federal Civilian Executive Branch (FCEB) agencies need to remediate this vulnerability by March 28, 2024 in order to protect their devices against active threats.

These two vulnerabilities allow an attacker to create new administrator accounts on the TeamCity server which have full control over all TeamCity projects, builds, agents and artifacts.

Exploitation code is readily available online and has already been integrated in offensive security tools like the MetaSploit framework.

So, it doesn’t come as a surprise that researchers are now reporting abuse of the vulnerabilities.

Bleeping Computer reports that attackers have already compromised more than 1,440 instances, while a scan for vulnerable instances by Shadowserver showed that the US and Germany are the most affected countries.

The vulnerabilities affect all TeamCity on-premises versions through 2023.11.3 and were fixed in version 2023.11.4. Customers of TeamCity Cloud have already had their servers patched, and according to JetBrains they weren’t attacked.

To update your server, download the latest version (2023.11.4) or use the automatic update option within TeamCity. 

JetBrains has also made a security patch plugin available for customers who are unable to upgrade to version 2023.11.4. There are two security patch plugins, one for TeamCity 2018.2 and newer and one for TeamCity 2018.1 and older. See the TeamCity plugin installation instructions for information on installing the plugin.

If your server is publicly accessible over the internet, and you are unable to immediately mitigate the issue you should probably make your server inaccessible until you can.

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