# CVE-2021-3657: isync: A flaw was found in mbsync versions prior to 1.4.4

CVE-2021-3657 is fixed in isync on 1 platform, first by DEBIAN-CVE-2021-3657, released February 18, 2022. It is not reported as exploited. Patch Radar priority: Urgent.

- Web page: https://patchblox.com/security/cve/CVE-2021-3657
- Priority: Urgent
- CVSS: 9.8 (critical)
- EPSS: 0.0349
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Feb 18, 2022

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | isync Debian 12.15 (bookworm) | 1.4.4-1 | [DEBIAN-CVE-2021-3657](https://patchblox.com/security/data/fix/linux/debian-cve-2021-3657.md) | Feb 18, 2022 |
| Linux | isync Debian 13.5 (trixie) | 1.4.4-1 | [DEBIAN-CVE-2021-3657](https://patchblox.com/security/data/fix/linux/debian-cve-2021-3657.md) | Feb 18, 2022 |
| Linux | isync Debian 14 | 1.4.4-1 | [DEBIAN-CVE-2021-3657](https://patchblox.com/security/data/fix/linux/debian-cve-2021-3657.md) | Feb 18, 2022 |

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Data as of 2026-11-08T13:06:02Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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