CVE-2018-20843 in WebLogic Server Proxy Plug-In
Summary
by MITRE
In libexpat in Expat before 2.2.7, XML input including XML names that contain a large number of colons could make the XML parser consume a high amount of RAM and CPU resources while processing (enough to be usable for denial-of-service attacks).
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Analysis
by VulDB Data Team • 05/30/2025
The vulnerability identified as CVE-2018-20843 affects the libexpat XML parsing library, specifically targeting versions prior to 2.2.7. This issue represents a classic denial-of-service attack vector that exploits the parser's handling of malformed XML input containing excessive colons within XML names. The vulnerability stems from insufficient input validation and resource management within the XML parsing algorithm, creating a scenario where an attacker can craft malicious XML documents designed to trigger excessive computational overhead.
The technical flaw manifests when the XML parser encounters XML names containing an excessive number of colons, which causes the parser to allocate disproportionate amounts of memory and processing cycles. This occurs because the parser's internal algorithms for handling XML names must process each colon character individually, leading to exponential growth in memory allocation and CPU usage. The parser's resource consumption grows rapidly with the number of colons present, creating a situation where even moderately sized input can cause significant system strain. This behavior aligns with CWE-400, which categorizes the vulnerability as an unchecked resource consumption issue, and reflects the broader category of resource exhaustion attacks.
The operational impact of this vulnerability is substantial for systems relying on XML processing, particularly those exposed to untrusted input from external sources. Attackers can exploit this weakness by submitting carefully crafted XML documents containing thousands or even millions of colons within XML names, causing the target system to consume excessive RAM and CPU resources. This resource exhaustion can lead to complete system unresponsiveness, application crashes, or cascading failures in systems where multiple XML parsers are running concurrently. The vulnerability is particularly dangerous in web applications, API endpoints, and middleware systems that process XML data from diverse sources without proper input sanitization.
Mitigation strategies for CVE-2018-20843 primarily focus on updating to libexpat version 2.2.7 or later, which includes fixes for the resource consumption issue. Organizations should also implement input validation measures that limit the length and complexity of XML names, particularly those containing special characters like colons. Network-level protections such as rate limiting and resource quotas can help prevent exploitation attempts from consuming excessive system resources. Additionally, implementing proper XML parsing configurations that disable unnecessary features and set strict limits on document size and nesting levels provides additional defense-in-depth measures. The vulnerability demonstrates the importance of adhering to security best practices outlined in the ATT&CK framework, specifically targeting the resource exhaustion techniques used in denial-of-service attacks and emphasizing the need for proper input validation and resource management controls.