CVE-2021-3657 A flaw was found in mbsync versions prior to 1.4.4.

What is CVE-2021-3657?

CVE-2021-3657 is a critical vulnerability in Debian Debian Linux, published on 18 Feb 2022.

Official 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.

Weakness: CWE-119

Which versions are affected by CVE-2021-3657?

It affects 4 products.

Products include Debian Debian Linux, Fedoraproject Fedora, Isync Project Isync, Redhat Enterprise Linux.

How do I fix CVE-2021-3657?

Upgrade to a fixed version.

Is CVE-2021-3657 being exploited?

There is no known exploitation of CVE-2021-3657 in the wild and no public exploit code.

Other vulnerabilities in Debian Debian Linux

• CVE-2015-3306 ProFTPD Improper Access Control Exploited Critical 10.0
• CVE-2025-49113 RoundCube Webmail Deserialization of Untrusted Data Exploited Critical 9.9
• CVE-2026-24061 GNU InetUtils Argument Injection Exploited Critical 9.8
• CVE-2025-39682 Linux Kernel Improper Check for Unusual or Exceptional Conditions Exploited Critical 9.8
• CVE-2024-9680 Mozilla Firefox Use-After-Free Exploited Critical 9.8

All Debian Debian Linux vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2021-3657, GHSA-gq8r-4g7c-69vw, UBUNTU-CVE-2021-3657.