CVE-2021-33913Public exploitNetwork attack vector

Remote code execution via undersized macro buffer

Published Jan 19, 2022 · Updated Aug 4, 2024

Heap-based buffer overflow in libspf2 before 1.2.11 allows remote attackers to execute arbitrary code via a crafted SPF DNS record. SPF_record_expand_data reuses the leftmost label length to size the URL-encoded reversed-domain buffer, then writes the full domain into it. Exposure requires an unauthenticated email path whose SPF processing is configured to use libspf2; most mail infrastructure does not include it.

CVSS severity9.8
Critical
EPSS probability9.64%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Heap-based buffer overflow in libspf2 before 1.2.11 allows remote attackers to execute arbitrary code via a crafted SPF DNS record. SPF_record_expand_data reuses the leftmost label length to size the URL-encoded reversed-domain buffer, then writes the full domain into it. Exposure requires an unauthenticated email path whose SPF processing is configured to use libspf2; most mail infrastructure does not include it.

The record

CVE
CVE-2021-33913
Published
Jan 19, 2022
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-787, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 19, 2022CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.8UbuntuCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-787Out-of-bounds Write
T1190Exploit Public-Facing Application

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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