CVE-2021-3657

Severity: critical (CVSS 9.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 3.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2022-02-18 · updated 2026-06-17
Weakness: CWE-119

Description

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.

Affected products

Debian Linux: 9.0
Fedoraproject Fedora: 35
Isync Project Isync: before 1.4.4 — fixed in 1.4.4
Red Hat Enterprise Linux: 7.0

How to fix

Upgrade to a fixed version: Isync Project Isync 1.4.4. Check the vendor advisory for mitigations.

References

https://bugzilla.redhat.com/show_bug.cgi?id=2028932
https://lists.debian.org/debian-lts-announce/2022/07/msg00001.html
https://security.gentoo.org/glsa/202208-15
https://www.openwall.com/lists/oss-security/2021/12/03/1
https://bugzilla.redhat.com/show_bug.cgi?id=2028932
https://lists.debian.org/debian-lts-announce/2022/07/msg00001.html
https://security.gentoo.org/glsa/202208-15
https://www.openwall.com/lists/oss-security/2021/12/03/1

Get alerted when CVE-2021-3657 or the next vulnerability affects your stack