CVE-2019-9815 in Firefox
Summary
by MITRE
If hyperthreading is not disabled, a timing attack vulnerability exists, similar to previous Spectre attacks. Apple has shipped macOS 10.14.5 with an option to disable hyperthreading in applications running untrusted code in a thread through a new sysctl. Firefox now makes use of it on the main thread and any worker threads. *Note: users need to update to macOS 10.14.5 in order to take advantage of this change.*. This vulnerability affects Thunderbird < 60.7, Firefox < 67, and Firefox ESR < 60.7.
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Analysis
by VulDB Data Team • 08/10/2025
The vulnerability described in CVE-2019-9815 represents a significant timing attack risk that exploits the presence of hyperthreading technology in modern processors. This issue falls under the broader category of side-channel attacks that leverage speculative execution mechanisms, similar to the well-known Spectre vulnerabilities that affected numerous processor architectures. The flaw specifically targets systems where hyperthreading remains enabled, creating a pathway for malicious actors to potentially extract sensitive information through carefully crafted timing measurements. The vulnerability demonstrates how contemporary processor optimizations can introduce security risks when not properly managed in software environments.
The technical implementation of this vulnerability stems from the way modern processors handle simultaneous multithreading through hyperthreading. When multiple threads share the same physical processor core, they can interfere with each other's cache states and execution timing patterns. Attackers can exploit this interference to perform cache timing attacks that reveal information about memory operations in other threads. The vulnerability is particularly concerning because it operates at the hardware level and can bypass traditional software security measures. This aligns with CWE-203, which describes "Observable Behavioral Vulnerability" where information leakage occurs through timing or other observable behaviors rather than direct data access.
Apple's response to this vulnerability involved implementing a sysctl parameter in macOS 10.14.5 that allows applications to disable hyperthreading for threads running untrusted code. This approach represents a system-level mitigation strategy that gives software developers and system administrators more control over security configurations. The implementation specifically targets the main thread and worker threads in Firefox, demonstrating how browser vendors can integrate hardware-level security controls into their applications. This solution addresses the underlying architectural issue by providing a mechanism to isolate execution contexts when processing potentially malicious content, which aligns with ATT&CK technique T1059.007 for execution through scripting and T1070.004 for indicator removal through system modification.
The operational impact of CVE-2019-9815 affects a broad range of software applications that rely on modern processor architectures. Thunderbird versions prior to 60.7, Firefox versions before 67, and Firefox ESR versions before 60.7 all remain vulnerable to this timing attack. This vulnerability requires specific user action to remediate, as the mitigation depends on users updating to macOS 10.14.5 and enabling the appropriate sysctl parameter. The dependency on system-level updates rather than application patches creates a more complex remediation process. Organizations must consider not only software updates but also hardware configuration changes when addressing this vulnerability, making it particularly challenging to implement comprehensive security coverage across enterprise environments.
The mitigation strategies for this vulnerability demonstrate a multi-layered approach to addressing hardware-level security issues. The sysctl-based solution provides a programmatic way to disable hyperthreading for specific execution contexts, which is particularly important for applications handling untrusted input. Firefox's implementation of this feature shows how browser vendors can proactively address emerging security concerns by integrating system-level controls into their applications. However, the requirement for users to update to macOS 10.14.5 creates a dependency that may not be immediately addressed in all environments. Security professionals should consider implementing additional monitoring and detection mechanisms to identify potential exploitation attempts, as well as developing policies for regular system updates and configuration management to ensure proper security controls are maintained. The vulnerability highlights the ongoing challenge of balancing performance optimizations with security requirements in modern computing environments.