CVE-2017-9233 in tvOS
Summary
by MITRE
XML External Entity vulnerability in libexpat 2.2.0 and earlier (Expat XML Parser Library) allows attackers to put the parser in an infinite loop using a malformed external entity definition from an external DTD.
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Analysis
by VulDB Data Team • 01/20/2021
The CVE-2017-9233 vulnerability represents a critical XML External Entity processing flaw within the Expat XML Parser Library version 2.2.0 and earlier. This vulnerability specifically targets the parser's handling of external entity definitions within Document Type Definitions, creating a potential denial of service condition that can be exploited by malicious actors. The flaw resides in the library's inability to properly validate and process malformed external entity references, which can lead to resource exhaustion and system instability. The vulnerability impacts a wide range of applications that rely on Expat for XML parsing operations, making it particularly concerning for web services, enterprise applications, and security-critical systems that process untrusted XML data.
The technical implementation of this vulnerability stems from the parser's failure to detect and prevent infinite recursion patterns when processing external entity declarations. When an attacker crafts a malicious XML document containing a malformed external entity definition that references an external DTD, the parser attempts to resolve these references in a manner that can lead to infinite loops. This occurs because the parser does not implement adequate recursion depth limiting or cycle detection mechanisms for external entity resolution. The flaw is classified under CWE-611 as an Improper Restriction of XML External Entity Reference, which directly relates to the insecure handling of external resources during XML parsing operations. The vulnerability operates at the application layer and can be exploited through various attack vectors including web services, file uploads, and API endpoints that accept XML input.
The operational impact of CVE-2017-9233 extends beyond simple denial of service conditions to potentially enable more sophisticated attacks depending on the system architecture. An attacker can exploit this vulnerability to consume excessive CPU cycles and memory resources, effectively causing a denial of service that disrupts legitimate system operations. The infinite loop condition can persist for extended periods, making it difficult to detect and mitigate. This vulnerability particularly affects systems where XML parsing is performed on untrusted input, such as web applications accepting user-submitted XML data, web services processing XML payloads, and enterprise applications handling XML-based configuration files. The vulnerability can be leveraged in distributed denial of service scenarios and may be combined with other attacks to amplify their impact, particularly in environments where multiple services rely on the same vulnerable XML parsing library.
Mitigation strategies for CVE-2017-9233 primarily involve upgrading to Expat version 2.2.1 or later, which includes fixes for the infinite loop condition in external entity processing. Organizations should implement comprehensive patch management procedures to ensure all systems using Expat are updated promptly. Additional defensive measures include implementing strict XML parsing configurations that disable external entity resolution entirely, configuring input validation filters to reject suspicious XML constructs, and deploying network monitoring solutions to detect unusual resource consumption patterns. The vulnerability aligns with ATT&CK technique T1210 for Exploitation of Remote Services and T1499 for Endpoint Denial of Service, highlighting the need for layered security approaches. Security teams should also consider implementing application firewalls and XML-specific security controls to prevent exploitation attempts. Regular security assessments and vulnerability scanning should be conducted to identify systems running vulnerable versions of Expat, and automated patch deployment systems should be implemented to minimize exposure windows.