CVE-2016-5300 in iTunes
Summary
by MITRE
The XML parser in Expat does not use sufficient entropy for hash initialization, which allows context-dependent attackers to cause a denial of service (CPU consumption) via crafted identifiers in an XML document. NOTE: this vulnerability exists because of an incomplete fix for CVE-2012-0876.
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Analysis
by VulDB Data Team • 11/24/2022
The vulnerability described in CVE-2016-5300 affects the Expat XML parser, a widely used open-source library for parsing XML documents across numerous applications and systems. This issue represents a sophisticated denial of service vulnerability that exploits weaknesses in the hash table implementation used by the parser to store and manage XML element and attribute names. The vulnerability stems from insufficient entropy during hash initialization, creating predictable hash values that can be exploited by malicious actors to manipulate the parser's performance characteristics.
The technical flaw resides in the hash table collision handling mechanism within Expat's XML parsing engine. When parsing XML documents, the parser creates hash tables to efficiently store and retrieve element and attribute names. In this case, the hash initialization process does not incorporate sufficient randomness or entropy, making it possible for attackers to craft specific XML documents containing identifiers that deliberately cause hash collisions. This weakness allows attackers to create XML content where multiple elements or attributes map to the same hash bucket, resulting in degraded performance as the parser must handle linear collision resolution instead of efficient O(1) hash lookups.
The operational impact of this vulnerability extends beyond simple resource exhaustion, as it can be leveraged to consume excessive CPU cycles and memory resources. When an attacker submits a crafted XML document designed to trigger hash collisions, the parser's performance degrades significantly, potentially leading to complete system unresponsiveness or application crashes. This vulnerability is particularly dangerous because it affects applications that process untrusted XML input from external sources, such as web services, content management systems, and various enterprise applications that rely on Expat for XML processing. The vulnerability exists because of an incomplete fix for CVE-2012-0876, indicating that previous attempts to address similar hash collision issues were insufficient and left residual weaknesses in the implementation.
From a cybersecurity perspective, this vulnerability aligns with CWE-330, which describes insufficient entropy in cryptographic or security-related functions, and represents a classic example of a resource exhaustion attack that can be classified under ATT&CK technique T1496. The vulnerability demonstrates how seemingly minor implementation details in security-critical libraries can create significant operational risks. Organizations using Expat in their applications face substantial risk from this vulnerability, as it can be exploited remotely without requiring authentication or special privileges, making it particularly dangerous in internet-facing applications. The impact is exacerbated by the fact that many applications and systems use Expat as a foundational component for XML processing, meaning that exploitation could affect a wide range of services and platforms. Mitigation strategies include updating to patched versions of Expat, implementing input validation and size limits for XML documents, and deploying application firewalls or intrusion detection systems that can identify and block suspicious XML patterns. The vulnerability serves as a critical reminder of the importance of proper entropy in cryptographic and hash-related functions, and highlights the need for thorough testing and validation of security fixes to prevent similar issues from persisting across multiple versions of software components.