CVE-2026-63077: JetBrains TeamCity On-Premises unauthenticated RCE through the agent-polling protocol, every on-prem version affected (critical)
JetBrains patches an unauthenticated remote-code-execution flaw reachable on every TeamCity On-Premises version ever shipped
Defender actions
- Upgrade TeamCity On-Premises to 2025.11.7 or 2026.1.3 on the matching branch, or install JetBrains' security-patch plugin where the version is older than that but at least 2017.1, and until that is done restrict external access to the TeamCity server, as JetBrains advises, because the flaw needs nothing but HTTP(S) reachability.
- Treat any TeamCity On-Premises server that was internet-reachable and still unpatched at any point after 2026-07-27 as a compromise-assessment target rather than a completed patch: review build-agent registrations for agents you did not enrol, and rotate the VCS credentials, artifact-repository tokens and signing keys the server holds, because an upgrade evicts the entry point but not what was taken through it.
Analysis
JetBrains' advisory is short on mechanism by design and unambiguous on reach: an unauthenticated attacker with HTTP(S) access to a TeamCity On-Premises server can bypass authentication checks and run arbitrary operating-system commands with the privileges of the TeamCity server process, and every On-Premises version ever shipped is affected (JetBrains, 2026-07-27). The reachable surface is the TeamCity agent polling protocol. JetBrains' own framing is that exploitation of the flaw requires no authentication and that the attacker bypasses authentication checks by way of that protocol, so there is no credential, session, or user interaction standing between a network-reachable server and command execution (JetBrains, 2026-07-27). The flaw was reported privately on 2026-07-10 by Antoni Tremblay, and JetBrains stated at publication that it was not aware of any active exploitation (JetBrains, 2026-07-27); on 2026-08-07 it reported "active exploitation, as well as attempted exploitation, targeting unpatched TeamCity servers" (JetBrains, 2026-08-07). CISA's catalog classifies it as CWE-502, deserialization of untrusted data, reachable through the agent polling protocol (CISA KEV catalog, 2026-08-05).
The reason this warrants moving ahead of the ordinary patch queue is what a build server is, not the score. JetBrains says a successful attack could expose TeamCity data, configurations and stored credentials and compromise the integrity of build artifacts and downstream CI/CD pipelines (JetBrains, 2026-07-27), so command execution as the server process converts into a durable ability to alter what future builds produce. At disclosure no public exploitation had been observed, and that clock ran out within nine days (see the updates below). JetBrains published no technical detail beyond the agent polling protocol, but "all versions affected" plus "no authentication" plus a widely deployed, easily fingerprinted product is a combination that, in analyst judgement, tends to close quickly once someone reconstructs the path.
Triage: on a server that already runs a fixed version or the security-patch plugin, com.thoughtworks.xstream.security.ForbiddenClassException in the log marks an exploitation attempt that was blocked, not a compromise (JetBrains, 2026-08-07). The agent-polling endpoint legitimately receives continuous check-in traffic from your build fleet, so request volume to it discriminates nothing on its own. Two things do. The source address set is finite and knowable: your registered agents are an inventory, so polling traffic from anything outside it is the anomaly. And on the server host, legitimate command execution belongs to agent processes running build steps; the server process itself spawning an interpreter or a network utility is not a quieter version of normal activity but a different thing entirely.
Cited evidence
A critical security vulnerability has been identified in TeamCity On-Premises. If exploited, this flaw may enable an unauthenticated attacker with HTTP(S) access to a TeamCity server to bypass authentication checks and execute arbitrary operating system commands with the privileges of the TeamCity server process.
This unauthenticated remote code execution vulnerability was reported to us privately on July 10, 2026, by Antoni Tremblay in accordance with our coordinated disclosure policy.
At the time of publishing this advisory, we are not aware of any active exploitation of this vulnerability.
based on evidence of active exploitation
Since our initial announcement on July 27, 2026, we have received reports of active exploitation, as well as attempted exploitation, targeting unpatched TeamCity servers.
Updates5
CISA added CVE-2026-63077 to its Known Exploited Vulnerabilities catalog on 2026-08-05, stating it did so based on evidence of active exploitation (CISA, 2026-08-05). Until this listing the flaw was patched with no confirmed exploitation, which was also JetBrains' own position: its advisory states it was not aware of any active exploitation of the vulnerability at the time of publishing, and as of 2026-08-06 that advisory had not been revised since 2026-07-27 (JetBrains, 2026-07-27). As of 2026-08-06 neither CISA's alert nor its catalog entry named an exploiting cluster, a victim set, or the observed intrusion path, so the confirmed fact then was exploitation itself and nothing beyond it; the ransomware use and the Cadence breach came later (2026-09-30 update).
Nothing about the underlying flaw has changed. JetBrains describes it as letting an unauthenticated attacker with HTTP(S) access to a TeamCity server bypass authentication checks and execute arbitrary operating-system commands with the privileges of the TeamCity server process, affecting every On-Premises version ever shipped, with TeamCity Cloud already mitigated so that its customers need take no action (JetBrains, 2026-07-27). CISA's catalog entry names the flaw a deserialization of untrusted data vulnerability (CISA, 2026-08-05); the vendor's own advisory describes the impact without using that term. What changes is the response owed by anyone who was slow to patch. A build server sits upstream of source code, artifact signing and deployment credentials, so the consequence of a week of exposure is not bounded by the server itself. The federal remediation deadline attached to the KEV listing is a US compliance date and carries no operational meaning for Swiss organisations; the exploitation confirmation is what does.
CISA's Known Exploited Vulnerabilities catalog now marks CVE-2026-63077 as used in ransomware campaigns (CISA KEV catalog, 2026-09-29). JetBrains has separately confirmed that its own Cadence service was compromised through this flaw between 2026-08-08 and 2026-08-24 because the server was never patched against the advisory (JetBrains, 2026-08-28). JetBrains itself reported active and attempted exploitation of unpatched servers on 2026-08-07 and named server-log indicators for checking a server (JetBrains, 2026-08-07).
JetBrains did not say it withheld a gadget chain: its advisory describes exploitation only "via the TeamCity agent polling protocol" and publishes no further technical detail (JetBrains, 2026-07-27). The deserialization class comes from CISA's catalog record, and the exploitation status rests on two independent statements, JetBrains' follow-up and CISA's listing.
JetBrains' blog advisory rates CVE-2026-63077 critical without a number (JetBrains, 2026-07-27), but the CVE record JetBrains published as numbering authority on 2026-07-27 scores it CVSS 3.1 9.8, for a network attack that needs no privileges and no user interaction (ENISA EUVD, 2026-07-27). JetBrains reports exploitation of unpatched servers since its 2026-07-27 announcement (JetBrains, 2026-08-07), so the compromise-assessment window starts on that date rather than at CISA's 2026-08-05 listing, and includes servers patched between the two. Until a fix is applied, JetBrains strongly recommends temporarily restricting external access to the server; as a separate, longer-term measure it says internet-facing servers should also consider requiring connections through a VPN or an additional security layer (JetBrains, 2026-08-07); it does not describe the agent-polling endpoint as something to close off separately. The advisory has carried a pointer to that follow-up since 2026-08-07 (JetBrains, 2026-07-27).
On TeamCity 2024.03 and newer, the server downloads available security patch plugins automatically and notifies administrators if notifications are configured; the pending patch for CVE-2026-63077 is applied from Administration | Updates, under Available security updates (JetBrains, 2026-07-27).
Sources6
Revision history
- Published 2026-07-29T0408Z-intel
- Update 2026-08-06T0411Z-intel
CISA added CVE-2026-63077 to its Known Exploited Vulnerabilities catalog on 2026-08-05 based on evidence of active exploitation, changing the status of the unauthenticated JetBrains TeamCity On-Premises remote-code- execution flaw covered here on 2026-07-29 from patch-available to confirmed exploited. JetBrains' advisory, unchanged since 2026-07-27, still records that it was not aware of any active exploitation at publication. No authority has named an exploiting cluster or the observed intrusion path. Because every On-Premises version ever shipped is affected and the flaw needs only HTTP(S) reachability, any TeamCity server that was internet-reachable and unpatched before 2026-08-05 now warrants a compromise assessment rather than only an upgrade.
Changed: actions cves evidence sources tags body
- Update 2026-09-30T0639Z-audit
CISA's Known Exploited Vulnerabilities catalog now marks CVE-2026-63077 as used in ransomware campaigns. Ransomware operators are now exploiting the unauthenticated TeamCity On-Premises RCE, so any server that was reachable and unpatched needs the compromise assessment, not only the upgrade. The summary and main text no longer say exploitation is unobserved. CWE-502 now cites CISA's KEV record, and a numeric score that no citable source publishes is removed. JetBrains' 2026-08-07 confirmation of active exploitation and its log indicators lead the detection and triage lines, an Exposure line gives the fixed versions, TeamCity Cloud is described as already mitigated rather than unaffected, the sourcing note describes only cited sources, and two citation dates follow the pages.
Changed: summary sourcing_note tags sources references body title cves evidence actions
- Correction 2026-10-11T1307Z-audit
The verification value moves to multi-source (JetBrains' exploitation follow-up and CISA's catalog listing are independent statements), and two sentences are reworded so they describe what JetBrains published rather than claim it withheld a gadget chain, and do not refer to the entry itself. The main analysis gains the current Defender takeaway (patch, then treat exposed unpatched servers as incident cases because of the recorded ransomware use), and the sentence on what a server holds now follows JetBrains' own wording. The sourcing note no longer says JetBrains withholds details.
Changed: verification body sourcing_note
- Correction 2026-10-20T0835Z-audit
JetBrains, as numbering authority, scores CVE-2026-63077 CVSS 3.1 9.8 in the CVE record, although its blog advisory gives no number. Because JetBrains reports exploitation since its 2026-07-27 announcement, the compromise-assessment window now starts on that date, and the interim mitigation follows JetBrains: restrict external access to the whole server rather than to one endpoint.
Changed: actions body cves sources sourcing_note
- Improvement 2026-10-21T0800Z-audit
On TeamCity 2024.03 and newer the security patch plugin for CVE-2026-63077 is downloaded automatically but still has to be applied from Administration | Updates, under Available security updates; until a fix is in place, JetBrains strongly recommends temporarily restricting external access to internet-facing servers.
Changed: body
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