CVE-2018-18651 in Xpdf
Summary
by MITRE
An issue was discovered in Xpdf 4.00. catalog->getNumPages() in AcroForm.cc allows attackers to launch a denial of service (hang caused by large loop) via a specific pdf file, as demonstrated by pdftohtml. This is mainly caused by a large number after the /Count field in the file.
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Analysis
by VulDB Data Team • 06/03/2023
The vulnerability CVE-2018-18651 represents a denial of service flaw in Xpdf 4.00 that specifically targets the AcroForm.cc component within the library's catalog handling functionality. This issue manifests when the catalog->getNumPages() function processes maliciously crafted PDF files that contain excessive numerical values following the /Count field. The flaw occurs during the parsing of interactive form elements within PDF documents, where the software fails to properly validate or limit the magnitude of numeric values that can appear in the document structure.
The technical implementation of this vulnerability stems from inadequate input validation within the PDF parsing engine. When pdftohtml or similar tools process a specially crafted PDF file, the software enters an infinite or extremely prolonged loop as it attempts to interpret the exaggerated count value that follows the /Count field in the document catalog. This behavior creates a condition where the application becomes unresponsive and effectively hangs, consuming system resources without making meaningful progress. The vulnerability specifically targets the AcroForm.cc file which handles interactive form elements, making it particularly dangerous in environments where PDF processing is automated or where users might encounter untrusted documents.
From an operational perspective, this vulnerability presents significant risks in automated PDF processing environments, web applications that convert PDFs to HTML, and document management systems that rely on Xpdf libraries. Attackers can exploit this flaw by crafting malicious PDF files that contain oversized count values, causing legitimate applications to hang indefinitely. The impact extends beyond simple service disruption as the hanging process can consume substantial CPU and memory resources, potentially leading to system instability or resource exhaustion attacks. This vulnerability is particularly concerning in server environments where multiple PDF processing tasks might be queued, as a single malicious document could block the entire processing pipeline and affect other legitimate users.
The root cause of this vulnerability aligns with CWE-835, which addresses the issue of infinite loops or excessive iteration without proper termination conditions. This weakness in the Xpdf library's input validation mechanism creates a path for attackers to manipulate the parsing behavior through crafted PDF structures. The vulnerability demonstrates poor adherence to defensive programming practices where numerical inputs are not properly bounded or validated before processing. In the context of the MITRE ATT&CK framework, this vulnerability could be categorized under T1499.004 for Network Denial of Service, as it allows attackers to consume system resources through malformed input, though the attack vector is more specifically related to application-level input processing rather than network-level attacks.
The recommended mitigations for this vulnerability include immediate patching of Xpdf libraries to version 4.01 or later, which contains the necessary fixes for proper input validation. Organizations should implement input sanitization measures that validate numeric values within PDF documents, particularly those following the /Count field in catalog structures. Additionally, deploying rate limiting and resource monitoring mechanisms can help detect and prevent exploitation attempts. Security teams should also consider implementing sandboxing or containerization for PDF processing applications to limit the impact of potential exploitation. Regular security assessments of PDF processing pipelines and input validation routines should be conducted to identify similar vulnerabilities in other components of the document handling stack.