CVE-2019-25036 – CVE-2019-25036 is an assertion failure vulnerab... (How to Fix)

Q: What is the severity of CVE-2019-25036?

CVE-2019-25036 has a CVSS score of 7.5 (HIGH). CVE-2019-25036 is an assertion failure vulnerability in Unbound DNS resolver's synth_cname function that can cause denial of service. The vendor disputes exploitability, stating running installations cannot be exploited remotely or locally. Systems running Unbound DNS resolver before version 1.9.5 are affected.

📋 TL;DR

CVE-2019-25036 is an assertion failure vulnerability in Unbound DNS resolver's synth_cname function that can cause denial of service. The vendor disputes exploitability, stating running installations cannot be exploited remotely or locally. Systems running Unbound DNS resolver before version 1.9.5 are affected.

💻 Affected Systems

Products:

Unbound DNS resolver

Versions: All versions before 1.9.5

Operating Systems: Linux, Unix-like systems, Any OS running Unbound

Default Config Vulnerable: ⚠️ Yes

Notes: Vendor disputes practical exploitability of running installations.

📦 What is this software?

Debian Linux by Debian

View all CVEs affecting Debian Linux →

Unbound by Nlnetlabs

View all CVEs affecting Unbound →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Unbound DNS resolver crashes due to assertion failure, causing complete DNS resolution failure for dependent services.

🟠

Likely Case

Denial of service through service crash if vulnerable code path is triggered, though vendor disputes practical exploitability.

🟢

If Mitigated

Minimal impact with proper monitoring and restart mechanisms in place.

🌐 Internet-Facing: LOW - Vendor disputes remote exploitability and no public exploits exist.

🏢 Internal Only: LOW - Vendor disputes local exploitability and requires specific conditions to trigger.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: NO

Unauthenticated Exploit: ✅ No

Complexity: HIGH

No known exploits in the wild. Vendor disputes exploitability of running installations.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 1.9.5 and later

Vendor Advisory: https://nlnetlabs.nl/projects/unbound/download/

Restart Required: Yes

Instructions:

1. Download Unbound 1.9.5 or later from official repository. 2. Stop Unbound service: 'systemctl stop unbound'. 3. Install updated version using package manager or compile from source. 4. Start Unbound service: 'systemctl start unbound'.

🔧 Temporary Workarounds

Restrict DNS queries

all

Limit DNS queries to trusted sources to reduce attack surface

Configure access-control in unbound.conf: access-control: 192.168.1.0/24 allow

🧯 If You Can't Patch

Implement monitoring and automatic restart for Unbound service
Deploy redundant DNS resolvers to maintain service availability

🔍 How to Verify

Check if Vulnerable:

Check Unbound version: 'unbound -V' or 'unbound -h' and verify if version is below 1.9.5

Check Version:

unbound -V

Verify Fix Applied:

Run 'unbound -V' and confirm version is 1.9.5 or higher

📡 Detection & Monitoring

Log Indicators:

Unbound service crashes
Assertion failure messages in logs
DNS resolution failures

Network Indicators:

DNS query timeouts
Increased DNS failure rates

SIEM Query:

source="unbound.log" AND ("assertion failure" OR "crash" OR "segmentation fault")

📊 Metadata

CVE ID: CVE-2019-25036

CVSS v3 Score: 7.5 (HIGH)

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

CWE: CWE-617

Published: April 27, 2021

Last Updated: November 21, 2024

🔗 References

https://lists.debian.org/debian-lts-announce/2021/05/msg00007.html Mailing List,Third Party Advisory
https://ostif.org/our-audit-of-unbound-dns-by-x41-d-sec-full-results/ Patch,Third Party Advisory
https://security.netapp.com/advisory/ntap-20210507-0007/ Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/05/msg00007.html Mailing List,Third Party Advisory
https://ostif.org/our-audit-of-unbound-dns-by-x41-d-sec-full-results/ Patch,Third Party Advisory
https://security.netapp.com/advisory/ntap-20210507-0007/ Third Party Advisory

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🔗 Related Vulnerabilities

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