CVE-2021-33913

Name: CVE-2021-33913
Creator: NVD / NIST
Published: 2022-01-19T18:15:07.83+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

CRITICALCVSS 9.8/10EPSS 9.64%

Last modified Jun 17, 2026

CVE-2021-33913 is a critical-severity vulnerability rated 9.8/10 on the CVSS scale. libspf2 before 1.2.11 has a 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 SPF_record_expand_data in spf_expand.c. The amount of overflowed data depends on the relationship between the length of an entire domain name and the length of its leftmost label. EPSS estimates a 9.64% chance of exploitation in the next 30 days.

Description

libspf2 before 1.2.11 has a 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 SPF_record_expand_data in spf_expand.c. The amount of overflowed data depends on the relationship between the length of an entire domain name and the length of its leftmost label. 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; the Postfix web site links to unofficial patches for use of libspf2 with Postfix; older versions of spfquery relied on libspf2) but most often is not.

Metrics

CVSS 3.1

9.8/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS Probability

9.64%

94.9th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-787

Affected Software

Vendor	Product	Versions
Libspf2 Project	Libspf2	< 1.2.11

References

https://github.com/shevek/libspf2/tree/8131fe140704eaae695e76b5cd09e39bd1dd220bThird Party Advisory
https://lists.debian.org/debian-lts-announce/2022/01/msg00015.htmlMailing List, Third Party Advisory
https://nathanielbennett.com/blog/libspf2-cve-jan-2022-disclosureExploit, Third Party Advisory
https://security.gentoo.org/glsa/202401-22
https://github.com/shevek/libspf2/tree/8131fe140704eaae695e76b5cd09e39bd1dd220bThird Party Advisory
https://lists.debian.org/debian-lts-announce/2022/01/msg00015.htmlMailing List, Third Party Advisory
https://nathanielbennett.com/blog/libspf2-cve-jan-2022-disclosureExploit, Third Party Advisory
https://security.gentoo.org/glsa/202401-22

Timeline

Published: Jan 19, 2022
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-33913?

How severe is CVE-2021-33913?

CVE-2021-33913 has a CVSS score of 9.8/10 (CRITICAL severity). The EPSS model estimates a 9.64% probability of exploitation in the next 30 days.

How do I fix CVE-2021-33913?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-33913?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST