CVE-2017-9208 in QPDF
Summary
by MITRE
libqpdf.a in QPDF 6.0.0 allows remote attackers to cause a denial of service (infinite recursion and stack consumption) via a crafted PDF document, related to releaseResolved functions, aka qpdf-infiniteloop1.
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Analysis
by VulDB Data Team • 12/07/2022
The vulnerability identified as CVE-2017-9208 affects QPDF version 6.0.0 and represents a critical denial of service flaw that can be exploited remotely through crafted PDF documents. This issue stems from improper handling of recursive structures within the libqpdf.a library, specifically during the processing of releaseResolved functions. The vulnerability manifests as infinite recursion that leads to excessive stack consumption, ultimately causing the application to crash or become unresponsive. This type of vulnerability falls under the category of resource exhaustion attacks where an attacker can consume system resources indefinitely to disrupt service availability.
The technical implementation of this flaw occurs when the QPDF library processes PDF documents containing maliciously constructed recursive references within the releaseResolved functions. These functions are responsible for managing object references and memory cleanup within PDF structures, but they fail to properly validate or limit recursive traversal depth. When encountering specially crafted PDF files with circular references or deeply nested structures, the library enters infinite recursion loops that consume stack memory until system resources are exhausted. This behavior aligns with CWE-674, which describes "Uncontrolled Recursion" as a vulnerability where recursion is not properly bounded or checked. The flaw demonstrates a classic stack overflow scenario where function call depth exceeds available stack space, leading to program termination.
From an operational perspective, this vulnerability presents significant risks for systems that process untrusted PDF documents, particularly those serving web applications, email servers, or document management systems. Attackers can exploit this vulnerability by simply crafting a malicious PDF file and presenting it to a vulnerable system, requiring no privileged access or complex exploitation techniques. The impact extends beyond simple service disruption as the infinite recursion can cause complete system instability, especially in environments where multiple PDF processing operations occur simultaneously. The vulnerability is particularly concerning for automated systems that process large volumes of PDF documents, as a single malicious file can bring down entire processing pipelines. This type of attack maps to ATT&CK technique T1499.004, which covers "Resource Hijacking" through denial of service attacks that consume system resources.
Mitigation strategies for CVE-2017-9208 primarily involve upgrading to patched versions of QPDF where the recursive function calls have been properly bounded and validated. Organizations should implement input validation measures that limit the depth of recursive structures during PDF processing and consider implementing sandboxing techniques to isolate PDF processing operations. Network-level defenses can include implementing PDF content filtering and scanning mechanisms to identify and block potentially malicious documents before they reach vulnerable systems. Additionally, monitoring and alerting should be configured to detect unusual resource consumption patterns that may indicate exploitation attempts. The vulnerability highlights the importance of proper input validation and recursion bounds checking in security-critical libraries, emphasizing that even seemingly benign operations like document processing can become attack vectors when proper safeguards are absent. System administrators should also consider implementing rate limiting and resource quotas for PDF processing services to minimize the impact of successful exploitation attempts.