CVE-2020-12663Network attack vector

Resolver hang via unvalidated CNAME target length

Published May 19, 2020 · Updated Aug 4, 2024

Infinite-loop denial of service in NLnet Labs Unbound 1.10.0 and earlier allows remote attackers to hang the resolver via malformed DNS answers. The response scrubber's parse_get_cname_target function accepted an unvalidated CNAME target and derived its length without parsing the encoded domain name. Exploitation requires a resolver to process an answer from an attacker-controlled upstream server; production builds hang, while debug builds can terminate on an assertion.

CVSS severity7.5
High
EPSS probability3.59%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Infinite-loop denial of service in NLnet Labs Unbound 1.10.0 and earlier allows remote attackers to hang the resolver via malformed DNS answers. The response scrubber's parse_get_cname_target function accepted an unvalidated CNAME target and derived its length without parsing the encoded domain name. Exploitation requires a resolver to process an answer from an attacker-controlled upstream server; production builds hang, while debug builds can terminate on an assertion.

The record

CVE
CVE-2020-12663
Published
May 19, 2020
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-835, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 19, 2020CVE 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

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/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 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 exploitUnconfirmed

Weakness, pattern, technique

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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