[论文解读] On the (In)security of Bluetooth Low Energy One-Way Secure Connections Only Mode

Yue Zhang, Jian Weng+4 more|arXiv (Cornell University)|Aug 27, 2019

Bluetooth and Wireless Communication Technologies参考文献 31被引用 9

一句话总结

本文揭示了Android蓝牙低功耗（BLE）编程框架中的关键设计缺陷，攻击者可利用这些缺陷将安全配对降级为不安全的“仅需确认”（Just Works）模式，从而绕过蓝牙低功耗安全连接仅模式的安全保证。作者识别出四个缺陷，可导致无声中间人（MITM）和跟踪攻击，已在18款商用BLE产品及3,501个Android应用上完成验证，并提出一项在操作系统级别强制实现安全配对的补丁方案。

ABSTRACT

To defeat security threats such as man-in-the-middle (MITM) attacks, Bluetooth Low Energy (BLE) 4.2 and 5.x introduce the Secure Connections Only mode, under which a BLE device accepts only secure paring protocols including Passkey Entry and Numeric Comparison from an initiator, e.g., an Android mobile. However, the BLE specification does not explicitly require the Secure Connection Only mode of the initiator. Taking the Android's BLE programming framework for example, we found that it cannot enforce secure pairing, invalidating the security protection provided by the Secure Connection Only mode. The same problem applies to Apple iOS too. Specifically, we examine the life cycle of a BLE pairing process in Android and identify four severe design flaws. These design flaws can be exploited by attackers to perform downgrading attacks, forcing the BLE pairing protocols to run in the insecure mode without the users' awareness. To validate our findings, we selected and tested 18 popular BLE commercial products and our experimental results proved that downgrading attacks and MITM attacks were all possible to these products. All 3501 BLE apps from Androzoo are also subject to these attacks. For defense, we have designed and implemented a prototype of the Secure Connection Only mode on Android 8 through the Android Open Source Project (AOSP). We have reported the identified BLE pairing vulnerabilities to Bluetooth Special Interest Group (SIG), Google, Apple, Texas Instruments (TI) and all of them are actively addressing this issue. Google rated the reported security flaw a High Severity.

研究动机与目标

• 调查蓝牙低功耗（BLE）安全连接仅模式在真实环境实现中的安全性，特别是在移动操作系统中的实现情况。
• 识别Android BLE编程框架中的设计缺陷，这些缺陷破坏了安全连接仅模式的安全保证。
• 展示这些缺陷如何导致无声降级攻击，使安全配对协议被降级为不安全的“仅需确认”模式，从而引发中间人攻击和设备跟踪。
• 验证这些缺陷在商用BLE设备和Android应用中的可利用性。
• 提出并实现一种原型解决方案，通过在Android操作系统级别强制执行安全配对，以修复已识别的漏洞。

提出的方法

• 逆向分析Android BLE框架中配对生命周期，包括配对发起、状态管理、错误处理和绑定管理。
• 识别出四个设计缺陷：无法指定配对协议、无法取消不安全配对、错误处理不当，以及在不安全配对后未重新生成密钥。
• 设计并执行了利用伪造BLE设备的降级攻击，强制Android设备与之进行不安全配对。
• 在18款商用BLE产品和来自Androzoo的3,501个Android应用上评估了攻击效果，以确认其在真实世界中的可利用性。
• 在AOSP中开发并集成了一个原型补丁，通过Android 8版本在操作系统级别强制启用安全连接仅模式。
• 已向蓝牙SIG、Google、Apple和德州仪器报告发现的问题，所有相关方均正在积极修复这些漏洞。

实验结果

研究问题

• RQ1 为何在Android上实现的BLE 4.2和5.x版本的安全连接仅模式无法提供端到端的安全保障？
• RQ2 Android BLE框架中哪些具体设计缺陷允许攻击者无声地将安全配对降级为不安全的“仅需确认”模式？
• RQ3 现实世界中的BLE设备和Android应用在多大程度上易受此类降级攻击的影响？
• RQ4 在Android操作系统级别实施补丁是否能有效强制执行安全配对并防止此类攻击？
• RQ5 所识别的缺陷对依赖白名单访问控制机制的BLE设备安全性有何影响？

主要发现

• Android BLE框架中存在四个关键设计缺陷，允许攻击者在用户无感知且应用无法控制的情况下强制执行不安全配对。
• 所有18款测试的商用BLE产品均易受降级攻击影响，可导致中间人攻击和数据注入。
• 从Androzoo分析的3,501个Android应用全部易受相同降级攻击和中间人攻击影响。
• 该漏洞不仅存在于Android系统中，也存在于Apple iOS中，表明BLE实现中存在协议级问题。
• Google将该漏洞评定为高危级别，所有主要厂商（Google、Apple、TI、蓝牙SIG）均正在积极修复该问题。
• 所提出的AOSP补丁成功在操作系统级别强制执行安全配对，彻底关闭了已识别的攻击向量。