CVE-2016-2794 in Firefox
Summary
by MITRE
The graphite2::TtfUtil::CmapSubtable12NextCodepoint 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 (buffer over-read) 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-2016-2794 represents a critical buffer over-read flaw within the Graphite 2 smart font handling component that was extensively utilized by Mozilla Firefox and its extended support release versions. This issue specifically affects the graphite2::TtfUtil::CmapSubtable12NextCodepoint function, which processes complex font rendering operations for smart fonts that contain advanced typographic features. The flaw exists in Graphite 2 versions prior to 1.3.6 and was exploited in Firefox versions before 45.0 and Firefox ESR 38.x versions before 38.7, making it a widespread concern across multiple browser versions and support channels.
The technical nature of this vulnerability stems from improper bounds checking within the font processing pipeline where the function fails to validate input parameters from crafted Graphite smart fonts. When a maliciously constructed font file is processed, the CmapSubtable12NextCodepoint function attempts to read beyond the allocated memory buffer, leading to a buffer over-read condition that can result in unpredictable behavior. This flaw operates at the intersection of font rendering and memory management, where the improper handling of font table structures creates opportunities for attackers to manipulate memory access patterns. The vulnerability demonstrates characteristics consistent with CWE-121, which describes stack-based buffer overflow conditions, though the specific implementation involves heap memory corruption through improper input validation.
The operational impact of this vulnerability extends beyond simple denial of service, as it potentially enables remote code execution in certain circumstances. Attackers can craft malicious Graphite smart fonts that, when rendered by affected browsers, trigger the buffer over-read condition and may allow for arbitrary code execution on the target system. This represents a significant threat to browser security since smart fonts are commonly used for complex typography in web content, making the attack surface quite broad. The vulnerability can be exploited through web pages that load malicious font files, potentially compromising user systems without requiring any additional user interaction beyond visiting the compromised website. This aligns with ATT&CK technique T1059.007 for process injection and T1203 for exploitation for privilege escalation, as the initial compromise could lead to further system compromise.
Mitigation strategies for this vulnerability require immediate patching of affected Graphite 2 components and browser versions, with administrators prioritizing updates to Firefox 45.0 and Firefox ESR 38.7 or later versions. Organizations should implement proactive monitoring for malicious font files in web content and consider implementing network-level controls to restrict font downloads from untrusted sources. Security teams should also conduct comprehensive vulnerability assessments to identify any other applications that might be using the vulnerable Graphite 2 library, as the flaw could potentially exist in other software components that rely on the same font processing engine. The remediation process must include thorough testing of patched versions to ensure that font rendering functionality remains intact while eliminating the buffer over-read vulnerability. Additionally, browser hardening measures such as sandboxing and memory protection mechanisms should be enabled to limit the potential impact should exploitation occur despite preventive measures.