Affected versions of conquer-once implements Sync for its OnceCell type without restricting it to Sendable types.

This allows non-Send but Sync types such as MutexGuard to be sent across threads leading to undefined behavior and memory corruption in concurrent programs.

The issue was fixed by adding a Send constraint to OnceCell.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-36208
• oliver-giersch/conquer-once#3
• https://rustsec.org/advisories/RUSTSEC-2020-0101.html