CVE-2021-33912: libspf2 before 1.2.11 has a four-byte heap-based buffer overflow that might allow remote attackers to execute

Q: How do I fix CVE-2021-33912?

libspf2: upgrade to 1.2.11; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-33912 being exploited?

CVE-2021-33912 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-2021-33912 affect?

CVE-2021-33912 affects libspf2, debian linux. Severity is critical (CVSS 9.8). libspf2 before 1.2.11 has a four-byte heap-based buffer overflow that might allow remote attackers to execute arbitrary code (via an unauthenticated e-mail message from anywhere on the Internet) with a crafted SPF DNS record, because of inc

Q: How urgent is CVE-2021-33912?

Priority 73/100. Critical severity; EPSS top 5%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

This week

CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATE

CVE-2021-33912 is a critical-severity flaw (CVSS 9.8) affecting libspf2, debian linux. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

libspf2 before 1.2.11 has a four-byte heap-based buffer overflow that might allow remote attackers to execute arbitrary code (via an unauthenticated e-mail message from anywhere on the Internet) with a crafted SPF DNS record, because of incorrect sprintf usage in SPF_record_expand_data in spf_expand.c. The vulnerable code may be part of the supply chain of a site's e-mail infrastructure (e.g., with additional configuration, Exim can use libspf2;

Why this priority

Critical severity
EPSS top 5%: very likely to be exploited
Public exploit reference
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	9.6% (top 5%)
CISA exploited list	No
Weakness	CWE-787
Published	2022-01-19
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: debian · libspf2 project

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

Am I affected?

Timeline

2022-01-19 18:15 UTCPublished CVE record published
2026-06-17 03:55 UTCUpdated Record last modified

libspf2 project libspf2 UPGRADE

Affected: < 1.2.11

Fixed in: 1.2.11

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy libspf2 || rpm -q libspf2 || libspf2 --version

2. UPGRADE to 1.2.11 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade libspf2

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade libspf2

Note

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

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.

Exact fixes by distribution

A distro patch exists: prefer upgrading over downgrading.

Fedora 46 · nemo · FIXED · 6.8.0-1.fc46 · FEDORA-2026-0edead723f · source

sudo dnf upgrade --advisory=FEDORA-2026-0edead723f

Fedora ELN · composer · FIXED · 2.11.1-1.eln159 · FEDORA-2026-cb4f992456 · source

sudo dnf upgrade --advisory=FEDORA-2026-cb4f992456

Ubuntu 22.04 · libspf2 · NOT AFFECTED

Ubuntu 20.04 · libspf2 · FIXED · 1.2.10-7+deb9u2build0.20.04.1

sudo apt update && sudo apt install --only-upgrade libspf2 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' libspf2)" ge "1.2.10-7+deb9u2build0.20.04.1" && echo PATCHED || echo STILL-VULNERABLE

Ubuntu 24.04 · libspf2 · NOT AFFECTED

Debian 12 · libspf2 · FIXED · 1.2.10-7.1

sudo apt update && sudo apt install --only-upgrade libspf2 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' libspf2)" ge "1.2.10-7.1" && echo PATCHED || echo STILL-VULNERABLE

Debian 13 · libspf2 · FIXED · 1.2.10-7.1

sudo apt update && sudo apt install --only-upgrade libspf2 dpkg --compare-versions "$(dpkg-query -W -f='${Version}' libspf2)" ge "1.2.10-7.1" && echo PATCHED || echo STILL-VULNERABLE

Alpine 3.20 · libspf2 · FIXED · libspf2-1.2.11-r0

apk add --upgrade libspf2 apk info -v libspf2

Alpine 3.21 · libspf2 · FIXED · libspf2-1.2.11-r0

apk add --upgrade libspf2 apk info -v libspf2

Alpine 3.22 · libspf2 · FIXED · libspf2-1.2.11-r0

apk add --upgrade libspf2 apk info -v libspf2

Alpine 3.23 · libspf2 · FIXED · libspf2-1.2.11-r0

apk add --upgrade libspf2 apk info -v libspf2

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-2021-33912?

libspf2: upgrade to 1.2.11; debian linux: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-33912 being exploited?

CVE-2021-33912 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-2021-33912 affect?

CVE-2021-33912 affects libspf2, debian linux. Severity is critical (CVSS 9.8). libspf2 before 1.2.11 has a four-byte heap-based buffer overflow that might allow remote attackers to execute arbitrary code (via an unauthenticated e-mail message from anywhere on the Internet) with a crafted SPF DNS record, because of inc

How urgent is CVE-2021-33912?

Priority 73/100. Critical severity; EPSS top 5%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist

Software pages

Debian vulnerabilities · Exim vulnerabilities

Related CVEs

CVE-2023-42118 HIGH 8.8 MITIGATE Exim libspf2 Integer Underflow Remote Code Execution Vulnerability
CVE-2021-33913 CRITICAL 9.8 UPGRADE libspf2 before 1.2.11 has a heap-based buffer overflow that might allow remote attackers to execute arbitrary
CVE-2021-20314 CRITICAL 9.8 UPGRADE MITIGATE Stack buffer overflow in libspf2 versions below 1.2.11 when processing certain SPF macros can lead to Denial o

References

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