CVE-2020-7595: Xmlsoft Libxml2 DOS Vulnerability

CVE-2020-7595 Overview

CVE-2020-7595 is a Denial of Service (DoS) vulnerability in libxml2 version 2.9.10, affecting the xmlStringLenDecodeEntities function in parser.c. The vulnerability causes an infinite loop when processing certain malformed XML input that triggers a specific end-of-file (EOF) condition. This flaw allows remote attackers to cause a denial of service by providing specially crafted XML documents that exhaust system resources.

libxml2 is a widely used XML parsing library that provides APIs for reading, writing, and manipulating XML documents. The library is used extensively across enterprise applications, Linux distributions, and embedded systems, making this vulnerability particularly impactful.

Critical Impact
Remote attackers can cause complete service unavailability by triggering an infinite loop in XML parsing, leading to CPU exhaustion and denial of service conditions across affected systems.

Affected Products

xmlsoft libxml2 2.9.10
Fedora 30, 31, 32
Canonical Ubuntu Linux 12.04, 14.04, 16.04, 18.04, 19.10
Debian Linux 9.0
Siemens SINEMA Remote Connect Server
NetApp Clustered Data ONTAP, SMI-S Provider, SnapDrive, SteelStore Cloud Integrated Storage
NetApp H300S/H500S/H700S/H300E/H500E/H700E/H410S/H410C series firmware
Oracle Real User Experience Insight, Enterprise Manager Base Platform, Enterprise Manager Ops Center
Oracle MySQL Workbench, PeopleSoft Enterprise PeopleTools
Oracle Communications Cloud Native Core Network Function Cloud Native Environment

Discovery Timeline

January 21, 2020 - CVE-2020-7595 published to NVD
December 03, 2025 - Last updated in NVD database

Technical Details for CVE-2020-7595

Vulnerability Analysis

The vulnerability exists in the xmlStringLenDecodeEntities function within the parser.c source file of libxml2. This function is responsible for decoding XML entities within strings during the parsing process. When the parser encounters a specific end-of-file condition while processing entity references, it fails to properly handle the boundary case, resulting in an infinite loop.

The vulnerability is exploitable over the network without requiring any privileges or user interaction. An attacker can craft a malicious XML document that triggers the vulnerable code path, causing the parsing thread or process to enter an infinite loop. This results in complete CPU core exhaustion for the affected process, effectively causing a denial of service condition.

The impact is limited to availability—there is no confidentiality breach or integrity violation. However, given the widespread use of libxml2 in web services, application servers, and system utilities, the potential for service disruption is significant.

Root Cause

The root cause is an improper loop termination condition (CWE-835: Loop with Unreachable Exit Condition) in the entity decoding logic. When processing XML entity references, the function enters a loop to decode embedded entities. Under certain EOF conditions, the loop's exit condition is never satisfied, causing the function to iterate indefinitely.

The fix, implemented in GNOME libxml2 commit 0e1a49c89076, adds proper boundary checks to ensure the loop terminates correctly when unexpected EOF conditions are encountered during entity processing.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction:

Delivery: Attacker sends a crafted XML document to a target application that uses libxml2 for parsing
Trigger: The malformed XML contains entity references that trigger the EOF handling bug
Exploitation: The parser enters an infinite loop, consuming CPU resources
Impact: The affected service becomes unresponsive, causing denial of service

Attack scenarios include:

Web services accepting XML input (SOAP, REST APIs with XML payloads)
XML-RPC endpoints
Document processing applications
Configuration parsers that accept user-supplied XML

The vulnerability can be exploited remotely by any attacker who can submit XML content to an application using vulnerable libxml2 versions.

Detection Methods for CVE-2020-7595

Indicators of Compromise

Sustained high CPU utilization (100% on single core) by processes using libxml2
Processes handling XML parsing becoming unresponsive or hung
Application logs showing XML parsing operations that never complete
Service timeouts coinciding with receipt of XML documents from external sources

Detection Strategies

Monitor for processes with abnormally high CPU usage that are linked to libxml2 shared libraries
Implement application-level logging to track XML parsing duration and flag operations exceeding normal thresholds
Deploy network-based detection for malformed XML documents targeting known vulnerable patterns
Use software composition analysis (SCA) tools to identify systems running libxml2 version 2.9.10

Monitoring Recommendations

Configure resource monitoring alerts for sustained 100% CPU utilization on processes handling XML
Implement watchdog timers on XML parsing operations to detect and terminate hung processes
Monitor application health endpoints for unresponsive services following XML processing
Review system logs for repeated process restarts indicating DoS conditions

How to Mitigate CVE-2020-7595

Immediate Actions Required

Upgrade libxml2 to a patched version that includes commit 0e1a49c89076
Apply vendor-specific security patches for affected operating systems and applications
Implement input validation to reject malformed XML documents before parsing
Configure resource limits (CPU, memory, timeouts) for XML processing operations

Patch Information

The vulnerability has been addressed through multiple vendor patches:

libxml2: The fix is available in GNOME libxml2 commit 0e1a49c89076
Ubuntu: Addressed in Ubuntu Security Notice USN-4274-1
Debian: Fixed in Debian LTS Security Announcement
Fedora: Updates available via Fedora package announcements
Oracle: Addressed in Oracle Critical Patch Updates
NetApp: See NetApp Security Advisory ntap-20200702-0005
Siemens: See Siemens Product Security Advisory SSA-292794
CISA: Industrial control systems guidance available in CISA ICS Advisory ICSA-21-103-08

Workarounds

Implement XML parsing timeouts to automatically terminate operations exceeding expected duration
Use process isolation (containers, sandboxing) to limit the impact of resource exhaustion
Deploy rate limiting on endpoints accepting XML input to reduce attack surface
Consider using alternative XML parsing libraries with proper resource management until patches can be applied
Implement input size limits and schema validation to reject potentially malicious XML documents

bash

# Configuration example - Set process resource limits
# Add to /etc/security/limits.conf to limit CPU time for processes
# handling XML parsing operations
xmluser         hard    cpu             60
xmluser         soft    cpu             30

# For systemd services, add to the service unit file:
# [Service]
# CPUQuota=50%
# TimeoutSec=30