Skip to content
Fencer Vulns
critical 9.8

CVE-2021-3657 json

A flaw was found in mbsync versions prior to 1.4.4.

published 2022-02-18 4 y ago modified 2026-06-17 4 mo ago also EUVD-2021-26952

cvss
9.8
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
3.5%
EPSS 3.5% chance of exploitation in 30 days, percentile 89%
public exploits
0
none known

Summary

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.

Patch / fix

  • isync_project isync 1.4.4
  • isync Debian 14, Debian 13, Debian 12 1.4.4-1

debian advisories DEBIAN-CVE-2021-3657

  • isync openSUSE Tumbleweed 1.4.4-1.1

opensuse advisories openSUSE-SU-2024:11779-1

  • isync Alpine 3.24, Alpine 3.23, Alpine 3.22 +7 more 1.4.4-r0

alpine advisories ALPINE-CVE-2021-3657

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • isync Arch Linux 1.4.4-1

arch linux advisories AVG-2579

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-3657 updated cvss 8.8

isync: buffer overflows due to inadequate handling of extremely large IMAP literals

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

3.49% chance of exploitation in the next 30 days, higher than 89% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 7.5 AV:N/AC:L/Au:N/C:P/I:P/A:P

Affected products

  • n/a isync

Classification

Weakness
  • CWE-119 Improper Restriction of Operations within the Bounds of a Memory Buffer

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.