CVE-2018-6544 in MuPDF
Summary
by MITRE
pdf_load_obj_stm in pdf/pdf-xref.c in Artifex MuPDF 1.12.0 could reference the object stream recursively and therefore run out of error stack, which allows remote attackers to cause a denial of service via a crafted PDF document.
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Analysis
by VulDB Data Team • 02/03/2023
The vulnerability identified as CVE-2018-6544 resides within the Artifex MuPDF document processing library version 1.12.0, specifically in the pdf_load_obj_stm function located in pdf/pdf-xref.c. This flaw represents a classic stack exhaustion issue that demonstrates poor handling of recursive object stream references within PDF documents. The vulnerability manifests when the PDF parser encounters a crafted document containing recursive references to object streams, creating a scenario where the error handling mechanism becomes overwhelmed by the recursive calls. Such recursive references can be constructed in a way that causes the parser to repeatedly process the same object stream without proper termination conditions, ultimately exhausting the available error stack space.
The technical implementation of this vulnerability stems from inadequate recursion detection and stack management within the PDF object stream loading process. When MuPDF attempts to parse an object stream, it follows a recursive pattern to resolve object references, but fails to implement proper depth limiting or cycle detection mechanisms. This allows an attacker to construct a malicious PDF document containing circular references within object streams, where each reference points back to a previous object stream in a loop. The error stack, which should be limited to prevent such exhaustion, becomes consumed by the recursive processing without proper bounds checking. This behavior directly aligns with CWE-674, which describes "Uncontrolled Recursion" as a weakness where a program fails to properly limit the number of recursive calls or detect circular references, leading to stack exhaustion.
The operational impact of CVE-2018-6544 is significant as it enables remote attackers to execute a denial of service attack against systems processing PDF documents through the affected MuPDF library. Any application that utilizes MuPDF for PDF rendering, parsing, or processing becomes vulnerable to this attack vector, including web browsers, document viewers, email clients, and server applications that handle PDF uploads. The attack requires only the delivery of a specially crafted PDF file, making it particularly dangerous for web-based applications that process untrusted document uploads. Successful exploitation results in complete service disruption as the target application crashes or becomes unresponsive due to stack exhaustion, potentially affecting availability for legitimate users and creating opportunities for more sophisticated attacks.
Mitigation strategies for this vulnerability should focus on immediate patching of affected systems, as the primary fix involves updating to a newer version of MuPDF that contains proper recursion detection and stack management. Organizations should implement comprehensive application hardening measures including input validation and sandboxing of PDF processing components to limit the impact of potential exploitation attempts. Network-based protections can include PDF content filtering and sandboxing solutions that analyze and process PDF documents in isolated environments before delivery to end users. Additionally, monitoring and logging should be enhanced to detect unusual processing patterns that might indicate exploitation attempts, and access controls should be implemented to limit PDF processing capabilities to trusted sources only. This vulnerability demonstrates the importance of robust error handling and recursion management in document processing libraries, as outlined in various security frameworks including the ATT&CK framework's focus on resource exhaustion techniques that attackers use to compromise system availability.