CVE-2017-18186 in QPDF
Summary
by MITRE
An issue was discovered in QPDF before 7.0.0. There is an infinite loop due to looping xref tables in QPDF.cc.
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Analysis
by VulDB Data Team • 02/06/2023
The vulnerability identified as CVE-2017-18186 represents a critical denial of service flaw in QPDF versions prior to 7.0.0. This issue stems from improper handling of cross-reference tables within PDF documents, specifically when these tables contain circular references or malformed structures that cause the processing logic to enter an infinite loop. The vulnerability affects the core PDF parsing functionality of the QPDF library, which is widely used for PDF manipulation and validation across various security tools and applications. When a maliciously crafted PDF document with problematic xref table structures is processed by an affected version of QPDF, the library's internal loop detection mechanisms fail to properly identify and terminate the infinite iteration, leading to complete system resource exhaustion and application hang.
The technical root cause of this vulnerability aligns with CWE-835, which describes the weakness of an infinite loop or infinite recursion that occurs without proper termination conditions. The flaw manifests in the QPDF.cc source file where the cross-reference table parsing logic does not adequately validate the structural integrity of xref entries before entering processing loops. When encountering circular references within the xref table, the parser continues to iterate through the same set of entries indefinitely, consuming CPU cycles and memory resources until the system becomes unresponsive. This behavior represents a classic example of a control flow flaw where the loop termination conditions are insufficient to handle malformed input data structures, particularly those that contain self-referencing or circular pointers within the PDF's internal indexing mechanism.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as it can be exploited in various attack vectors that leverage PDF processing capabilities. Systems that automatically process or validate PDF documents, including email security gateways, document management systems, and security scanning tools that rely on QPDF for PDF analysis, become vulnerable to this attack. The infinite loop condition causes the affected applications to become unresponsive, potentially leading to complete system crashes or resource exhaustion that can be leveraged by attackers to perform resource exhaustion attacks. This vulnerability is particularly concerning in environments where PDF processing is automated or occurs in high-volume scenarios, as it can effectively bring down entire document processing pipelines and impact service availability for legitimate users.
Mitigation strategies for CVE-2017-18186 primarily involve upgrading to QPDF version 7.0.0 or later, which includes proper loop detection and termination mechanisms for xref table processing. Security administrators should also implement input validation measures that scan PDF documents for suspicious xref table structures before processing them through QPDF libraries. Additionally, organizations should consider implementing resource limits and timeouts for PDF processing operations to prevent complete system exhaustion even if the vulnerability is not patched immediately. From an ATT&CK framework perspective, this vulnerability maps to the technique T1499.004 - Network Denial of Service, as it can be used to exhaust system resources and render services unavailable. The vulnerability also relates to T1566.001 - Phishing, since attackers could potentially craft malicious PDF documents to exploit this weakness in email security systems. Organizations should also consider implementing sandboxing techniques for PDF processing and monitoring for unusual CPU usage patterns that might indicate exploitation attempts.