CVE-2016-2797 in Firefox
Summary
by MITRE
The graphite2::TtfUtil::CmapSubtable12Lookup 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, a different vulnerability than CVE-2016-2801.
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Analysis
by VulDB Data Team • 07/09/2022
The vulnerability identified as CVE-2016-2797 represents a critical buffer over-read flaw within the Graphite 2 smart font handling mechanism, specifically affecting the graphite2::TtfUtil::CmapSubtable12Lookup function. This issue exists in Graphite 2 versions prior to 1.3.6 and has significant implications for web browsers that utilize this library for font rendering, particularly Mozilla Firefox versions before 45.0 and Firefox ESR 38.x before 38.7. The vulnerability stems from inadequate input validation and memory management when processing crafted Graphite smart fonts, creating a pathway for remote attackers to exploit the system through malicious font files.
The technical flaw manifests when the Graphite 2 library processes certain font tables, specifically within the cmap subtable 12 lookup mechanism that handles Unicode variation selectors and complex text rendering scenarios. When a maliciously crafted font file is loaded, the function fails to properly bounds-check array accesses or validate the structure of the incoming data, leading to memory corruption that can result in buffer over-read conditions. This type of vulnerability falls under CWE-129, which addresses insufficient bounds checking, and specifically relates to CWE-787, representing out-of-bounds write operations that can occur when improper input validation allows attackers to manipulate memory layout.
The operational impact of this vulnerability extends beyond simple denial of service, as demonstrated by the potential for unspecified other impacts mentioned in the description. Remote attackers can leverage this flaw to cause Firefox browsers to crash through buffer over-read conditions, effectively creating a denial of service scenario that disrupts user experience and potentially provides a foothold for more sophisticated attacks. The vulnerability is particularly concerning because it operates at the font rendering layer, which is frequently accessed during web page loading, making it an attractive target for exploit development. Attackers can craft Graphite smart fonts that, when rendered by vulnerable browsers, trigger the buffer over-read condition and potentially lead to arbitrary code execution or further system compromise.
The attack vector for this vulnerability involves delivering a maliciously crafted Graphite smart font to a victim's browser through web content, email attachments, or other delivery mechanisms. The ATT&CK framework categorizes this as a technique involving "Exploitation for Client Execution" under the broader category of software exploitation, where the attack targets client-side applications rather than server components. The vulnerability's persistence in multiple Firefox versions indicates a widespread impact across different user bases, requiring coordinated patch management and security updates. Organizations should prioritize updating to patched versions of Firefox and Graphite 2 libraries, as well as implementing content filtering measures that can detect and block suspicious font files. The vulnerability highlights the importance of robust input validation in font rendering libraries and demonstrates how seemingly benign font processing operations can become attack vectors when proper memory management and bounds checking are absent from the implementation.