CVE-2019-9503 in brcmfmac WiFi Driver
Summary
by MITRE
The Broadcom brcmfmac WiFi driver prior to commit a4176ec356c73a46c07c181c6d04039fafa34a9f is vulnerable to a frame validation bypass. If the brcmfmac driver receives a firmware event frame from a remote source, the is_wlc_event_frame function will cause this frame to be discarded and unprocessed. If the driver receives the firmware event frame from the host, the appropriate handler is called. This frame validation can be bypassed if the bus used is USB (for instance by a wifi dongle). This can allow firmware event frames from a remote source to be processed. In the worst case scenario, by sending specially-crafted WiFi packets, a remote, unauthenticated attacker may be able to execute arbitrary code on a vulnerable system. More typically, this vulnerability will result in denial-of-service conditions.
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Analysis
by VulDB Data Team • 01/08/2026
The vulnerability described in CVE-2019-9503 affects the Broadcom brcmfmac WiFi driver implementation across multiple Linux kernel versions, representing a critical security flaw in wireless network stack processing. This issue stems from improper validation of firmware event frames received through different communication buses, creating a pathway for malicious actors to exploit the driver's frame handling logic. The vulnerability specifically impacts systems utilizing the brcmfmac driver with USB-based WiFi adapters, where the normal frame validation mechanisms fail to properly distinguish between legitimate host-generated frames and potentially malicious remote frames.
The technical flaw resides in the is_wlc_event_frame function's conditional logic that governs how firmware event frames are processed based on their source. When frames arrive via USB bus connections, the validation mechanism becomes ineffective, allowing remote attackers to inject specially crafted frames that would normally be discarded by the driver. This bypass occurs because the driver's validation routine does not properly account for the differences in frame processing between different bus types, specifically failing to maintain consistent security checks across USB and other communication interfaces. The vulnerability operates at the kernel level, affecting the core WiFi driver functionality and potentially enabling privilege escalation or code execution.
The operational impact of this vulnerability extends beyond simple denial-of-service conditions to potentially enable remote code execution on affected systems. Attackers can exploit this flaw by sending crafted WiFi packets that bypass the normal validation checks, allowing malicious firmware event frames to be processed by the vulnerable driver. The attack surface is particularly concerning for systems utilizing USB WiFi adapters, as these devices are commonly deployed in enterprise and consumer environments where wireless connectivity is essential. The vulnerability can be exploited without authentication, making it particularly dangerous as it allows attackers to compromise systems simply by being within wireless range.
Systems running Linux kernels with the affected brcmfmac driver versions are at risk, particularly those using USB-based WiFi hardware such as USB dongles and external WiFi adapters. The vulnerability affects various Linux distributions including Ubuntu, Debian, and other systems that utilize the standard Linux kernel WiFi stack. Organizations with wireless infrastructure or users relying on USB WiFi adapters should be particularly concerned about this vulnerability, as it can be exploited from remote locations without requiring physical access to target systems. The impact is amplified in environments where wireless networks are used for critical operations, as the vulnerability could potentially lead to complete system compromise.
Mitigation strategies include updating to kernel versions that contain the fix for commit a4176ec356c73a46c07c181c6d04039fafa34a9f, which properly addresses the frame validation bypass issue. System administrators should prioritize patching affected systems and verifying that the updated kernel versions correctly implement frame validation across all supported bus types. Additionally, network administrators can implement network segmentation to limit wireless access points that may be vulnerable, and consider disabling unnecessary wireless functionality on systems that do not require it. The vulnerability aligns with CWE-284 Access Control Issues and can be mapped to ATT&CK technique T1059 Command and Scripting Interpreter for potential exploitation paths, though the primary concern remains the bypass of kernel-level security controls.