16万美元资产被盗竟是乌龙事件？ | Yeld.finance“闪电贷攻击”事件

一、事件概览

北京时间2021年2月27日，【链必安-区块链安全态势感知平台（Beosin-OSINT）】舆情监测到，DeFi知名项目Yeld.finance官方发出通告，表示该项目的DAI池遭受到闪电贷攻击，原文链接如下： <u style="text-decoration: none; border-bottom: 1px dashed grey;"><a href="https://link.zhihu.com/?target=https%3A//yeldf.medium.com/the-yeld-dai-earn-vault-has-been-hacked-93f27d475b1b" class=" external" target="_blank" rel="nofollow noreferrer" data-za-detail-view-id="1043" style="color: inherit; text-decoration: none; cursor: pointer; border-bottom: 1px solid grey;"><span class="invisible" style="font: 0px / 0 a; color: transparent; text-shadow: none; background-color: transparent;">https://&lt;/span>&lt;span class="visible">yeldf.medium.com/the-ye</span><span class="invisible" style="font: 0px / 0 a; color: transparent; text-shadow: none; background-color: transparent;">ld-dai-earn-vault-has-been-hacked-93f27d475b1b</span><span class="ellipsis"></span></a></u>

成都链安（Beosin）安全团队第一时间介入响应，对原文中所提及的交易(0x57b378f8d20d3945ab40cd62aa24063f375bcfc5693c2e788dc193ffa1a5cc3a)进行分析。经分析后发现，该笔交易为Yeld.finance项目自身的策略机制而导致的资金转移，与闪电贷攻击无关。闪电贷攻击表示不背这个锅。

二、事件分析

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图1 交易信息

如图1所示，该笔交易是名为0xf0f225e0的用户，调用了0xe780cab7ca8014543f194fc431e6bf7dc5c16762合约的deposit函数。经确认，0xef80cab7合约正是项目方的DAI池。该笔交易一共产生了6笔代币转移，分别用T1到T6表示。那么，这些代币转移究竟是什么操作导致的呢？下面通过代码进行分析：

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图2 deposit函数源代码

很明显，第538行代码，产生导致了序号为T1的代币转移，将token（即DAI）转移到yDAI合约。这是一笔普通的代币转账，表示用户存入了9,377 DAI到yDAI合约。

第541-553行代码，是yDAI合约用于计算用户存入的DAI应返回给用户多少yDAI，并在第554行进行铸币，对应序号为T2的代币转账，表示yDAI合约向用户铸了9,306 yDAI。

然后进入第555行的rebalance函数，分析该函数的逻辑。

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图3 rebalance函数源码

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图4 recommend函数

第732行代码会计算newProvider，该函数会调用recommend函数(如图4所示)，recommend函数会调用IEarnAPRWithPool合约查询4个Defi项目DYDX,COMPOUND,AAVE,FULCRUM中，年利率(APR)最高的项目，查询结果如图5所示：

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图5 recommend查询结果

其中dYdX池的APR最高，newProvider被设置为dYdX池。当前池为AAVE池，进入736行的if代码块，调用内部函数_withdrawAll。

<figure data-size="normal" style="margin: 1.4em 0px; color: rgb(18, 18, 18); font-family: -apple-system, BlinkMacSystemFont, "Helvetica Neue", "PingFang SC", "Microsoft YaHei", "Source Han Sans SC", "Noto Sans CJK SC", "WenQuanYi Micro Hei", sans-serif; font-size: medium; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; background-color: rgb(255, 255, 255); text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;"><img src="https://pic4.zhimg.com/80/v2-71e4678ebc85355956d9302af98495e3_720w.jpg" data-caption="" data-size="normal" data-rawwidth="779" data-rawheight="657" class="origin_image zh-lightbox-thumb lazy" width="779" data-original="https://pic4.zhimg.com/v2-71e4678ebc85355956d9302af98495e3_r.jpg" data-actualsrc="https://pic4.zhimg.com/v2-71e4678ebc85355956d9302af98495e3_b.jpg" data-lazy-status="ok" style="display: block; max-width: 100%; margin: 0px auto; cursor: zoom-in; background-color: transparent; animation: 0.5s ease-in 0s 1 normal none running fxRichTextFadeIn;"/></figure>

图6 _withdrawAll函数源代码

第778行代码将会提出AAVE池中的所有DAI，产生了序号为T3-T5的代币转移，具体代码可参考AAVE(0xfC1E690f61EFd961294b3e1Ce3313fBD8aa4f85d)合约redeem函数相关代码，此处不再详述。

最后是第741行代码，将从AAVE中提出的16.6余万枚DAI存入dYdX合约，产生了序号为T6的代币转移，即将16.6万枚DAI存入dYdX池。

整个交易就此结束，可以看到，这次所谓的“闪电贷攻击”只是虚惊一场。用户只是单纯的存入了一笔DAI，然后刚好触发了Yeld.finance项目的策略机制，并不是所谓的“闪电贷攻击”，可谓是闹了场乌龙事件。值得注意的是，dYdX在该事件中充当了一个“良心商家”的角色，并不是以往闪电贷攻击中的帮凶。

三、安全建议

尽管本次事件经成都链安（Beosin）安全团队分析后被判断为虚假一场，但在这里还是有必要提醒各项目方，依然需要在日常的安全防护工作中，对闪电贷攻击加以预警和防范。

同时，作为致力于区块链生态安全建设的成都链安（Beosin）也在此建议，项目方的安全预警机制和安全加固工作切不可等闲视之。寻求第三方安全公司的力量，搭建覆盖全生命周期的一站式安全解决方案方为万全之策。