Critical RCE Vulnerability in Marimo Exploited Within 10 Hours of Disclosure

• •CVE-2026-39987 allows unauthenticated RCE on Marimo instances via WebSocket.
• •Exploitation occurred within 9 hours and 41 minutes of the vulnerability disclosure.
• •Attackers focused on credential theft, highlighting the need for immediate remediation.

On April 8, 2026, a critical pre-authenticated remote code execution vulnerability (CVE-2026-39987) was disclosed in Marimo, an open-source Python notebook platform. The flaw allows unauthenticated attackers to gain a full interactive shell via the /terminal/ws WebSocket endpoint. Exploitation was observed within 9 hours and 41 minutes of the advisory's publication, with attackers executing credential theft operations shortly after. The vulnerability affects all Marimo versions prior to 0.23.0 and has a CVSS score of 9.3. Researchers noted that no public proof-of-concept code existed at the time of the attack, indicating that attackers were able to construct exploits directly from the advisory's technical details. The attack pattern suggests professional threat actor involvement, focusing on credential harvesting rather than deploying malware. Organizations using Marimo are urged to implement emergency remediation to mitigate risks associated with this vulnerability.