CVE-2016-2796 in Firefox
Summary
by MITRE
Heap-based buffer overflow in the graphite2::vm::Machine::Code::Code function in Graphite 2 before 1.3.6, as used in Mozilla Firefox before 45.0 and Firefox ESR 38.x before 38.7, allows remote attackers to cause a denial of service or possibly have unspecified other impact via a crafted Graphite smart font.
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Analysis
by VulDB Data Team • 07/09/2022
The vulnerability identified as CVE-2016-2796 represents a critical heap-based buffer overflow within the Graphite 2 smart font rendering library, specifically affecting the graphite2::vm::Machine::Code::Code function. This flaw exists in Graphite 2 versions prior to 1.3.6 and has significant implications for web browsers that utilize this library for font processing. The vulnerability is particularly concerning because it affects Mozilla Firefox versions before 45.0 and Firefox ESR 38.x versions before 38.7, making it a widespread concern across multiple browser versions and supported release channels. The buffer overflow occurs during the processing of crafted Graphite smart fonts, which are sophisticated font formats designed to support complex text layouts and advanced typography features.
The technical nature of this vulnerability stems from improper bounds checking within the Graphite 2 library's virtual machine implementation. When Firefox encounters a maliciously crafted Graphite smart font, the code execution path within the graphite2::vm::Machine::Code::Code function fails to validate input data length against allocated memory boundaries. This allows an attacker to write beyond the intended buffer limits, potentially corrupting adjacent memory regions. The heap-based nature of the overflow means that memory allocation occurs dynamically during runtime, making the exploitation landscape more complex and potentially more dangerous than stack-based alternatives. The vulnerability manifests when the font rendering engine processes font data that contains malformed or oversized data structures, leading to memory corruption that can result in application instability.
The operational impact of CVE-2016-2796 extends beyond simple denial of service conditions to potentially enable more severe security consequences. While the primary effect described is denial of service, the nature of heap corruption typically opens possibilities for arbitrary code execution, making this vulnerability particularly dangerous in the context of web browsers where users may encounter malicious content without explicit warnings. Attackers could potentially leverage this vulnerability to execute malicious code within the browser context, potentially leading to complete system compromise. The widespread adoption of Graphite 2 in font processing across multiple browser implementations means that exploitation could affect a large user base, particularly given that Firefox was one of the most widely used browsers at the time of this vulnerability's discovery. The vulnerability's presence in both regular Firefox releases and Extended Support Release versions indicates the severity and persistence of the issue across different maintenance channels.
Mitigation strategies for CVE-2016-2796 primarily focus on updating to patched versions of both the Graphite 2 library and affected browser implementations. System administrators and users should prioritize upgrading to Firefox 45.0 or later versions, or Firefox ESR 38.7 and later releases, which contain the necessary patches to address the buffer overflow vulnerability. The patch implementation typically involves adding proper bounds checking mechanisms within the graphite2::vm::Machine::Code::Code function to prevent memory overflows when processing font data. Additional protective measures include implementing content security policies that limit font loading from untrusted sources, utilizing sandboxing mechanisms that isolate font processing components, and employing web application firewalls that can detect and block malicious font content. Organizations should also consider implementing regular security assessments and vulnerability scanning to identify any potential exploitation attempts targeting this specific vulnerability. From a cybersecurity perspective, this vulnerability aligns with CWE-121, which describes heap-based buffer overflow conditions, and could potentially map to ATT&CK technique T1059.007 for command and scripting interpreter usage in exploitation scenarios. The vulnerability demonstrates the importance of proper input validation in font processing libraries and highlights the risks associated with complex font rendering systems in modern web browsers.