CVE-2021-44143: A flaw was found in mbsync in isync 1.4.0 through 1.4.3

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CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2021-44143 is a critical-severity flaw (CVSS 9.8) affecting isync, debian linux, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A flaw was found in mbsync in isync 1.4.0 through 1.4.3. Due to an unchecked condition, a malicious or compromised IMAP server could use a crafted mail message that lacks headers (i.e., one that starts with an empty line) to provoke a heap overflow, which could conceivably be exploited for remote code execution.

Why this priority

Critical severity
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	9.8 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	3.8% (top 10%)
CISA exploited list	No
Weakness	CWE-787
Published	2021-11-22
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · fedoraproject · isync project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-11-22 20:15 UTCPublished CVE record published
2026-06-17 04:11 UTCUpdated Record last modified

isync project isync UPGRADE

Affected: >= 1.4.0 and <= 1.4.3

Fixed in: newer than 1.4.3

Safe to downgrade to: a release before 1.4.0

1. Check installed version

apt-cache policy isync || rpm -q isync || isync --version

2. UPGRADE to newer than 1.4.3 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade isync

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade isync

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 1.4.0 (Debian / Ubuntu)

apt-cache madison isync sudo apt install isync=<VERSION_BEFORE_1.4.0> sudo apt-mark hold isync

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list isync sudo dnf downgrade isync-<VERSION_BEFORE_1.4.0> sudo dnf versionlock add isync

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

debian debian linux MITIGATE

Affected: 9.0 · 10.0 · 11.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 9.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

fedoraproject fedora MITIGATE

Affected: 34 · 35

Fixed in: no fixed release known

Safe to downgrade to: a release before 34

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-44143?

isync: upgrade to newer than 1.4.3; debian linux: mitigate or apply the listed mitigation; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-44143 being exploited?

CVE-2021-44143 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-44143 affect?

CVE-2021-44143 affects isync, debian linux, fedora. Severity is critical (CVSS 9.8). A flaw was found in mbsync in isync 1.4.0 through 1.4.3. Due to an unchecked condition, a malicious or compromised IMAP server could use a crafted mail message that lacks headers (i.e., one that starts with an empty line) to provoke a heap

How urgent is CVE-2021-44143?

Priority 67/100. Critical severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

Related CVEs

CVE-2021-3657 CRITICAL 9.8 UPGRADE MITIGATE A flaw was found in mbsync versions prior to 1.4.4
CVE-2021-3578 HIGH 7.8 UPGRADE MITIGATE A flaw was found in mbsync before v1.3.6 and v1.4.2, where an unchecked pointer cast allows a malicious or com

References

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