CVE-2019-15903 in Outside In Technology
Summary
by MITRE
In libexpat before 2.2.8, crafted XML input could fool the parser into changing from DTD parsing to document parsing too early; a consecutive call to XML_GetCurrentLineNumber (or XML_GetCurrentColumnNumber) then resulted in a heap-based buffer over-read.
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Analysis
by VulDB Data Team • 05/30/2025
The vulnerability identified as CVE-2019-15903 represents a critical heap-based buffer over-read flaw in the libexpat XML parser library affecting versions prior to 2.2.8. This vulnerability stems from improper handling of XML document structure during parsing operations, specifically when the parser encounters certain crafted input that manipulates the internal state machine controlling parsing mode transitions. The issue manifests when maliciously constructed XML content causes the parser to prematurely transition from Document Type Definition (DTD) parsing mode to regular document parsing mode, creating a dangerous state where subsequent parsing operations can access memory beyond allocated buffer boundaries.
The technical exploitation of this vulnerability occurs through carefully crafted XML input that manipulates the parser's internal state management. When the parser processes such input, it fails to properly validate the transition between parsing modes, allowing a subsequent call to XML_GetCurrentLineNumber or XML_GetCurrentColumnNumber functions to access memory locations that have already been freed or are otherwise inaccessible. This heap-based buffer over-read represents a classic memory safety vulnerability that can lead to arbitrary code execution, information disclosure, or denial of service conditions depending on the specific memory layout and execution environment. The vulnerability aligns with CWE-125: Out-of-bounds Read, which specifically addresses situations where applications read memory beyond the bounds of allocated buffers, and falls under the ATT&CK technique T1203: Exploitation for Client Execution when exploited in web applications or services that process XML input.
The operational impact of CVE-2019-15903 extends across numerous applications and systems that rely on libexpat for XML processing, including web servers, application frameworks, and enterprise software solutions. Attackers can leverage this vulnerability by submitting malicious XML content to applications that utilize the affected parser, potentially leading to complete system compromise or data breaches. The vulnerability is particularly concerning because it can be exploited through common web application attack vectors such as file uploads, XML-based web services, or API endpoints that accept structured data input. Organizations using affected versions of libexpat should prioritize immediate patching, as the vulnerability can be exploited remotely without authentication and can result in significant security breaches. The remediation strategy involves upgrading to libexpat version 2.2.8 or later, which includes proper validation of parsing state transitions and memory access controls that prevent the buffer over-read condition from occurring.