CVE-2026-120338 PrefectHQ CVE debrief

Who should care

System administrators, security teams, and developers using Prefab UI's Generative UI feature should be aware of this vulnerability and take immediate action to upgrade to version 0.21.0 or later.

Why it matters

This critical vulnerability allows an attacker to execute arbitrary code as the server OS user, potentially leading to file read and write access, persistence, and other security consequences. System administrators, security teams, and developers using Prefab UI's Generative UI feature should immediately upgrade to version 0.21.0 or later and restrict access to the feature.

• Code execution as server OS user
• File read and write access as server OS user
• Potential for persistence via .pth file
• Verification of Deno permissions and Generative UI configuration required

Technical summary

The Prefab UI's Generative UI sandbox runs caller-supplied Python inside Pyodide, which is compiled to WebAssembly and runs in a Deno subprocess. However, the launcher starts Deno with unrestricted --allow-read, --allow-write, and --allow-net permissions, and the runner leaves the Deno API on the JavaScript global object. This allows an attacker to read and write files as the server OS user by exploiting the Generative UI sandbox. The fix in version 0.21.0 revokes Deno read, write, and network permission before accepting code, starts the process with --no-prompt, and limits bootstrap network access to cdn.jsdelivr.net.

Defensive priority

High

Recommended defensive actions

• Immediately upgrade to version 0.21.0 or later
• Restrict access to the Generative UI feature
• Monitor for suspicious activity
• Implement additional security measures to prevent exploitation
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The vulnerability allows an attacker to read and write files as the server OS user by exploiting the Generative UI sandbox. The sandbox runs caller-supplied Python inside Pyodide, which is compiled to WebAssembly and runs in a Deno subprocess. However, the launcher starts Deno with unrestricted --allow-read, --allow-write, and --allow-net permissions, and the runner leaves the Deno API on the JavaScript global object.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.