CVE-2021-3672Public exploitNetwork attack vector

DNS cache injection via unescaped NUL bytes

Published Nov 23, 2021 · Updated Oct 15, 2024

Hostname validation failure in c-ares 1.0.0 through 1.17.1 allows remote attackers to induce incorrect DNS names via crafted replies. The ares_expand_name function copied nonprintable bytes from RFC 1035 names into a NUL-terminated C string without escaping them, so an embedded zero byte truncated the visible CNAME or record name. Exploitation requires a DNS response containing the crafted name; applications that cache returned names can accept an attacker-chosen domain as a different name, enabling DNS cache injection.

CVSS severity5.6
Medium
EPSS probability2.81%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Hostname validation failure in c-ares 1.0.0 through 1.17.1 allows remote attackers to induce incorrect DNS names via crafted replies. The ares_expand_name function copied nonprintable bytes from RFC 1035 names into a NUL-terminated C string without escaping them, so an embedded zero byte truncated the visible CNAME or record name. Exploitation requires a DNS response containing the crafted name; applications that cache returned names can accept an attacker-chosen domain as a different name, enabling DNS cache injection.

The record

CVE
CVE-2021-3672
Published
Nov 23, 2021
Updated
Oct 15, 2024
Vendor
c-ares Project
Product
c-ares
Classifications
CWE-79, T1565.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 23, 2021CVE publishedPublication date reported by the CVE source.
  2. Oct 15, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=c-ares 1.17.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

c-ares Project · c-aresversion=c-ares 1.17.2

Published CVSS scores

5.6NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L
6.8NIST NVDAV:N/AC:M/Au:N/C:P/I:P/A:P

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1565.002Transmitted Data Manipulation

Public exploit references

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

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo