CVE-2026-44211 Overview
CVE-2026-44211 is a cross-origin WebSocket hijack vulnerability affecting Cline Kanban servers in versions 2.13.0 and prior. Cline is an autonomous coding agent distributed as a software development kit (SDK), an integrated development environment (IDE) extension, and a command-line interface (CLI) assistant. The flaw stems from missing authentication on the WebSocket endpoint [CWE-306], allowing a malicious web origin to establish a connection to a victim's local Kanban server. At the time of publication, no patches are publicly available.
Critical Impact
An attacker who lures a developer to a malicious web page can hijack the WebSocket channel of the local Cline Kanban server and issue commands to the autonomous coding agent, leading to confidentiality, integrity, and availability loss.
Affected Products
- Cline (cline:cline) versions 2.13.0 and prior
- Cline IDE extension deployments running the Kanban server
- Cline CLI assistant deployments exposing the Kanban WebSocket
Discovery Timeline
- 2026-06-01 - CVE-2026-44211 published to the National Vulnerability Database (NVD)
- 2026-06-03 - Last updated in NVD database
Technical Details for CVE-2026-44211
Vulnerability Analysis
The vulnerability resides in the WebSocket interface exposed by the Cline Kanban server component. The server accepts WebSocket connections without validating the Origin header or requiring an authentication token. Browsers do not apply the Same-Origin Policy to WebSocket handshakes the same way they apply it to XMLHttpRequest or Fetch requests, so any web page can open a WebSocket to ws://localhost endpoints running on the developer's machine.
Once the connection is established, the attacker's JavaScript can send and receive messages on the channel. Because Cline operates as an autonomous coding agent with access to source repositories and the local environment, control of the Kanban WebSocket exposes agent operations to the attacker. The issue is tracked under [CWE-306: Missing Authentication for Critical Function].
Root Cause
The Cline Kanban server binds a WebSocket endpoint on the local host without enforcing origin validation or session authentication. Trust is implicitly granted to any client able to reach the listening port, which includes browser contexts loaded from arbitrary external origins.
Attack Vector
Exploitation requires user interaction. A developer running a vulnerable Cline version must visit an attacker-controlled web page while the Kanban server is active. The malicious page opens a cross-origin WebSocket to the local Cline endpoint and issues commands through the established channel. The attack is network-reachable through the victim's browser and does not require prior credentials. See the GitHub Security Advisory GHSA-5c57-rqjx-35g2 for technical details.
// No verified proof-of-concept code is available at time of publication.
// Refer to the vendor advisory for technical details.
Detection Methods for CVE-2026-44211
Indicators of Compromise
- Unexpected inbound WebSocket connections to local Cline Kanban listener ports originating from browser processes
- HTTP Upgrade: websocket requests carrying an Origin header that does not match the IDE or CLI host
- Cline agent command activity occurring without a corresponding developer-initiated session
Detection Strategies
- Inspect process and network telemetry on developer endpoints for WebSocket handshakes targeting Cline ports from non-local origins
- Correlate browser child-process network activity with Cline Kanban server access events
- Alert on Cline agent actions, such as repository writes or shell commands, that lack an associated IDE foreground event
Monitoring Recommendations
- Capture endpoint network flows for loopback WebSocket traffic and retain Origin header values where available
- Monitor developer workstations for outbound connections to newly observed domains immediately preceding local WebSocket activity
- Track Cline version inventory across the developer fleet to identify hosts running 2.13.0 or earlier
How to Mitigate CVE-2026-44211
Immediate Actions Required
- Stop or disable the Cline Kanban server on affected developer workstations until a patched release is available
- Restrict developer browser usage on hosts running the Cline IDE extension or CLI assistant
- Block the Cline Kanban listener port at the host firewall so only the local IDE process can connect
Patch Information
At the time of CVE publication, the vendor advisory states that no public patch is available. Monitor the Cline GitHub Security Advisory GHSA-5c57-rqjx-35g2 for the fixed release and upgrade as soon as a corrected version is published.
Workarounds
- Terminate the Cline Kanban server process when not actively using the Kanban feature
- Configure host-based firewall rules to deny WebSocket traffic to the Cline port from any process other than the trusted IDE binary
- Use a separate browser profile or dedicated browser for general web browsing while Cline is running
# Example: block external access to a local Cline Kanban port on Linux
# Replace <PORT> with the actual Cline Kanban listener port
sudo iptables -A INPUT -p tcp --dport <PORT> ! -s 127.0.0.1 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
