CVE-2021-3516
📋 TL;DR
CVE-2021-3516 is a use-after-free vulnerability in libxml2's xmllint tool that allows attackers to execute arbitrary code or cause denial of service by submitting specially crafted XML files. This affects systems running xmllint versions before 2.9.11, particularly those that process untrusted XML input. The vulnerability impacts confidentiality, integrity, and availability of affected systems.
💻 Affected Systems
- libxml2
- xmllint
- applications using libxml2 library
📦 What is this software?
Clustered Data Ontap Antivirus Connector by Netapp
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Fedora by Fedoraproject
Fedora by Fedoraproject
Ontap Select Deploy Administration Utility by Netapp
View all CVEs affecting Ontap Select Deploy Administration Utility →
Xmllint by Xmlsoft
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution leading to complete system compromise, data theft, and persistent backdoor installation.
Likely Case
Application crash or denial of service affecting XML processing functionality.
If Mitigated
Limited impact if proper input validation and sandboxing are implemented.
🎯 Exploit Status
Proof-of-concept available in bug reports; exploitation requires ability to submit crafted XML to xmllint.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: libxml2 2.9.11 and later
Vendor Advisory: https://bugzilla.redhat.com/show_bug.cgi?id=1954225
Restart Required: No
Instructions:
1. Update libxml2 package to version 2.9.11 or later. 2. For RHEL/CentOS: 'yum update libxml2'. 3. For Debian/Ubuntu: 'apt update && apt upgrade libxml2'. 4. Rebuild any applications statically linked with libxml2.
🔧 Temporary Workarounds
Disable xmllint processing of untrusted input
allRestrict xmllint usage to trusted XML sources only
Implement XML input validation
allAdd XML schema validation and size limits before processing
🧯 If You Can't Patch
- Isolate xmllint usage to secure environments with no network access
- Implement strict access controls and monitoring on systems using xmllint
🔍 How to Verify
Check if Vulnerable:
Check libxml2 version: 'xmllint --version' or 'rpm -q libxml2' or 'dpkg -l libxml2'Check Version:
xmllint --versionVerify Fix Applied:
Confirm version is 2.9.11 or later: 'xmllint --version | grep -q "2.9.1[1-9]\|2.9.[2-9]\|2.[1-9][0-9]" && echo "Patched"'📡 Detection & Monitoring
Log Indicators:
- Unexpected xmllint process crashes
- Large or malformed XML file processing
- Memory access violation errors
Network Indicators:
- Unusual XML file uploads to systems using xmllint
- XML processing from untrusted sources
SIEM Query:
process_name:"xmllint" AND (event_type:"crash" OR memory_violation:"true")🔗 References
- https://bugzilla.redhat.com/show_bug.cgi?id=1954225
- https://gitlab.gnome.org/GNOME/libxml2/-/commit/1358d157d0bd83be1dfe356a69213df9fac0b539
- https://gitlab.gnome.org/GNOME/libxml2/-/issues/230
- https://lists.debian.org/debian-lts-announce/2021/05/msg00008.html
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/BZOMV5J4PMZAORVT64BKLV6YIZAFDGX6/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QVM4UJ3376I6ZVOYMHBNX4GY3NIV52WV/
- https://security.gentoo.org/glsa/202107-05
- https://security.netapp.com/advisory/ntap-20210716-0005/
- https://www.oracle.com/security-alerts/cpujan2022.html
- https://bugzilla.redhat.com/show_bug.cgi?id=1954225
- https://gitlab.gnome.org/GNOME/libxml2/-/commit/1358d157d0bd83be1dfe356a69213df9fac0b539
- https://gitlab.gnome.org/GNOME/libxml2/-/issues/230
- https://lists.debian.org/debian-lts-announce/2021/05/msg00008.html
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/BZOMV5J4PMZAORVT64BKLV6YIZAFDGX6/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/QVM4UJ3376I6ZVOYMHBNX4GY3NIV52WV/
- https://security.gentoo.org/glsa/202107-05
- https://security.netapp.com/advisory/ntap-20210716-0005/
- https://www.oracle.com/security-alerts/cpujan2022.html
