Machine speed, hold the AI: Hand-rolled marimo CVE-2026-39987 exploit

AI is lowering the barrier to entry for attackers; that much is settled. But what’s still up for debate is whether skilled threat actors can keep up with their LLM-driven competitors. Recently, the Sysdig Threat Research Team (TRT) watched a single threat actor go from an open WebSocket to a live SSH session on a bastion host in eight seconds. There was no agent in the loop, nor was there any sign of LLM-generated scripts or tooling. Instead, the operator used a Python toolkit they wrote and debugged by hand, in-session, over the preceding four hours. The attacker exploited CVE-2026-39987, a pre-authentication remote code execution (RCE) vulnerability in marimo. They ran a complete credential-pivot chain end to end: initial access through the unauthenticated WebSocket terminal, an AWS Secrets Manager call using credentials harvested from the compromised instance, and SSH access to a bastion host with the retrieved private key. Over the course of a nine-hour session, they issued more than 850 interactive commands, used no recognizable publicly available offensive tooling, and hand-rolled their scripts in-session. Eight seconds is the kind of speed we expect to see in AI-assisted attacks. This operator got there on skill alone, and along the way…