CVE-2026-59795: JetBrains TeamCity Stored XSS in Unauthenticated Agent Registration (CVSS 8.1)

Technical summary

A stored XSS vulnerability exists in TeamCity's agent registration endpoint, which does not require authentication. The vulnerability allows injection of arbitrary HTML and JavaScript into the agent registration parameters. These malicious inputs are stored in the TeamCity database and reflected to authenticated users viewing agent details or registration logs, enabling execution of attacker-controlled scripts in the context of the TeamCity application. The attack vector is network-based with low complexity; exploitation requires user interaction (a logged-in administrator or operator viewing the poisoned agent data).

Business impact

Compromise of TeamCity instances can severely impact CI/CD pipelines, which are critical to software delivery. An attacker exploiting this vulnerability could inject malicious code into builds, exfiltrate secrets stored in TeamCity (API keys, credentials), compromise build artifacts delivered to customers, or pivot to development infrastructure. For organizations using TeamCity to manage enterprise deployments, this creates risk of supply chain contamination and operational disruption.

Affected systems

JetBrains TeamCity versions before 2026.1.2 are affected. Organizations running any version of TeamCity on or before 2026.1.1 should assume they are vulnerable. Both on-premises and cloud-hosted TeamCity deployments are at risk. The vulnerability is accessible from the network without authentication, making it exploitable from the internet if TeamCity is exposed.

Exploitability

Exploitability is moderate to high in practical scenarios. While the vulnerability requires unauthenticated access to the agent registration endpoint and user interaction (an authenticated user must view the malicious agent registration), the agent registration process is typically accessible and routine in TeamCity environments. An attacker can craft a registration request with XSS payload and wait for administrators to inspect agents, making this a practical attack path in many deployments. No special tooling or advanced techniques are required.

Remediation

Upgrade JetBrains TeamCity to version 2026.1.2 or later. This version contains fixes that sanitize agent registration inputs and prevent stored XSS. Organizations unable to patch immediately should restrict network access to TeamCity's agent registration endpoint (typically port 8111) to trusted IP ranges, disable agent auto-registration if not required, and monitor agent registration logs for suspicious entries containing script tags or HTML entities.

Patch guidance

Apply the JetBrains TeamCity 2026.1.2 update or later as soon as feasible. Verify compatibility with your current deployment topology and test in a staging environment before production rollout. Review release notes for any breaking changes or configuration updates. JetBrains typically provides upgrade documentation and may offer migration tools for large deployments. If you operate a disconnected or airgapped TeamCity instance, obtain the patch through JetBrains' offline channels.

Detection guidance

Search TeamCity agent registration logs and database for entries containing HTML tags, JavaScript event handlers (onclick, onerror, onload), or URL-encoded script sequences (%3Cscript, %22, %27). Monitor for unusual agent names or descriptions containing special characters or encoded payloads. Implement web application firewalls (WAF) rules to block requests to the agent registration endpoint containing suspicious patterns. Log and alert on failed agent registration attempts or registrations from unexpected source IPs. Review TeamCity audit logs for users viewing agent details shortly after unusual registrations.

Why prioritize this

This vulnerability merits urgent patching due to its CVSS score of 8.1 (HIGH), the critical nature of CI/CD infrastructure, and the low barrier to exploitation. TeamCity is a foundational tool in most software development organizations, making it a high-value target. Stored XSS in this context can compromise build pipelines and software supply chain, extending impact far beyond the TeamCity instance itself. The unauthenticated attack vector and lack of exploitation complexity amplify risk.

Risk score, explained

The CVSS 3.1 score of 8.1 reflects a HIGH severity due to: (1) Network attack vector (AV:N) — exploitable remotely without special access; (2) Low attack complexity (AC:L) — no special conditions required; (3) No authentication required (PR:N) — attacker does not need credentials; (4) User interaction required (UI:R) — an authenticated user must view the malicious payload; (5) Confidentiality and integrity impact (C:H, I:H) — attacker can read sensitive data and modify application state; (6) No availability impact (A:N) — the attack does not cause denial of service. The score appropriately captures the severity for a stored XSS in a privileged application, though note that this vulnerability is not yet tracked in the CISA KEV catalog.

Frequently asked questions