CVE-2026-76956 in Libexpat
Summary
by MITRE • 08/20/2026
In libexpat 2.8.2 and 2.8.3 before 2.8.4, misinterpretation of getentropy's return code leads to insufficient entropy, which results in being vulnerable to hash flooding attacks, causing a denial of service via crafted XML content.
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Analysis
by VulDB Data Team • 08/20/2026
The vulnerability identified in libexpat versions 2.8.2 and 2.8.3 stems from an improper handling of the return value returned by the getentropy system call during the initialization of random number generation for hash table seeding. In secure software design, particularly within parsers that process untrusted input like XML, it is critical to ensure that internal data structures are randomized to prevent predictable memory layouts. This randomness serves as a primary defense against hash flooding attacks, where an attacker deliberately crafts inputs designed to cause excessive collisions in hash tables, thereby degrading performance from average case linear time complexity to worst-case quadratic or worse. The specific flaw arises because the code fails to correctly interpret the return status of getentropy, which is responsible for providing cryptographically secure random bytes. When this function returns a non-zero value indicating an error, such as insufficient entropy availability on the host system, the library does not adequately handle this failure state. Instead of aborting or falling back to a safer alternative with explicit warning, it proceeds with potentially low-entropy or static seed values derived from flawed logic.
This misinterpretation directly compromises the effectiveness of hash randomization mechanisms employed by libexpat. Hash flooding is a well-documented class of denial-of-service attacks that target applications using hash-based data structures without adequate entropy for key selection. By exploiting this weakness, an attacker can construct malicious XML documents containing specific patterns of elements and attributes that trigger maximum collision rates within the parser's internal hash tables. As the parser processes these crafted inputs, the computational cost increases exponentially due to the repeated resolution of collisions in linked lists or similar structures used by many standard library implementations. This results in significant CPU consumption and latency spikes, effectively rendering the application unresponsive or causing it to crash under load. The impact is particularly severe for services that parse XML from external sources without strict input validation prior to parsing, such as web applications, API gateways, and document processing systems.
From a classification perspective, this vulnerability aligns with CWE-203: Observable Discrepancy, where the system's behavior differs based on internal state or error conditions in a way that can be exploited by an attacker. It also relates to CWE-754: Improper Check for Unusual or Exceptional Conditions, as the software fails to properly validate and handle exceptional return codes from critical security functions like getentropy. Furthermore, this flaw facilitates attacks categorized under MITRE ATT&CK technique T1496: Resource Hijacking, specifically through CPU exhaustion via algorithmic complexity attacks. The lack of robust error handling for entropy sources represents a fundamental weakness in the application's resilience against targeted denial-of-service efforts that do not require complex exploitation chains but rely solely on precise input construction to trigger resource exhaustion.
Mitigation strategies must focus on both immediate patching and long-term architectural improvements. The primary remediation is to upgrade libexpat to version 2.8.4 or later, where the handling of getentropy return codes has been corrected to ensure that random number generation fails safely when entropy cannot be securely obtained. In environments where upgrading immediately is not feasible, administrators should implement input validation layers at the network perimeter to filter out suspicious XML patterns known to trigger hash collisions before they reach the parser. Additionally, deploying rate-limiting and resource monitoring tools can help detect and mitigate denial-of-service attempts in real-time by identifying sudden spikes in CPU usage associated with parsing operations. Developers integrating libexpat should also ensure that their build environments provide adequate entropy sources, such as /dev/urandom on Linux systems or BCryptGenRandom on Windows, to minimize the likelihood of encountering low-entropy conditions during initialization. Regular security audits focusing on error handling paths for cryptographic primitives are essential to prevent similar oversights in future updates.