CVE-2021-3594: An invalid pointer initialization issue was found in the SLiRP networking implementation of QEMU

Q: How do I fix CVE-2021-3594?

libslirp: upgrade to 4.6.0; enterprise linux: mitigate or apply the listed mitigation; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-3594 being exploited?

CVE-2021-3594 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-3594 affect?

CVE-2021-3594 affects libslirp, enterprise linux, debian linux. Severity is low (CVSS 3.8). An invalid pointer initialization issue was found in the SLiRP networking implementation of QEMU. The flaw exists in the udp_input() function and could occur while processing a udp packet that is smaller than the size of the 'udphdr' struct

Q: How urgent is CVE-2021-3594?

Priority 25/100. On your watchlist

Routine

LOW 3.8WATCHLISTUPGRADEMITIGATEMITIGATE

CVE-2021-3594 is a low-severity flaw (CVSS 3.8) affecting libslirp, enterprise linux, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

An invalid pointer initialization issue was found in the SLiRP networking implementation of QEMU. The flaw exists in the udp_input() function and could occur while processing a udp packet that is smaller than the size of the 'udphdr' structure. This issue may lead to out-of-bounds read access or indirect host memory disclosure to the guest. The highest threat from this vulnerability is to data confidentiality. This flaw affects libslirp versions

Why this priority

On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	3.8 (Low)
Attack vector	Local
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	0.3% (top 74%)
CISA exploited list	No
Weakness	CWE-824
Published	2021-06-15
Last modified	2026-06-17

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

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

Am I affected?

Timeline

2021-06-15 21:15 UTCPublished CVE record published
2026-06-17 04:05 UTCUpdated Record last modified

libslirp project libslirp UPGRADE

Affected: < 4.6.0

Fixed in: 4.6.0

Safe to downgrade to: no unaffected older release

1. Check installed version

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

2. UPGRADE to 4.6.0 (Debian / Ubuntu)

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

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade libslirp

Note

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

redhat enterprise linux MITIGATE

Affected: 8.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 8.0

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.

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: 33 · 34

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-2021-3594?

libslirp: upgrade to 4.6.0; enterprise linux: mitigate or apply the listed mitigation; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-3594 being exploited?

CVE-2021-3594 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-2021-3594 affect?

CVE-2021-3594 affects libslirp, enterprise linux, debian linux. Severity is low (CVSS 3.8). An invalid pointer initialization issue was found in the SLiRP networking implementation of QEMU. The flaw exists in the udp_input() function and could occur while processing a udp packet that is smaller than the size of the 'udphdr' struct

How urgent is CVE-2021-3594?

Priority 25/100. On your watchlist

Software pages

Debian vulnerabilities · QEMU vulnerabilities

Related CVEs

CVE-2021-3595 LOW 3.8 UPGRADE MITIGATE An invalid pointer initialization issue was found in the SLiRP networking implementation of QEMU
CVE-2021-3593 LOW 3.8 UPGRADE MITIGATE An invalid pointer initialization issue was found in the SLiRP networking implementation of QEMU
CVE-2021-3592 LOW 3.8 UPGRADE MITIGATE An invalid pointer initialization issue was found in the SLiRP networking implementation of QEMU
CVE-2020-8608 MEDIUM 5.6 DOWNGRADE MITIGATE In libslirp 4.1.0, as used in QEMU 4.2.0, tcp_subr.c misuses snprintf return values, leading to a buffer overf
CVE-2020-7211 HIGH 7.5 DOWNGRADE DOWNGRADE tftp.c in libslirp 4.1.0, as used in QEMU 4.2.0, does not prevent ..\ directory traversal on Windows.

References

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