CVE-2024-27198 is an authentication bypass (CWE-288) in JetBrains TeamCity, JetBrains' self-hosted on-premises continuous integration/continuous delivery (CI/CD) server, that allows an unauthenticated attacker to circumvent login and perform administrative actions. It is triggered by sending specially crafted requests to the TeamCity web interface that dodge the authentication check, after which the attacker gains full administrator access. With admin access, an attacker can create administrative accounts, alter build configurations, plant code that runs on the server and its build agents, and reach source code, credentials, and secrets stored in the CI/CD environment, which ransomware operators have already used as an entry point. Any TeamCity deployment an attacker can reach over the network is affected, with internet-facing instances at the highest risk. Exploitation is confirmed in the wild: CISA added the flaw to the KEV catalog on 2024-03-07 with known ransomware use, and EPSS estimates a 99.9% probability of exploitation within 30 days.
What to do: Upgrade immediately to the patched TeamCity release per JetBrains' advisory (2023.11.4 or later fixes this vulnerability), or apply vendor mitigations or discontinue use per CISA guidance if patching is not possible. Audit instances for unexpected administrator accounts, modified build configurations, and other signs of compromise, since ransomware operators are known to be actively exploiting this bug. Restrict TeamCity's network exposure to trusted users and networks until the update is applied.
Affected
JetBrains TeamCity
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Estimated exposure
largetens of thousands of internet-exposed TeamCity servers (roughly 20,000–30,000 per public scans at disclosure) — Public internet-wide scans around the March 2024 disclosure identified on the order of 20,000+ TeamCity servers reachable from the internet, and total deployments are higher because many instances run only on internal networks.
Order-of-magnitude estimate by the model from install counts, market share and public scan data it knows; verify before quoting.
Description
In JetBrains TeamCity before 2023.11.4 authentication bypass allowing to perform admin actions was possible
CISA Known Exploited Vulnerability
Affected
JetBrains TeamCity
Required action
Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.
Microsoft reports China-linked ransomware group Storm-1175 switched from Medusa to a new C++ strain, StormEncryptor, likely exploiting N-able flaw CVE-2026-18577.
Microsoft Threat Intelligence reports that the financially motivated, China-linked group Storm-1175 began deploying a new ransomware strain called StormEncryptor on August 2, 2026, replacing its previous Medusa ransomware. StormEncryptor is written in C++, appends the .encrypted extension to files, and drops a !!!README_FIRST!!!.txt ransom note in each scanned directory. Microsoft assesses the group is likely exploiting CVE-2026-18577, an authentication bypass in N-able disclosed on August 2, 2026 and added to CISA's Known Exploited Vulnerabilities catalog the next day. Since 2023, Storm-1175 has exploited more than 16 vulnerabilities in products including Microsoft Exchange, Ivanti, ConnectWise ScreenConnect, JetBrains TeamCity, SimpleHelp, CrushFTP, and GoAnywhere MFT, often moving from initial access to data theft and ransomware deployment within days.