CVE-2021-37311: Buffer Overflow vulnerability in fcitx5 5.0.8 allows attackers to cause a denial of service via crafted messag

This week

HIGH 7.5REMOTEDOWNGRADE

CVE-2021-37311 is a high-severity flaw (CVSS 7.5) affecting fcitx 5. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Buffer Overflow vulnerability in fcitx5 5.0.8 allows attackers to cause a denial of service via crafted message to the application's listening port.

Why this priority

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

No fix is published yet: follow the downgrade or mitigation steps.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.9% (top 40%)
CISA exploited list	No
Weakness	CWE-120
Published	2023-02-03
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: fcitx 5 project

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

Am I affected?

Timeline

2023-02-03 18:15 UTCPublished CVE record published
2026-06-17 04:00 UTCUpdated Record last modified

fcitx 5 project fcitx 5 DOWNGRADE

Affected: 5.0.8

Fixed in: no fixed release known

Safe to downgrade to: a release before 5.0.8

1. Check installed version

apt-cache policy fcitx-5 || rpm -q fcitx-5 || fcitx-5 --version

2. No fixed release: DOWNGRADE to a release before 5.0.8 (Debian / Ubuntu)

apt-cache madison fcitx-5 sudo apt install fcitx-5=<VERSION_BEFORE_5.0.8> sudo apt-mark hold fcitx-5

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

dnf --showduplicates list fcitx-5 sudo dnf downgrade fcitx-5-<VERSION_BEFORE_5.0.8> sudo dnf versionlock add fcitx-5

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.

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-2021-37311?

fcitx 5: downgrade or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-37311 being exploited?

CVE-2021-37311 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-2021-37311 affect?

CVE-2021-37311 affects fcitx 5. Severity is high (CVSS 7.5). Buffer Overflow vulnerability in fcitx5 5.0.8 allows attackers to cause a denial of service via crafted message to the application's listening port.

How urgent is CVE-2021-37311?

Priority 51/100. High severity; Public exploit reference; Remotely exploitable, no login, no user action

References

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