MIT Lifelong Kindergarten Scratch scratch-vm before 0.2.0-prerelease.20200714185213 loads extension URLs from untrusted project.json files with certain _ characters, resulting in remote code execution because the URL's content is treated as a script and is executed as a worker. The responsible code is getExtensionIdForOpcode in serialization/sb3.js. The use of _ is incompatible with a protection mechanism in older versions, in which URLs were split and consequently deserialization attacks were prevented.

NOTE: the scratch.mit.edu hosted service is not affected because of the lack of worker scripts.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-14000
• scratchfoundation/scratch-vm#2476
• scratchfoundation/scratch-vm@90b9da4
• https://scratch.mit.edu/discuss/topic/422904/?page=1#post-4223443