CVE-2021-3404: In ytnef 1.9.3, the SwapWord function in lib/ytnef.c allows remote attackers to cause a denial-of-service (and

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HIGH 7.8WATCHLISTDOWNGRADEMITIGATEMITIGATE

CVE-2021-3404 is a high-severity flaw (CVSS 7.8) affecting ytnef, enterprise linux, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

In ytnef 1.9.3, the SwapWord function in lib/ytnef.c allows remote attackers to cause a denial-of-service (and potentially code execution) due to a heap buffer overflow which can be triggered via a crafted file.

Why this priority

High severity
Public exploit reference
On your watchlist

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

Risk facts

CVSS score	7.8 (High)
Attack vector	Local
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	1.9% (top 20%)
CISA exploited list	No
Weakness	CWE-119, CWE-787
Published	2021-03-04
Last modified	2026-06-17

Publisher (CNA): Red Hat, Inc. · Vendor: fedoraproject · redhat · ytnef project

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

Am I affected?

Timeline

2021-03-04 22:15 UTCPublished CVE record published
2026-06-17 04:05 UTCUpdated Record last modified

ytnef project ytnef DOWNGRADE

Affected: 1.9.3

Fixed in: no fixed release known

Safe to downgrade to: a release before 1.9.3

1. Check installed version

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

2. No fixed release: DOWNGRADE to a release before 1.9.3 (Debian / Ubuntu)

apt-cache madison ytnef sudo apt install ytnef=<VERSION_BEFORE_1.9.3> sudo apt-mark hold ytnef

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

dnf --showduplicates list ytnef sudo dnf downgrade ytnef-<VERSION_BEFORE_1.9.3> sudo dnf versionlock add ytnef

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.

redhat enterprise linux MITIGATE

Affected: 7.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 7.0

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: 33

Fixed in: no fixed release known

Safe to downgrade to: a release before 33

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-2021-3404?

ytnef: downgrade or apply the listed mitigation; enterprise 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-2021-3404 being exploited?

CVE-2021-3404 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-2021-3404 affect?

CVE-2021-3404 affects ytnef, enterprise linux, fedora. Severity is high (CVSS 7.8). In ytnef 1.9.3, the SwapWord function in lib/ytnef.c allows remote attackers to cause a denial-of-service (and potentially code execution) due to a heap buffer overflow which can be triggered via a crafted file.

How urgent is CVE-2021-3404?

Priority 57/100. High severity; Public exploit reference; On your watchlist

Related CVEs

CVE-2021-3403 HIGH 7.8 DOWNGRADE MITIGATE In ytnef 1.9.3, the TNEFSubjectHandler function in lib/ytnef.c allows remote attackers to cause a denial-of-se
CVE-2017-9474 MEDIUM 5.5 DOWNGRADE In ytnef 1.9.2, the DecompressRTF function in lib/ytnef.c allows remote attackers to cause a denial of service
CVE-2017-9473 MEDIUM 5.5 DOWNGRADE MITIGATE In ytnef 1.9.2, the TNEFFillMapi function in lib/ytnef.c allows remote attackers to cause a denial of service
CVE-2017-9472 MEDIUM 5.5 DOWNGRADE In ytnef 1.9.2, the SwapDWord function in lib/ytnef.c allows remote attackers to cause a denial of service (he
CVE-2017-9471 MEDIUM 5.5 DOWNGRADE MITIGATE In ytnef 1.9.2, the SwapWord function in lib/ytnef.c allows remote attackers to cause a denial of service (hea

References

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