CVE-2017-8691 in Windows
Summary
by MITRE
Windows Server 2008 SP2 and R2 SP1, and Windows 7 SP1 allow an attacker to execute code remotely on a target system when the Windows font library fails to properly handle specially crafted embedded fonts, aka "Express Compressed Fonts Remote Code Execution Vulnerability."
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Analysis
by VulDB Data Team • 01/08/2021
The vulnerability identified as CVE-2017-8691 represents a critical remote code execution flaw within Microsoft Windows operating systems that affects Windows Server 2008 SP2 and R2 SP1, as well as Windows 7 SP1. This vulnerability resides in the Windows font library component responsible for processing embedded fonts, specifically when handling Express Compressed Fonts format. The flaw enables attackers to execute arbitrary code on target systems without requiring authentication, making it particularly dangerous in enterprise environments where multiple systems may be exposed to untrusted network traffic.
The technical root cause of this vulnerability stems from improper validation and handling of specially crafted embedded font files within the Windows font rendering subsystem. When the system encounters a malformed or maliciously constructed font file, the font library fails to properly sanitize the input data, leading to memory corruption that can be exploited by attackers to gain remote code execution privileges. This type of vulnerability falls under CWE-121, which describes heap-based buffer overflow conditions, and more specifically aligns with CWE-125, out-of-bounds read conditions that can lead to arbitrary code execution. The vulnerability is particularly insidious because it can be triggered through normal font processing operations that occur when viewing web pages, opening email attachments, or rendering documents containing embedded fonts.
The operational impact of CVE-2017-8691 extends beyond simple remote code execution, as it provides attackers with a vector for establishing persistent access to compromised systems. Attackers can leverage this vulnerability to install backdoors, steal sensitive data, or use the compromised system as a launching point for further attacks within the network infrastructure. The vulnerability is especially concerning in enterprise environments where Windows systems are frequently exposed to untrusted content from the internet, making it a prime target for advanced persistent threat actors. According to ATT&CK framework, this vulnerability maps to T1059.007 for command and scripting interpreter and T1068 for exploit for privilege escalation, as successful exploitation typically results in elevated privileges on the target system.
Mitigation strategies for CVE-2017-8691 should include immediate deployment of Microsoft security patches, which address the underlying font library processing flaw. Organizations should also implement network segmentation to limit exposure to potentially malicious font content and configure email filters to block suspicious attachments containing embedded fonts. Additionally, disabling automatic font rendering for untrusted content and implementing application whitelisting can provide additional layers of protection. Security monitoring should focus on detecting unusual font processing activities and potential exploitation attempts, particularly in web browsers and document viewers that may encounter embedded font content during normal operations. The vulnerability highlights the importance of keeping font processing libraries updated and demonstrates how seemingly benign components like font rendering can become critical attack vectors in modern cybersecurity landscapes.