CVE-2026-50219 in libexpat
Summary
by MITRE • 06/04/2026
libexpat before 2.8.2 lacks handler call depth tracking for calls to XML_GetBuffer, XML_Parse, XML_ParseBuffer, XML_ParserFree, or XML_ParserReset from within handlers in cases of a policy violation. Thus, a use-after-free can occur,
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Analysis
by VulDB Data Team • 06/05/2026
The vulnerability in libexpat versions prior to 2.8.2 represents a critical security flaw that stems from inadequate handler call depth tracking mechanisms within the XML parsing library. This issue specifically manifests when parsing operations involve calls to XML_GetBuffer, XML_Parse, XML_ParseBuffer, XML_ParserFree, or XML_ParserReset functions from within handler callbacks during policy violation scenarios. The absence of proper depth tracking creates a condition where recursive or nested handler invocations can lead to improper memory management states.
When an XML parser encounters malformed input or violates established parsing policies, the library's internal state tracking fails to properly monitor the nesting level of handler calls. This deficiency allows for situations where a handler function may attempt to free or reset parser resources while other handler functions are still actively executing or have not yet completed their execution cycle. The lack of depth tracking means that the parser cannot accurately determine whether it is safe to perform resource cleanup operations or whether such operations would result in accessing memory that has already been freed.
The operational impact of this vulnerability extends beyond simple memory corruption, as it creates a use-after-free condition that can be exploited by malicious actors to execute arbitrary code or cause denial-of-service scenarios. Attackers can craft specially crafted XML documents that trigger policy violations during parsing, subsequently leveraging the improper memory management to manipulate the parser's internal state. This vulnerability is particularly dangerous because it operates at the core parsing layer where applications process untrusted input, making it a prime target for exploitation in web applications, XML processing services, and any system that relies on XML data interchange.
From a cybersecurity perspective, this vulnerability aligns with common attack patterns documented in the attack tactics and techniques framework, particularly those involving memory corruption and privilege escalation. The flaw demonstrates characteristics consistent with CWE-415 which addresses double free conditions and CWE-416 which covers use after free vulnerabilities. The vulnerability represents a classic example of how inadequate state management in parsing libraries can create exploitable conditions that bypass normal security controls. Organizations using vulnerable versions of libexpat face significant risk as this flaw can be leveraged to compromise systems through XML external entity (XXE) attacks or other injection-based exploits that target the underlying parsing infrastructure.
Mitigation strategies for this vulnerability require immediate patching of libexpat to version 2.8.2 or later, which implements proper handler call depth tracking to prevent recursive calls from interfering with resource management operations. Additionally, organizations should implement input validation measures that reduce the likelihood of triggering policy violations during parsing operations, including XML schema validation and strict input sanitization. Network segmentation and application firewalls can help limit the attack surface by restricting access to XML processing endpoints. Regular security assessments and vulnerability scanning should be conducted to identify systems running outdated versions of the library, while also implementing monitoring for unusual parsing behavior that might indicate exploitation attempts. The fix addresses the root cause by ensuring that parser resources are only freed or reset when no handler functions are active, thereby preventing the use-after-free condition that enables exploitation.