CVE-2019-25041Public exploit

Assertion-enabled build crash via oversized DNS label

Published Apr 27, 2021 · Updated Aug 5, 2024

Reachable assertion in NLnet Labs Unbound before 1.9.5 allows attackers to crash assertion-enabled builds during direct testing with malformed DNS packets. dname_pkt_copy follows DNS compression pointers, reads a label length, and asserts that it does not exceed LDNS_MAX_LABELLEN instead of rejecting the name first. The audit reproduced the assertion in instrumented code, but NLnet Labs states that earlier parsing checks prevent remote or local exploitation in a running installation; builds without assertions reject the input without harm.

CVSS severity7.5
High
EPSS probability2.13%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Reachable assertion in NLnet Labs Unbound before 1.9.5 allows attackers to crash assertion-enabled builds during direct testing with malformed DNS packets. dname_pkt_copy follows DNS compression pointers, reads a label length, and asserts that it does not exceed LDNS_MAX_LABELLEN instead of rejecting the name first. The audit reproduced the assertion in instrumented code, but NLnet Labs states that earlier parsing checks prevent remote or local exploitation in a running installation; builds without assertions reject the input without harm.

The record

CVE
CVE-2019-25041
Published
Apr 27, 2021
Updated
Aug 5, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-617, T1499.004
Attack vector
Unavailable
Privileges
Unavailable

Timeline

How it unfolded

  1. Apr 27, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 5, 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 vectorUnavailable in this record.
Required privilegesUnavailable in this record.

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

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

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)
VectorUnavailable
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-617Reachable Assertion
T1499.004Application or System Exploitation

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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