CVE-2026-120338 (GCVE-0-2026-120338)

Vulnerability from cvelistv5 – Published: 2026-11-01 15:24 – Updated: 2026-11-01 15:24
VLAI
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 0.14.0 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).
Severity
10 (Critical)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
9.9 (Critical)
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
CWE
  • CWE-693 - Protection Mechanism Failure
  • CWE-94 - Improper Control of Generation of Code ('Code Injection')
  • CWE-269 - Improper Privilege Management
Assigner
JFROG CNA under the mitre root
CNA scorecard B 76/100 over 66 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
PrefectHQ prefab-ui Affected: 0.14.0 , < 0.21.0 (semver)
guessed Create a notification for this product.
Credits
Yuval Moravchick JFrog Security Research
Show details on NVD website
To clipboard 

{
  "containers": {
    "cna": {
      "affected": [
        {
          "collectionURL": "https://pypi.org/project/prefab-ui/",
          "defaultStatus": "unaffected",
          "packageName": "prefab-ui",
          "product": "prefab-ui",
          "programFiles": [
            "src/prefab_ui/sandbox/_pyodide.py",
            "src/prefab_ui/sandbox/runner.js"
          ],
          "repo": "git://github.com/PrefectHQ/prefab",
          "vendor": "PrefectHQ",
          "versions": [
            {
              "lessThan": "0.21.0",
              "status": "affected",
              "version": "0.14.0",
              "versionType": "semver"
            }
          ]
        }
      ],
      "credits": [
        {
          "lang": "en",
          "type": "finder",
          "value": "Yuval Moravchick"
        },
        {
          "lang": "en",
          "type": "coordinator",
          "value": "JFrog Security Research"
        }
      ],
      "descriptions": [
        {
          "lang": "en",
          "supportingMedia": [
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              "type": "text/html",
              "value": "\u003cp\u003ePrefab UI\u0027s 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\u0027s 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\u0027s site-packages. The usual remote path is FastMCP\u0027s GenerativeUI provider, which registers the MCP tool generate_prefab_ui and forwards that tool\u0027s 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).\u003c/p\u003e"
            }
          ],
          "value": "Prefab UI\u0027s 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\u0027s 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\u0027s site-packages. The usual remote path is FastMCP\u0027s GenerativeUI provider, which registers the MCP tool generate_prefab_ui and forwards that tool\u0027s 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)."
        }
      ],
      "metrics": [
        {
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            "attackComplexity": "LOW",
            "attackVector": "NETWORK",
            "availabilityImpact": "HIGH",
            "baseScore": 10,
            "baseSeverity": "CRITICAL",
            "confidentialityImpact": "HIGH",
            "integrityImpact": "HIGH",
            "privilegesRequired": "NONE",
            "scope": "CHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H",
            "version": "3.1"
          },
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          "scenarios": [
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              "value": "FastMCP server with GenerativeUI enabled, Deno installed, and no authentication configured"
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            "attackVector": "NETWORK",
            "availabilityImpact": "HIGH",
            "baseScore": 9.9,
            "baseSeverity": "CRITICAL",
            "confidentialityImpact": "HIGH",
            "integrityImpact": "HIGH",
            "privilegesRequired": "LOW",
            "scope": "CHANGED",
            "userInteraction": "NONE",
            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H",
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          ]
        },
        {
          "descriptions": [
            {
              "cweId": "CWE-94",
              "description": "CWE-94: Improper Control of Generation of Code (\u0027Code Injection\u0027)",
              "lang": "en",
              "type": "CWE"
            }
          ]
        },
        {
          "descriptions": [
            {
              "cweId": "CWE-269",
              "description": "CWE-269: Improper Privilege Management",
              "lang": "en",
              "type": "CWE"
            }
          ]
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      "providerMetadata": {
        "dateUpdated": "2026-11-01T15:24:54.158Z",
        "orgId": "48a46f29-ae42-4e1d-90dd-c1676c1e5e6d",
        "shortName": "JFROG"
      },
      "references": [
        {
          "name": "Prefab repository",
          "tags": [
            "product"
          ],
          "url": "https://github.com/PrefectHQ/prefab"
        },
        {
          "name": "prefab-ui on PyPI",
          "tags": [
            "product"
          ],
          "url": "https://pypi.org/project/prefab-ui/"
        },
        {
          "name": "GHSA-595q-jv4r-q7j9",
          "tags": [
            "vendor-advisory"
          ],
          "url": "https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9"
        },
        {
          "name": "Refresh dependencies and fix renderer and sandbox regressions (fix pull request)",
          "tags": [
            "patch"
          ],
          "url": "https://github.com/PrefectHQ/prefab/pull/538"
        },
        {
          "name": "Fix commit 030f074",
          "tags": [
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          "url": "https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2"
        },
        {
          "name": "v0.21.0",
          "tags": [
            "release-notes"
          ],
          "url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0"
        },
        {
          "name": "prefab-ui 0.21.0",
          "tags": [
            "release-notes"
          ],
          "url": "https://pypi.org/project/prefab-ui/0.21.0/"
        },
        {
          "name": "v0.20.2 (confirmed affected)",
          "tags": [
            "technical-description"
          ],
          "url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2"
        },
        {
          "name": "v0.14.0 (first release with the sandbox)",
          "tags": [
            "technical-description"
          ],
          "url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0"
        }
      ],
      "source": {
        "discovery": "EXTERNAL"
      },
      "timeline": [
        {
          "lang": "en",
          "time": "2026-03-24T17:21:00.000Z",
          "value": "Pyodide sandbox added with --allow-read, --allow-write, and --allow-net (commit fc4362f3, pull request 334)"
        },
        {
          "lang": "en",
          "time": "2026-03-27T02:36:00.000Z",
          "value": "prefab-ui 0.14.0 published on PyPI, the first release that includes the sandbox"
        },
        {
          "lang": "en",
          "time": "2026-06-03T02:13:00.000Z",
          "value": "prefab-ui 0.20.2 published. This is the release confirmed in testing."
        },
        {
          "lang": "en",
          "time": "2026-08-05T16:12:00.000Z",
          "value": "Reported to PrefectHQ as GHSA-595q-jv4r-q7j9. The advisory is still in triage and has no CVE id."
        },
        {
          "lang": "en",
          "time": "2026-10-25T18:48:00.000Z",
          "value": "Fix merged in pull request 538 (commit 030f074). The runner revokes read, write, and network permission before accepting code."
        },
        {
          "lang": "en",
          "time": "2026-10-26T19:00:00.000Z",
          "value": "prefab-ui 0.21.0 published on PyPI with the fix. No vendor advisory text was published with the release."
        }
      ],
      "title": "Prefab UI Pyodide sandbox escape to host RCE",
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  "cveMetadata": {
    "assignerOrgId": "48a46f29-ae42-4e1d-90dd-c1676c1e5e6d",
    "assignerShortName": "JFROG",
    "cveId": "CVE-2026-120338",
    "datePublished": "2026-11-01T15:24:54.158Z",
    "dateReserved": "2026-11-01T15:12:50.987Z",
    "dateUpdated": "2026-11-01T15:24:54.158Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
  "dataVersion": "5.2",
  "vulnerability-lookup:meta": {
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        "affected": [
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            "affectedData": [
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                "collectionURL": "https://pypi.org/project/prefab-ui/",
                "defaultStatus": "unaffected",
                "packageName": "prefab-ui",
                "product": "prefab-ui",
                "programFiles": [
                  "src/prefab_ui/sandbox/_pyodide.py",
                  "src/prefab_ui/sandbox/runner.js"
                ],
                "repo": "git://github.com/PrefectHQ/prefab",
                "vendor": "PrefectHQ",
                "versions": [
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                    "status": "affected",
                    "version": "0.14.0",
                    "versionType": "semver"
                  }
                ]
              }
            ],
            "source": "reefs@jfrog.com"
          }
        ],
        "cveTags": [],
        "descriptions": [
          {
            "lang": "en",
            "value": "Prefab UI\u0027s 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\u0027s 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\u0027s site-packages. The usual remote path is FastMCP\u0027s GenerativeUI provider, which registers the MCP tool generate_prefab_ui and forwards that tool\u0027s 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)."
          }
        ],
        "id": "CVE-2026-120338",
        "lastModified": "2026-11-01T16:18:14.970",
        "metrics": {
          "cvssMetricV31": [
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                "baseSeverity": "CRITICAL",
                "confidentialityImpact": "HIGH",
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                "privilegesRequired": "NONE",
                "scope": "CHANGED",
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              "source": "reefs@jfrog.com",
              "type": "Secondary"
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        "published": "2026-11-01T16:18:14.970",
        "references": [
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            "source": "reefs@jfrog.com",
            "url": "https://github.com/PrefectHQ/prefab"
          },
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            "url": "https://github.com/PrefectHQ/prefab/commit/030f074f85e0b1be0dd44b0283185aecb26337a2"
          },
          {
            "source": "reefs@jfrog.com",
            "url": "https://github.com/PrefectHQ/prefab/pull/538"
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          {
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            "url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.14.0"
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            "url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.20.2"
          },
          {
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            "url": "https://github.com/PrefectHQ/prefab/releases/tag/v0.21.0"
          },
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            "url": "https://github.com/PrefectHQ/prefab/security/advisories/GHSA-595q-jv4r-q7j9"
          },
          {
            "source": "reefs@jfrog.com",
            "url": "https://pypi.org/project/prefab-ui/"
          },
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            "source": "reefs@jfrog.com",
            "url": "https://pypi.org/project/prefab-ui/0.21.0/"
          }
        ],
        "sourceIdentifier": "reefs@jfrog.com",
        "vulnStatus": "Received",
        "weaknesses": [
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                "lang": "en",
                "value": "CWE-94"
              },
              {
                "lang": "en",
                "value": "CWE-269"
              },
              {
                "lang": "en",
                "value": "CWE-693"
              }
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            "source": "reefs@jfrog.com",
            "type": "Primary"
          }
        ]
      }
    }
  }
}



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