CVE-2016-2792 in Firefox
Summary
by MITRE
The graphite2::Slot::getAttr function in Slot.cpp 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 (buffer over-read) or possibly have unspecified other impact via a crafted Graphite smart font, a different vulnerability than CVE-2016-2800.
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Analysis
by VulDB Data Team • 07/09/2022
The vulnerability identified as CVE-2016-2792 represents a critical buffer over-read flaw within the Graphite 2 text rendering library, specifically affecting the graphite2::Slot::getAttr function in Slot.cpp. This issue manifests in versions of Graphite 2 prior to 1.3.6 and has significant implications for web browsers that utilize this library for smart font rendering. The vulnerability arises from insufficient input validation when processing crafted Graphite smart fonts, creating a scenario where maliciously constructed font files can trigger memory access violations. The affected software ecosystem includes Mozilla Firefox versions before 45.0 and Firefox ESR 38.x versions before 38.7, making this a widespread concern for users of these browsers. This vulnerability operates at the intersection of font processing and memory safety, representing a classic example of how font rendering libraries can become attack vectors in modern web browsers.
The technical implementation of this vulnerability stems from improper bounds checking within the graphite2::Slot::getAttr function, which fails to validate the size and structure of attribute data retrieved from Graphite smart fonts. When a malicious font file is processed, the function attempts to read memory locations beyond the allocated buffer boundaries, leading to unpredictable behavior including denial of service conditions or potential information disclosure. The flaw operates as a buffer over-read because the function accesses memory locations that have not been properly validated against the actual size of the allocated buffer, creating a scenario where adjacent memory regions can be accessed without proper authorization. This type of vulnerability falls under the CWE-121 category of stack-based buffer overflow, though the specific manifestation here involves heap memory access patterns typical of modern font rendering engines. The vulnerability's classification aligns with ATT&CK technique T1059.007 for execution through font rendering processes, where adversaries leverage legitimate system functions to execute malicious code or cause system instability.
The operational impact of CVE-2016-2792 extends beyond simple denial of service scenarios, as it can potentially enable more sophisticated attacks depending on the execution environment and memory layout. Remote attackers can exploit this vulnerability by delivering maliciously crafted Graphite smart fonts through web content, making it particularly dangerous in browser environments where users may inadvertently encounter such content. The vulnerability's exploitation can lead to browser crashes, rendering the affected application unusable until restart, while also potentially exposing sensitive memory contents through information disclosure. Given that Graphite 2 is used in multiple applications beyond web browsers, including desktop publishing software and mobile applications, the potential attack surface is broader than initially apparent. The vulnerability's relationship to CVE-2016-2800 demonstrates the interconnected nature of font rendering vulnerabilities, where multiple flaws in related components can compound the overall security risk. Security researchers have noted that such buffer over-read vulnerabilities often serve as stepping stones for more complex exploitation techniques, particularly when combined with other memory corruption vulnerabilities in the same software ecosystem.
Mitigation strategies for CVE-2016-2792 primarily focus on updating to patched versions of Graphite 2 library and the affected browser software. Organizations should prioritize immediate deployment of Graphite 2 version 1.3.6 or later, which includes proper bounds checking and memory validation mechanisms. Browser vendors have addressed this vulnerability through security updates, and users should ensure their Firefox installations are updated to versions 45.0 or later for the main release channel and 38.7 or later for the ESR channel. Additionally, implementing content security policies that restrict font loading from untrusted sources can provide defense-in-depth protection, though this approach may impact legitimate functionality. System administrators should also consider monitoring for unusual font processing activities and implementing sandboxing measures to limit the potential impact of successful exploitation attempts. The vulnerability highlights the importance of regular security updates and proper input validation in font processing libraries, as these components often receive less scrutiny than core application functions but can provide significant attack vectors. Organizations should also review their font handling processes and implement automated scanning for potentially malicious font files in their content delivery pipelines.