CVE-2016-1521 in Graphite
Summary
by MITRE
The directrun function in directmachine.cpp in Libgraphite in Graphite 2 1.2.4, as used in Mozilla Firefox before 43.0 and Firefox ESR 38.x before 38.6.1, does not validate a certain skip operation, which allows remote attackers to execute arbitrary code, obtain sensitive information, or cause a denial of service (out-of-bounds read and application crash) via a crafted Graphite smart font.
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Analysis
by VulDB Data Team • 10/23/2024
The vulnerability identified as CVE-2016-1521 represents a critical security flaw in the Libgraphite library component of the Graphite 2 text rendering system. This issue specifically affects Mozilla Firefox versions prior to 43.0 and Firefox ESR 38.x versions prior to 38.6.1, where the library's directrun function in directmachine.cpp fails to properly validate skip operations during font processing. The flaw resides in the handling of Graphite smart fonts, which are advanced font formats that support complex text layouts and language-specific typographic features. When a maliciously crafted smart font is processed by the affected Firefox versions, the insufficient validation allows attackers to manipulate memory operations that should be constrained within safe boundaries.
The technical nature of this vulnerability stems from an out-of-bounds read condition that occurs when the directrun function processes skip operations without adequate parameter validation. This type of flaw falls under the CWE-125 weakness category, which describes out-of-bounds read vulnerabilities that can lead to information disclosure, application crashes, or potentially code execution. The vulnerability is particularly dangerous because it operates at the font rendering level, where malicious input can be seamlessly integrated into normal user activities. When an attacker crafts a Graphite smart font with malformed skip operations, the function fails to validate the operation parameters, allowing the execution to proceed beyond the intended memory boundaries. This condition can result in reading memory locations that contain sensitive data, causing application instability or providing attackers with information that could be leveraged for further exploitation.
The operational impact of CVE-2016-1521 extends beyond simple denial of service conditions to encompass potential remote code execution capabilities. Attackers can leverage this vulnerability to execute arbitrary code on affected systems simply by persuading users to view web content that includes the malicious font. The vulnerability affects a wide range of Firefox installations, including both regular release versions and extended support releases, making it particularly dangerous for enterprise environments where older versions may still be in use. The out-of-bounds read condition can trigger application crashes that may be exploited to achieve remote code execution, while the information disclosure aspect allows attackers to potentially extract sensitive memory contents. This vulnerability is particularly concerning in the context of the ATT&CK framework's privilege escalation and code execution tactics, as it can be used to gain unauthorized access to systems through the browser's text rendering pipeline.
Mitigation strategies for CVE-2016-1521 primarily focus on updating affected Firefox installations to versions that include patches addressing the validation issue in the Libgraphite library. System administrators should prioritize immediate deployment of Firefox 43.0 or later releases for regular users, and Firefox ESR 38.6.1 or later for extended support environments. The patch implementation addresses the core validation flaw by ensuring that all skip operations are properly checked before execution, preventing the out-of-bounds memory access that enables exploitation. Organizations should also consider implementing additional security measures such as content security policies that restrict font loading from untrusted sources, though the primary defense remains the software update. The vulnerability highlights the importance of proper input validation in text processing libraries and demonstrates how seemingly benign font rendering operations can become attack vectors when validation is insufficient. Security monitoring should include detection of unusual font processing patterns and memory access violations that may indicate exploitation attempts.