CVE-2019-25038 – CVE-2019-25038 is an integer overflow vulnerabi... (How to Fix)

Q: What is the severity of CVE-2019-25038?

CVE-2019-25038 has a CVSS score of 9.8 (CRITICAL). CVE-2019-25038 is an integer overflow vulnerability in Unbound DNS resolver's dnscrypt component that could allow memory corruption. The vulnerability affects Unbound installations before version 1.9.5, though the vendor disputes exploitability in real-world deployments.

📋 TL;DR

CVE-2019-25038 is an integer overflow vulnerability in Unbound DNS resolver's dnscrypt component that could allow memory corruption. The vulnerability affects Unbound installations before version 1.9.5, though the vendor disputes exploitability in real-world deployments.

💻 Affected Systems

Products:

Unbound DNS resolver

Versions: All versions before 1.9.5

Operating Systems: Linux, BSD, Unix-like systems

Default Config Vulnerable: ✅ No

Notes: Only vulnerable when dnscrypt feature is enabled and configured

📦 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

Remote code execution leading to complete system compromise of the DNS resolver server

🟠

Likely Case

Denial of service through Unbound crash or instability

🟢

If Mitigated

No impact if dnscrypt is not enabled or proper memory protections are in place

🌐 Internet-Facing: MEDIUM - Only affects systems with dnscrypt enabled and exposed to untrusted networks

🏢 Internal Only: LOW - Requires dnscrypt configuration and specific conditions to trigger

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: NO

Unauthenticated Exploit: ⚠️ Yes

Complexity: HIGH

Vendor disputes exploitability in running installations; theoretical vulnerability requires specific dnscrypt configuration

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 1.9.5 and later

Vendor Advisory: https://nlnetlabs.nl/downloads/unbound/CVE-2019-25038.txt

Restart Required: Yes

Instructions:

1. Download Unbound 1.9.5 or later from nlnetlabs.nl 2. Stop Unbound service 3. Install updated version 4. Restart Unbound service

🔧 Temporary Workarounds

Disable dnscrypt

all

Disable the dnscrypt feature if not required

Edit unbound.conf and remove or comment dnscrypt configuration
Set 'dnscrypt: no' in unbound.conf

🧯 If You Can't Patch

Disable dnscrypt feature entirely
Implement network segmentation to limit access to Unbound service

🔍 How to Verify

Check if Vulnerable:

Check Unbound version with 'unbound -V' and verify if below 1.9.5 and dnscrypt is enabled in configuration

Check Version:

unbound -V

Verify Fix Applied:

Verify version is 1.9.5 or higher with 'unbound -V' and check service is running

📡 Detection & Monitoring

Log Indicators:

Unbound crash logs
Memory allocation errors in system logs
DNS service interruption events

Network Indicators:

Unusual dnscrypt traffic patterns
DNS resolution failures

SIEM Query:

source="unbound.log" AND ("crash" OR "segmentation fault" OR "memory error")

📊 Metadata

CVE ID: CVE-2019-25038

CVSS v3 Score: 9.8 (CRITICAL)

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

CWE: CWE-190

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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