CVE-2016-2793 in Firefox
Summary
by MITRE
CachedCmap.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.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 07/09/2022
The vulnerability identified as CVE-2016-2793 represents a critical buffer over-read flaw in the Graphite 2 library's CachedCmap.cpp component, which serves as a font handling mechanism within the Mozilla Firefox browser ecosystem. This issue affects Firefox versions prior to 45.0 and Firefox ESR 38.x versions prior to 38.7, creating a significant security risk for users of these older browser versions. The vulnerability specifically manifests when the browser processes crafted Graphite smart fonts, which are advanced font formats designed to handle complex text rendering for various writing systems including those with complex ligatures and contextual shaping requirements.
The technical flaw stems from inadequate input validation within the CachedCmap.cpp file where the Graphite 2 library fails to properly bounds-check data structures when parsing Graphite smart font files. When a maliciously crafted font file is processed, the library attempts to read beyond the allocated memory boundaries, resulting in a buffer over-read condition that can trigger unexpected program behavior. This vulnerability is classified under CWE-129 as an Improper Validation of Array Index, specifically manifesting as an insufficient bounds check in array access operations. The flaw operates at the intersection of font rendering and memory management, where the complex data structures used to map character codes to glyph indices in smart fonts create opportunities for attackers to manipulate memory access patterns.
The operational impact of this vulnerability extends beyond simple denial of service, as the buffer over-read condition could potentially be exploited to execute arbitrary code or cause more severe system instability. Attackers can craft Graphite smart fonts that, when loaded by the vulnerable Firefox browser, trigger the buffer over-read condition and potentially lead to memory corruption. This type of vulnerability falls under the ATT&CK framework's T1059.007 technique for Command and Scripting Interpreter: PowerShell, though in this case the attack vector involves font manipulation rather than scripting. The vulnerability's potential for unspecified other impacts suggests that the memory corruption could be leveraged for more sophisticated exploitation techniques including privilege escalation or information disclosure, making it particularly dangerous in targeted attack scenarios.
Mitigation strategies for CVE-2016-2793 primarily focus on immediate browser version updates to patched releases that contain fixes for the Graphite 2 library. Organizations should prioritize updating Firefox installations to versions 45.0 or later for regular releases and 38.7 or later for ESR versions, as these releases incorporate the necessary patches to prevent the buffer over-read condition. Additionally, implementing browser security policies that restrict font loading from untrusted sources can provide an additional layer of protection. System administrators should consider disabling Graphite font support entirely in environments where the risk is high, though this may impact display capabilities for certain languages. The vulnerability also underscores the importance of regular security assessments and keeping third-party libraries updated, as the issue originates from a component outside the core browser codebase but within its dependencies. Network administrators should monitor for potential exploitation attempts through web traffic analysis, particularly when encountering unusual font file requests or when users access websites known to host malicious content.