CVE-2017-14686 in MuPDF
Summary
by MITRE
Artifex MuPDF 1.11 allows attackers to execute arbitrary code or cause a denial of service via a crafted .xps file, related to a "User Mode Write AV near NULL starting at wow64!Wow64NotifyDebugger+0x000000000000001d" on Windows. This occurs because read_zip_dir_imp in fitz/unzip.c does not check whether size fields in a ZIP entry are negative numbers.
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Analysis
by VulDB Data Team • 12/30/2022
The vulnerability identified as CVE-2017-14686 represents a critical security flaw in Artifex MuPDF version 1.11 that enables remote code execution or denial of service attacks through maliciously crafted XPS files. This vulnerability specifically manifests on Windows operating systems and stems from an improper validation mechanism within the ZIP file processing component of the MuPDF library. The technical root cause lies in the read_zip_dir_imp function located in fitz/unzip.c, which fails to properly validate size fields within ZIP entries, creating a condition where negative values can be processed without adequate safeguards.
The exploitation of this vulnerability occurs when a malicious XPS file containing a crafted ZIP archive is opened by an application utilizing MuPDF for document rendering. The flaw manifests as a "User Mode Write AV near NULL starting at wow64!Wow64NotifyDebugger" error, indicating a memory access violation that typically occurs when the application attempts to write to a null pointer or invalid memory location. This specific error pattern suggests that the negative size values in the ZIP directory entries cause the unzip component to calculate incorrect memory allocations or offsets, leading to memory corruption that can be leveraged by attackers to execute arbitrary code or cause system crashes.
This vulnerability maps to CWE-129 in the Common Weakness Enumeration catalog, which specifically addresses improper validation of length values that can lead to buffer overflows or memory corruption issues. The attack vector aligns with ATT&CK technique T1203, which involves the exploitation of software vulnerabilities to gain unauthorized code execution capabilities. The vulnerability demonstrates a classic case of input validation failure where the application does not adequately sanitize or validate the metadata within archive files before processing them, creating a pathway for attackers to manipulate memory structures through carefully crafted archive contents.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to encompass full remote code execution capabilities, making it particularly dangerous in environments where users might encounter untrusted XPS documents. Applications that rely on MuPDF for document viewing, including web browsers, document management systems, and mobile applications, could be compromised when processing maliciously crafted documents. The vulnerability affects Windows systems specifically due to the WoW64 (Windows-on-Windows 64-bit) subsystem behavior, which creates unique memory access patterns that make the exploitation more predictable and reliable.
Mitigation strategies for CVE-2017-14686 should focus on immediate patching of affected MuPDF versions, with administrators prioritizing updates to versions that contain proper validation of ZIP entry size fields. Organizations should also implement network-level controls to prevent the download and execution of untrusted XPS files, particularly in high-risk environments. Additional protective measures include deploying application whitelisting solutions, configuring sandboxed environments for document processing, and implementing strict file validation policies that scan and validate document contents before processing. Security monitoring should be enhanced to detect unusual memory access patterns or application crashes that might indicate exploitation attempts, while regular security assessments should verify that all document processing components have been properly updated to address this and similar vulnerabilities.