{"id":"CVE-2026-120338","title":"Prefab UI Pyodide sandbox escape to host RCE","summary":"Prefab UI's Generative UI sandbox runs caller-supplied Python inside Pyodide, Python compiled to WebAssembly, in a Deno subprocess. The feature is documented as isolation for untrusted code, including LLM-written UI. From prefab-ui 0.14.…","severity":"critical","cvss":10,"cvssVector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H","cvssSource":"cna","cwe":["CWE-693","CWE-94","CWE-269"],"vendor":"PrefectHQ","product":"prefab-ui","affected":["prefab-ui >= 0.14.0 < 0.21.0"],"ssvc":{"exploitation":"none","automatable":"no","technicalImpact":"total","timestamp":"2026-11-02T19:42:39.981153Z"},"published":"2026-11-01","updated":"2026-11-02","sourceUpdated":"2026-11-02T19:42:56.540Z","source":"CVEORG","sourceUrl":"https://www.cve.org/CVERecord?id=CVE-2026-120338","references":[{"url":"https://github.com/PrefectHQ/prefab","label":"Prefab repository"},{"url":"https://pypi.org/project/prefab-ui/","label":"prefab-ui on PyPI"},{"url":"https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9","label":"GHSA-595q-jv4r-q7j9"},{"url":"https://github.com/PrefectHQ/prefab/pull/538","label":"Refresh dependencies and fix renderer and sandbox regressions (fix pull request)"},{"url":"https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2","label":"Fix commit 030f074"},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0","label":"v0.21.0"},{"url":"https://pypi.org/project/prefab-ui/0.21.0/","label":"prefab-ui 0.21.0"},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2","label":"v0.20.2 (confirmed affected)"},{"url":"https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0","label":"v0.14.0 (first release with the sandbox)"}],"tags":["cve.org"],"epss":0.00771,"epssPercentile":0.55782,"ingestedAt":"2026-11-05T19:30:42.807Z","slug":"CVE-2026-120338","body":"## Overview\n\nPrefab UI's Generative UI sandbox runs caller-supplied Python inside Pyodide, Python compiled to WebAssembly, in a Deno subprocess. The feature is documented as isolation for untrusted code, including LLM-written UI. From prefab-ui 0.14.0 through 0.20.2 the launcher starts Deno with unrestricted --allow-read, --allow-write, and --allow-net, and the runner leaves the Deno API on the JavaScript global object. Pyodide's js module proxies that global, so the Python can call Deno.readTextFileSync and Deno.writeTextFileSync. A caller who can supply the code can read and write files as the server OS user, and can persist code execution by writing a .pth file into the server virtualenv's site-packages. The usual remote path is FastMCP's GenerativeUI provider, which registers the MCP tool generate_prefab_ui and forwards that tool's code argument into this sandbox. On a FastMCP server with no authentication configured, any client who can reach the server can call the tool. With authentication, any client allowed to call tools can. Generative UI is opt-in, and the host must have Deno installed. Confirmed on prefab-ui 0.20.2. Fixed in 0.21.0: after the runtime and Prefab sources load, the runner revokes Deno read, write, and network permission before it accepts code, the process is started with --no-prompt so a permission request cannot grant them again, and bootstrap network access is limited to cdn.jsdelivr.net. The fix is commit 030f074 (pull request 538).\n\n## Affected\n\n- `prefab-ui >= 0.14.0 < 0.21.0`\n\n## Remediation\n\nRefer to the linked advisories for vendor-supplied fixes and affected version ranges.","depth":"midnight","depthScore":55,"depthScoreParts":{"impact":55,"likelihood":0.2,"exploitation":0,"ransomware":0},"changes":[]}