There is 1 open security issue in buster.
1 issue left for the package maintainer to handle:
A flaw was found in mbsync versions prior to 1.4.4. Due to inadequate handling of extremely large (>=2GiB) IMAP literals, malicious or compromised IMAP servers, and hypothetically even external email senders, could cause several different buffer overflows, which could conceivably be exploited for remote code execution.
You can find information about how to handle this issue in the security team's documentation.