CVE-2018-9918 in QPDF
Summary
by MITRE
libqpdf.a in QPDF through 8.0.2 mishandles certain "expected dictionary key but found non-name object" cases, allowing remote attackers to cause a denial of service (stack exhaustion), related to the QPDFObjectHandle and QPDF_Dictionary classes.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 02/27/2023
The vulnerability identified as CVE-2018-9918 affects QPDF library version 8.0.2 and earlier, specifically within the libqpdf.a component that handles PDF document processing. This issue manifests as a stack exhaustion denial of service condition that occurs when the library encounters malformed PDF content during dictionary key processing. The flaw exists in how the QPDFObjectHandle and QPDF_Dictionary classes manage parsing of PDF objects, particularly when they encounter unexpected data structures during dictionary key validation.
The technical root cause of this vulnerability stems from inadequate input validation and error handling within the PDF parsing logic. When the library processes a PDF document containing malformed dictionary structures where a dictionary key is expected but a non-name object is encountered, the parsing routine fails to properly handle this exceptional condition. This failure leads to recursive or iterative processing patterns that consume excessive stack memory, ultimately resulting in stack exhaustion and application termination. The vulnerability is classified under CWE-772, which deals with missing Release of Resource After Effective Lifetime, specifically manifesting as stack exhaustion through improper handling of parsing errors.
The operational impact of CVE-2018-9918 extends beyond simple service disruption as it represents a remote code execution risk when combined with other attack vectors. Attackers can exploit this vulnerability by crafting malicious PDF files that trigger the specific parsing error condition, causing applications using the affected QPDF library to consume excessive stack resources and eventually crash. This makes the vulnerability particularly dangerous in web applications, email servers, and any system that processes untrusted PDF content. The attack surface is broad as numerous applications rely on QPDF for PDF manipulation and processing, including document management systems, web browsers, and security scanning tools. The vulnerability aligns with ATT&CK technique T1203, which involves exploiting software vulnerabilities to gain system access through resource exhaustion attacks.
Mitigation strategies for CVE-2018-9918 should focus on immediate library updates to version 8.1.0 or later, where the parsing logic has been corrected to properly handle malformed dictionary structures without consuming excessive stack resources. Organizations should implement input validation measures that sanitize PDF content before processing, particularly when handling untrusted documents. Additionally, deploying application-level stack protection mechanisms and implementing resource limits on PDF processing applications can help prevent complete service disruption. System administrators should also consider implementing network-based filtering to block suspicious PDF content and monitor for unusual processing patterns that may indicate exploitation attempts. The fix addresses the underlying parsing error handling by introducing proper bounds checking and limiting recursive processing depth, which aligns with security best practices for preventing stack-based buffer overflows and resource exhaustion attacks.