CVE-2018-5686 in MuPDF
Summary
by MITRE
In MuPDF 1.12.0, there is an infinite loop vulnerability and application hang in the pdf_parse_array function (pdf/pdf-parse.c) because EOF is not considered. Remote attackers could leverage this vulnerability to cause a denial of service via a crafted pdf file.
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Analysis
by VulDB Data Team • 09/12/2024
The vulnerability identified as CVE-2018-5686 represents a critical denial of service weakness in MuPDF version 1.12.0 that stems from improper handling of end-of-file conditions during PDF parsing operations. This flaw specifically affects the pdf_parse_array function located within the pdf/pdf-parse.c source file, where the parser fails to adequately check for end-of-file markers when processing array structures in maliciously crafted PDF documents. The absence of proper EOF validation creates a condition where the parsing loop continues indefinitely, leading to application hang and system resource exhaustion that effectively renders the affected software unusable.
The technical implementation of this vulnerability demonstrates a classic control flow issue that falls under CWE-835, which specifically addresses the problem of infinite loops or iterations without proper termination conditions. When MuPDF encounters a malformed PDF array structure that lacks proper termination markers, the pdf_parse_array function enters an infinite loop where it continuously processes the same data segment without advancing the file pointer or recognizing the end of the array structure. This behavior constitutes a fundamental flaw in the parser's state management and input validation mechanisms, creating a predictable path for exploitation that remote attackers can leverage through carefully constructed malicious PDF files.
From an operational perspective, this vulnerability presents a significant risk to organizations relying on MuPDF for PDF processing, particularly in environments where automated PDF handling occurs or where users might encounter untrusted PDF content. The denial of service impact extends beyond simple application unresponsiveness to potentially consume excessive system resources including CPU cycles and memory allocation, which could lead to broader system performance degradation or even complete system exhaustion in resource-constrained environments. The remote exploitation capability means that attackers can trigger this vulnerability without requiring local access, making it particularly dangerous for web applications, email gateways, or any system that automatically processes PDF documents from untrusted sources.
The attack surface for this vulnerability is extensive given MuPDF's widespread adoption across various platforms and applications, including mobile devices, web browsers, and document management systems. Security practitioners should consider this vulnerability in the context of ATT&CK technique T1499.004 which covers network denial of service attacks, and more specifically the broader category of T1499 for denial of service tactics. The mitigation strategy should focus on immediate patching of affected MuPDF versions, implementation of input validation measures that enforce proper EOF checking, and deployment of additional defensive measures such as PDF sandboxing or content filtering systems that can detect and block malformed PDF structures before they reach the vulnerable parser component. Organizations should also consider implementing network-level protections and monitoring for unusual CPU usage patterns that might indicate exploitation attempts.