CVE-2021-20247: A flaw was found in mbsync before v1.3.5 and v1.4.1

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HIGH 7.4REMOTEWATCHLISTUPGRADEDOWNGRADEMITIGATE

CVE-2021-20247 is a high-severity flaw (CVSS 7.4) affecting mbsync, extra packages for enterprise linux, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A flaw was found in mbsync before v1.3.5 and v1.4.1. Validations of the mailbox names returned by IMAP LIST/LSUB do not occur allowing a malicious or compromised server to use specially crafted mailbox names containing '..' path components to access data outside the designated mailbox on the opposite end of the synchronization channel. The highest threat from this vulnerability is to data confidentiality and integrity.

Why this priority

High severity
Public exploit reference
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	7.4 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	1.9% (top 21%)
CISA exploited list	No
Weakness	CWE-20, CWE-22
Published	2021-02-23
Last modified	2026-06-17

Publisher (CNA): Red Hat, Inc. · Vendor: debian · fedoraproject · mbsync project

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

Am I affected?

Timeline

2021-02-23 19:15 UTCPublished CVE record published
2026-06-17 03:33 UTCUpdated Record last modified

mbsync project mbsync UPGRADE

Affected: < 1.3.5 · >= 1.4.0 and < 1.4.1

Fixed in: 1.3.5, 1.4.1

Safe to downgrade to: a release before 1.4.0

1. Check installed version

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

2. UPGRADE to 1.3.5 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade mbsync

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 mbsync sudo apt install mbsync=<VERSION_BEFORE_1.4.0> sudo apt-mark hold mbsync

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

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

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.

fedoraproject extra packages for enterprise linux DOWNGRADE

Affected: 8.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

1. Check installed version

apt-cache policy extra-packages-for-enterprise-linux || rpm -q extra-packages-for-enterprise-linux || extra-packages-for-enterprise-linux --version

2. No fixed release: DOWNGRADE to a release before 8.0 (Debian / Ubuntu)

apt-cache madison extra-packages-for-enterprise-linux sudo apt install extra-packages-for-enterprise-linux=<VERSION_BEFORE_8.0> sudo apt-mark hold extra-packages-for-enterprise-linux

2. No fixed release: DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list extra-packages-for-enterprise-linux sudo dnf downgrade extra-packages-for-enterprise-linux-<VERSION_BEFORE_8.0> sudo dnf versionlock add extra-packages-for-enterprise-linux

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

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: 32 · 33

Fixed in: no fixed release known

Safe to downgrade to: a release before 32

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-2021-20247?

mbsync: upgrade to 1.3.5, 1.4.1; extra packages for enterprise linux: downgrade or apply the listed mitigation; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-20247 being exploited?

CVE-2021-20247 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-2021-20247 affect?

CVE-2021-20247 affects mbsync, extra packages for enterprise linux, debian linux. Severity is high (CVSS 7.4). A flaw was found in mbsync before v1.3.5 and v1.4.1. Validations of the mailbox names returned by IMAP LIST/LSUB do not occur allowing a malicious or compromised server to use specially crafted mailbox names containing '..' path components

How urgent is CVE-2021-20247?

Priority 61/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities

References

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