[论文解读] Flood & Loot: A Systemic Attack On The Lightning Network

Jona Harris, Aviv Zohar|arXiv (Cornell University)|Jun 15, 2020

Blockchain Technology Applications and Security参考文献 23被引用 14

一句话总结

本文提出了一种针对闪电网络的系统性威胁——洪水与掠夺攻击（Flood & Loot），该攻击利用费用估算机制和通道关闭时机，通过在比特币区块链上充斥低费用交易，使受害者无法及时关闭通道，从而实现对受害者资金的窃取。攻击成本极低——仅需针对85条通道即可保证成功窃取资金，且攻击者可通过操控费用动态进一步提高成功率。

ABSTRACT

The Lightning Network promises to alleviate Bitcoin's known scalability problems. The operation of such second layer approaches relies on the ability of participants to turn to the blockchain to claim funds at any time, which is assumed to happen rarely. One of the risks that was identified early on is that of a wide systemic attack on the protocol, in which an attacker triggers the closure of many Lightning channels at once. The resulting high volume of transactions in the blockchain will not allow for the proper settlement of all debts, and attackers may get away with stealing some funds. This paper explores the details of such an attack and evaluates its cost and overall impact on Bitcoin and the Lightning Network. Specifically, we show that an attacker is able to simultaneously cause victim nodes to overload the Bitcoin blockchain with requests and to steal funds that were locked in channels. We go on to examine the interaction of Lightning nodes with the fee estimation mechanism and show that the attacker can continuously lower the fee of transactions that will later be used by the victim in its attempts to recover funds - eventually reaching a state in which only low fractions of the block are available for lightning transactions. Our attack is made easier even further as the Lightning protocol allows the attacker to increase the fee offered by his own transactions. We continue to empirically show that the vast majority of nodes agree to channel opening requests from unknown sources and are therefore susceptible to this attack. We highlight differences between various implementations of the Lightning Network protocol and review the susceptibility of each one to the attack. Finally, we propose mitigation strategies to lower the systemic attack risk of the network.

研究动机与目标

• 分析一种新型的闪电网络系统性攻击向量，该向量结合区块链拥堵与资金窃取。
• 评估费用估算机制及实现特定行为如何放大攻击的成功率。
• 识别不同闪电网络实现中通道关闭时机与费用管理方面的漏洞。
• 提出缓解策略，以降低系统性风险，同时承认协议固有的局限性。

提出的方法

• 攻击者在低费用时期，向受害者节点建立通道，并设置较低的链上费用率。
• 攻击者通过受害者通道发起多跳支付，以增加所需关闭交易的数量。
• 攻击者将攻击时机与区块链拥堵期对齐，以最大化受害者关闭交易的延迟。
• 攻击者使用高费用交易替换受害者的低费用关闭尝试，确保自身交易优先被处理。
• 该攻击利用了受害者在通道关闭前未充分利用全部可用超时时间窗口的事实。
• 攻击者可提高自身交易的费用率，而受害者则无法在事后调整其费用。

实验结果

研究问题

• RQ1 攻击者如何同时制造区块链拥堵并从闪电网络参与者处窃取资金？
• RQ2 费用估算机制在促成或放大该攻击中扮演何种角色？
• RQ3 为何某些闪电网络实现比其他实现更易受此攻击影响？
• RQ4 该攻击在多大程度上可仅以极低的初始投资和通道数量执行？
• RQ5 哪些根本性的协议设计选择导致或加剧了该攻击的系统性风险？

主要发现

• 该攻击仅需针对85条目标通道即可成功，由于区块链拥堵和时间约束，可保证资金被窃取。
• 受害者关闭交易被延迟，因攻击者操控费用动态，导致其低费用交易被排除在区块之外。
• 攻击者可提高自身关闭交易的费用，从而在内存池中替换受害者的交易。
• LND及其他实现的通道关闭机制启动过晚（例如仅在超时前10个区块），增加了脆弱性。
• 该攻击因受害者预设费用且无法事后调整，而攻击者可动态调整，从而被进一步放大。
• 实证证据表明，大多数节点接受来自未知对等方的通道开启请求，从而扩大了攻击面。