CVE-2021-21367 in Switchboard Bluetooth Plug
Summary
by MITRE • 03/13/2021
Switchboard Bluetooth Plug for elementary OS from version 2.3.0 and before version version 2.3.5 has an incorrect authorization vulnerability. When the Bluetooth plug is running (in discoverable mode), Bluetooth service requests and pairing requests are automatically accepted, allowing physically proximate attackers to pair with a device running an affected version of switchboard-plug-bluetooth without the active consent of the user. By default, elementary OS doesn't expose any services via Bluetooth that allow information to be extracted by paired Bluetooth devices. However, if such services (i.e. contact list sharing software) have been installed, it's possible that attackers have been able to extract data from such services without authorization. If no such services have been installed, attackers are only able to pair with a device running an affected version without authorization and then play audio out of the device or possibly present a HID device (keyboard, mouse, etc...) to control the device. As such, users should check the list of trusted/paired devices and remove any that are not 100% confirmed to be genuine. This is fixed in version 2.3.5. To reduce the likelihood of this vulnerability on an unpatched version, only open the Bluetooth plug for short intervals when absolutely necessary and preferably not in crowded public areas. To mitigate the risk entirely with unpatched versions, do not open the Bluetooth plug within switchboard at all, and use a different method for pairing devices if necessary (e.g. `bluetoothctl` CLI).
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Analysis
by VulDB Data Team • 04/01/2021
The vulnerability identified as CVE-2021-21367 affects the switchboard-plug-bluetooth component in elementary OS versions prior to 2.3.5, representing a critical authorization flaw that undermines the security of Bluetooth device pairing operations. This issue manifests when the Bluetooth plug operates in discoverable mode, automatically accepting service requests and pairing attempts without requiring explicit user consent. The flaw creates a significant attack surface that allows physically proximate adversaries to establish unauthorized Bluetooth connections with affected systems, bypassing the intended security controls that should require active user approval for device pairing operations.
The technical implementation of this vulnerability stems from improper access control mechanisms within the Bluetooth plug's authorization flow. When the system operates in discoverable mode, the application fails to enforce proper user interaction requirements for pairing decisions, effectively eliminating the user consent step that should be mandatory for establishing Bluetooth connections. This misconfiguration aligns with CWE-668, which describes "Exposure of Resource to Wrong Sphere" where a resource is made available to a sphere for which it was not intended, specifically in this case where Bluetooth pairing operations are exposed without proper authorization checks. The flaw essentially removes the user's ability to make informed decisions about which devices can connect to their system, creating a scenario where any nearby Bluetooth device can establish a connection without explicit approval.
The operational impact of this vulnerability extends beyond simple unauthorized pairing, though the immediate risk appears relatively contained due to elementary OS default security configurations. While the operating system does not expose information extraction services through Bluetooth by default, the vulnerability creates opportunities for attackers to establish connections that could potentially enable data exfiltration if additional Bluetooth services have been installed. The threat model encompasses both passive and active attack vectors, including the ability to play audio through the device's speakers, present HID devices to gain control of the system, or potentially exploit other installed Bluetooth services that might provide unauthorized access to sensitive information. This vulnerability operates within the ATT&CK framework under the T1133 technique for "External Remote Services" where adversaries establish connections to systems through external service interfaces.
The security implications of this vulnerability become particularly concerning when considering the default configuration of elementary OS systems, which typically do not expose services that would allow for information extraction through Bluetooth connections. However, the presence of third-party applications or services that do expose Bluetooth interfaces creates potential attack vectors that could be exploited through this authorization flaw. The attack surface expands significantly if users have installed Bluetooth services that share contact lists, file systems, or other sensitive data, as these could be accessed by attackers who establish unauthorized connections through the vulnerable Bluetooth plug. Users must actively maintain their device pairing lists and remove any unknown or untrusted connections that may have been established through this vulnerability.
Mitigation strategies for this vulnerability focus on both immediate operational changes and long-term system hardening approaches. The most effective immediate solution involves updating to version 2.3.5 or later, which addresses the authorization flaw through proper implementation of user consent requirements for Bluetooth pairing operations. For users who cannot immediately update, the recommended approach includes limiting the exposure of the Bluetooth plug to brief intervals when absolutely necessary, particularly avoiding use in crowded public areas where proximity attacks are more likely. The operational recommendation to avoid opening the Bluetooth plug within switchboard entirely represents a complete mitigation strategy that eliminates the attack surface associated with this vulnerability. Alternative pairing methods such as using the bluetoothctl command-line interface provide more secure pairing mechanisms that do not suffer from the same authorization flaws, though they require more technical expertise from users. Additionally, system administrators should conduct regular audits of paired devices and maintain strict controls over which Bluetooth services are installed and enabled on systems to minimize the potential impact of this vulnerability.