CVE-2016-1977 in Firefox
Summary
by MITRE
The Machine::Code::decoder::analysis::set_ref 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 execute arbitrary code or cause a denial of service (stack memory corruption) via a crafted Graphite smart font.
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Analysis
by VulDB Data Team • 07/09/2022
The vulnerability identified as CVE-2016-1977 represents a critical heap-based buffer overflow in the Graphite smart font processing component of Mozilla Firefox. This flaw exists within the Machine::Code::decoder::analysis::set_ref function of Graphite 2 library versions prior to 1.3.6, which was subsequently integrated into Firefox browser versions before 45.0 and Firefox ESR 38.x before 38.7. The vulnerability stems from inadequate input validation and memory management when processing specially crafted Graphite smart fonts, creating a dangerous condition that can be exploited remotely by attackers to execute arbitrary code or induce denial of service conditions. The issue manifests through stack memory corruption that occurs during the font analysis phase, making it particularly dangerous as it can lead to complete system compromise when a user loads a malicious font file.
The technical exploitation of this vulnerability follows a well-defined pattern that aligns with CWE-121, heap-based buffer overflow conditions, and demonstrates characteristics consistent with ATT&CK technique T1059.007 for process injection and T1203 for exploitation for execution. Attackers craft malicious Graphite smart font files that contain malformed data structures designed to trigger the buffer overflow during font processing. When Firefox attempts to decode and analyze these fonts through the vulnerable set_ref function, the lack of proper bounds checking allows attackers to overwrite adjacent memory locations on the stack, potentially leading to arbitrary code execution with the privileges of the affected user. The vulnerability's remote exploitability means that simply visiting a malicious website or opening a compromised document containing the malicious font can trigger the attack vector without requiring user interaction beyond normal browsing behavior.
The operational impact of this vulnerability extends beyond simple code execution to encompass significant security implications for web browser users and organizations. The widespread use of Firefox as a primary web browser means that this vulnerability could affect millions of users globally, particularly those running affected versions of the browser. The memory corruption aspect of the vulnerability makes it particularly challenging to detect and prevent, as it can manifest in unpredictable ways depending on memory layout and system configuration. Organizations using Firefox ESR versions were especially at risk since these extended support releases often run in enterprise environments where patching cycles are longer and more controlled. The vulnerability's classification as a remote code execution flaw means that attackers could potentially establish persistent access to systems, steal sensitive information, or deploy additional malware payloads through this attack vector.
Mitigation strategies for CVE-2016-1977 primarily focus on immediate remediation through version updates and browser security hardening measures. The most effective immediate solution involves upgrading to Firefox 45.0 or later versions, or Firefox ESR 38.7 and later, which contain the patched Graphite 2 library components. Organizations should implement robust patch management processes to ensure timely deployment of security updates across all browser installations. Additional defensive measures include implementing content security policies that restrict font loading from untrusted sources, utilizing sandboxing mechanisms within the browser, and monitoring for suspicious font-related network activity. Network-based mitigations such as web application firewalls and intrusion detection systems can help detect and block exploitation attempts, though these measures are less effective than proper patching. Security teams should also consider implementing browser hardening configurations that disable unnecessary font processing capabilities and establish monitoring procedures for detecting potential exploitation attempts through stack memory corruption patterns.