Description
The package integrity validation in yarn < 1.19.0 contains a TOCTOU vulnerability where the hash is computed before writing a package to cache. It's not computed again when reading from the cache. This may lead to a cache pollution attack.
Remediation
References
https://github.com/yarnpkg/yarn/blob/master/CHANGELOG.md#1190
https://github.com/yarnpkg/yarn/commit/0474b8c66a8ea298f5e4dedc67b2de464297ad1c
https://hackerone.com/reports/703138
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