CVE-2023-52425 Vulnerability

Published: 2/20/2025

Red caution sign overlaid on a picture of a laptop with a hand on the trackpad

CVEs are Common Vulnerabilities and Exposures - unique identifiers assigned to publicly known cybersecurity vulnerabilities.

CVE-2023-52425 is a high-severity Denial of Service (DoS) vulnerability affecting libexpat, one of the most fundamental XML parsing libraries used in software development.

Because libexpat is embedded in countless applications—including Python, Node.js, Apache, and major Linux distributions—this vulnerability has a massive attack surface. It allows attackers to crash services by feeding them specially crafted, resource-intensive XML files.

This page explains how this "re-parsing" flaw works, which systems are at risk, and how to secure your infrastructure.

What is CVE-2023-52425 vulnerability?

CVE-2023-52425 is an Uncontrolled Resource Consumption vulnerability (CWE-400) in the libexpat library (versions up to 2.5.0).

The flaw exists in how the library handles large XML tokens. When parsing a token that spans multiple memory buffers, libexpat is forced to re-parse the data from the beginning repeatedly. An attacker can exploit this by sending an XML file containing extremely large or complex tokens, triggering an exponential spike in CPU and memory usage.

This effectively freezes or crashes the application processing the XML, resulting in a Denial of Service.

When was it discovered?

The vulnerability was publicly disclosed on February 4, 2024. It was addressed with the release of libexpat version 2.6.0, which introduced better heuristics to prevent excessive re-parsing.

Affected products & versions

Because libexpat is a dependency for thousands of software projects, the list of affected products is extensive. Key affected platforms include:

Product	Versions Affected	Fixed Versions / Patches
libexpat (Upstream)	Versions 2.5.0 and earlier	2.6.0 or later
Python	Versions prior to 3.8.19, 3.9.19, 3.10.14	Python 3.10.14+, 3.11+
Debian Linux	Debian 10 (Buster), 11 (Bullseye), 12 (Bookworm)	Updated expat packages (Feb 2024)
Ubuntu Linux	16.04 ESM, 18.04 ESM, 20.04 LTS, 22.04 LTS	Updated expat packages
Amazon Linux	AL1, AL2 (Core)	AL2023 patched in Nov 2024

CVE-2023-52425 technical description

The vulnerability is a variation of an algorithmic complexity attack.

When libexpat parses an XML file, it reads data into a buffer. If a single XML token (like a very long attribute value or tag name) is larger than the buffer, the parser must "grow" the buffer and restart the parsing of that token.

In vulnerable versions, an attacker can craft a token that forces this "fill buffer -> fail -> re-parse" cycle to happen thousands of times. This consumes vast amounts of CPU cycles relative to the size of the input file. A relatively small malicious file (kilobytes in size) can tie up a server's resources for minutes or hours, denying access to legitimate users.

Tactics, techniques & procedures (TTPs)

Attackers exploit this vulnerability to disrupt services (Availability impact).

Delivery: Attackers submit malicious XML files to web applications that accept XML input (e.g., SOAP endpoints, file uploaders, configuration importers).
Exploitation: The payload typically contains XML elements with incredibly long names or attribute values designed to span multiple parser buffers.
Impact: The target service becomes unresponsive. In cloud environments, this can also lead to financial impact by spiking resource consumption (CPU autoscaling) before the service eventually crashes.

Indicators of compromise

System Performance: A sudden, unexplained spike in CPU usage (often 100%) on web servers or application servers handling XML data.
Application Logs: "Timeout" errors or "Out of Memory" (OOM) crashes in applications that rely on XML parsing (like Python apps or Apache).
Network Traffic: Repeated HTTP POST requests containing XML data from a single source, followed immediately by server unresponsiveness.

Known proof-of-concepts & exploits

While there are no famous "named" malware campaigns (like ransomware) specifically leveraging this bug yet, the exploit method is well-understood.

Proof-of-Concept (PoC) scripts demonstrating the "billion laughs" style expansion or large-token crashes are available in security research repositories. Security tools and scanners have integrated checks for this vulnerability, making it easy for both defenders and attackers to identify vulnerable systems.

How to detect CVE-2023-52425 vulnerability?

Software Composition Analysis (SCA): Scan your codebases and containers for outdated versions of libexpat or python.
Vulnerability Scanning: Tools like Tenable Nessus, Qualys, and Amazon Inspector can identify vulnerable packages on Linux hosts.
Endpoint Detection: Use Managed EDR to monitor for abnormal process behavior, such as a web server process (httpd, python, node) unexpectedly consuming 100% CPU for extended periods.

Impact & risk of CVE-2023-52425 vulnerability

The risk is categorized as High (CVSS 7.5).

While it does not allow for Remote Code Execution (RCE) or data theft, the Availability risk is severe. For critical infrastructure or high-traffic web services, a successful DoS attack can cause significant operational downtime and financial loss.

Mitigation & remediation strategies

Patching is the only definitive solution.

Update libexpat: Upgrade the system-wide libexpat library to version 2.6.0 or later immediately.
Update Runtime Environments: Update Python, Node.js, and other runtime environments that bundle their own copies of libexpat.
Input Validation: As a temporary mitigation, configure your Web Application Firewall (WAF) or application logic to reject XML files that exceed a specific size or token length, though this is difficult to implement perfectly.
Resource Limits: Configure CPU and memory limits (e.g., in Docker or Kubernetes) for services that parse XML to prevent a single crashed process from taking down the entire host.

CVE-2023-52425 Vulnerability FAQs

It is a Denial of Service vulnerability in the libexpat XML parser. Attackers use specially formatted XML files with large tokens to force the parser into an infinite loop of re-parsing, consuming all available CPU and memory.

It is not a virus or malware, so it doesn't "infect" files. Instead, it is an exploit used to crash a system. An attacker sends a bad file, and the system crashes while trying to read it.

Yes. Because libexpat is buried deep in many software dependencies (OS-level and application-level), many systems remain unpatched. It remains a viable vector for attackers wishing to take a service offline.

The primary protection is to apply security updates to your Operating System (Linux/Windows) and language runtimes (Python, etc.) to ensure they are using libexpat version 2.6.0 or newer.