The cyberweapon that could take down the internet
A new cyberweapon could take down the entire internet – and there's not much that current defences can do to stop it. So say Max Schuchard at the University of Minnesota in Minneapolis and his colleagues, the masterminds who have created the digital ordnance. But thankfully they have no intention of destroying the net just yet. Instead, they are suggesting improvements to its defences.
Schuchard's new attack pits the structure of the internet against itself. Hundreds of connection points in the net fall offline every minute, but we don't notice because the net routes around them. It can do this because the smaller networks that make up the internet, known as autonomous systems, communicate with each other through routers. When a communication path changes, nearby routers inform their neighbours through a system known as the border gateway protocol (BGP). These routers inform other neighbours in turn, eventually spreading knowledge of the new path throughout the internet.
A previously discovered method of attack, dubbed ZMW – after its three creators Zhang, Mao and Wang, researchers in the US who came up with their version four years ago – disrupts the connection between two routers by interfering with BGP to make it appear that the link is offline. Schuchard and colleagues worked out how to spread this disruption to the entire internet and simulated its effects.
Surgical strike
The attack requires a large botnet – a network of computers infected with software that allows them to be externally controlled: Schuchard reckons 250,000 such machines would be enough to take down the internet. Botnets are often used to perform distributed denial-of-service (DDoS) attacks, which bring web servers down by overloading them with traffic, but this new line of attack is different.
"Normal DDoS is a hammer; this is more of a scalpel," says Schuchard. "If you cut in the wrong places then the attack won't work."
An attacker deploying the Schuchard cyberweapon would send traffic between computers in their botnet to build a map of the paths between them. Then they would identify a link common to many different paths and launch a ZMW attack to bring it down. Neighbouring routers would respond by sending out BGP updates to reroute traffic elsewhere. A short time later, the two sundered routers would reconnect and send out their own BGP updates, upon which attack traffic would start flowing in again, causing them to disconnect once more. This cycle would repeat, with the single breaking and reforming link sending out waves of BGP updates to every router on the internet. Eventually each router in the world would be receiving more updates than it could handle – after 20 minutes of attacking, a queue requiring 100 minutes of processing would have built up.
Clearly, that's a problem. "Routers under extreme computational load tend to do funny things," says Schuchard. With every router in the world preoccupied, natural routing outages wouldn't be fixed, and eventually the internet would be so full of holes that communication would become impossible. Shuchard thinks it would take days to recover.
"Once this attack got launched, it wouldn't be solved by technical means, but by network operators actually talking to each other," he says. Each autonomous system would have to be taken down and rebooted to clear the BGP backlog.
Meltdown not expected
So is internet meltdown now inevitable? Perhaps not. The attack is unlikely to be launched by malicious hackers, because mapping the network to find a target link is a highly technical task, and anyone with a large enough botnet is more likely to be renting it out for a profit.
An alternative scenario would be the nuclear option in a full-blown cyberwar – the last resort in retaliation to other forms of cyberattack. A nation state could pull up the digital drawbridge by adjusting its BGP to disconnect from the internet, just as Egypt did two weeks ago. An agent in another country could then launch the attack, bringing down the internet while preserving the attacking nation's internal network.
Sitting duck
Whoever launched the attack, there's little we could do about it. Schuchard's simulation shows that existing fail-safes built into BGP do little to protect against his attack – they weren't designed to. One solution is to send BGP updates via a separate network from other data, but this is impractical as it would essentially involve building a shadow internet.
Another is to alter the BGP system to assume that links never go down, but this change would have to be made by at least 10 per cent of all autonomous systems on the internet, according to the researchers' model, and would require network operators to monitor the health of connections in other ways. Schuchard says that convincing enough independent operators to make the change could be difficult.
"Nobody knows if it's possible to bring down the global internet routing system," says Mark Handley, an expert in networked systems at University College London. He suggests that the attack could cause "significant disruption" to the internet, with an effect greater than the Slammer worm of 2003, but it is unlikely to bring the whole thing down.
"The simulations in the paper make a lot of simplifying assumptions, which is necessary to simulate on this scale," he explains. "I doubt the internet would behave as described."
Schuchard and colleagues presented their findings at the Network and Distributed System Security Symposium in San Diego, California, on Tuesday.
:可以摧毁互联网的网络武器(记者 雅各布·阿伦)
一种新的网络武器可以摧毁整个互联网——并且目前几乎没有什么防御措施可以阻挡它。马克斯·舒哈德在明尼苏达大学对他的同事说了这番话,正是他们创造了这种“数字大炮”。但万幸的是,他们还不打算摧毁互联网。相反,他们正建议改进互联网的防御。
互联网结构存漏洞
舒哈德的新攻击方法利用互联网的结构来攻击其自身。在网络上,每分钟都有许多节点脱机,但我们不会注意到,因为网络会绕过它们。它能做到这一点是因为组成互联网的那些较小的网络——也就是人们所知的“自治系统”——通过路由器互相通讯。当一个通讯路线发生改变,附近的路由器会通过一个所谓的“边界网关协议”(BGP)系统向其附近的路由器发出通知。这些路由器又接着向其他邻近路由器发出通知,最后将新路径的情况发布到整个互联网。
此前发现的一种攻击方法叫作ZMW攻击,它是通过扰乱BGP,使两个路由器之间的连接显示为脱机,从而切断这两个路由器之间的连接。舒哈德和他的同事们研究出了如何将这种方法扩大到整个互联网,并模拟了其效果。
这种攻击需要一个巨大的“僵尸网络”——一个由被木马感染的计算机组成的网络。舒哈德估计25万台这样的电脑将足以摧毁互联网。僵尸网络经常被用来发动分布式拒绝服务(DDoS)攻击,这种攻击方式通过让网络服务器流量超载而使其死机。但舒哈德的这种新攻击方法与此不同。
“数字大炮”运作机制
发动舒哈德网络武器的攻击者要在僵尸网络中的计算机之间发送流量,建立它们之间的“路径地图”。然后他们要找到众多路径共用的一个连接,发动ZMW攻击摧毁它。附近的路由器会对此作出回应,发送BGP更新消息,将流量导向别的地方。很短的时间之后,这两个被切断的路由器会重新连接,并发送它们自己的BGP更新信息,攻击流量由此会再次流入,让它们再次断开。这一循环不断重复,每次断开和重建连接都会向互联网上的每一台路由器发送 BGP更新消息。最后全世界每一台路由器都会接收到超出自身处理能力的更新消息。
在世界上每一台路由器都被占用的情况下,正常的路由中断无法得到修复,最终互联网会变得千疮百孔,无法进行通讯。舒哈德认为这种情况需要数天时间才能恢复。
他说:“这种攻击一旦发动,就无法通过技术手段解决,只能由网络运营者互相口头交流。”每个自治系统都必须关闭并重启,以清除那些BGP积压处理任务。
如何防止网络崩溃
那么,互联网的崩溃是不是不可避免?可能不是。这种攻击不太可能由黑客蓄意发动,因为绘制网络地图、找到目标连接是一项技术性很强的工作,而且任何拥有足够大的僵尸网络的人更有可能将其出租来赢利。
不管是谁发动这样的攻击,我们对此都做不了什么。舒哈德的模拟显示,现有的BGP内置故障保护措施对于他的攻击几乎无能为力。一种解决办法是通过一个独立网络来发送BGP更新消息,但这不太现实,因为这必然涉及建立一个影子互联网。
另一个办法就是改变BGP系统,让其假定连接永不断开,但根据研究者的模型,此方法必须让互联网至少10%的自治系统作出这种改变,并且要求网络运营者寻找其他方法监控连接的健康状况。舒哈德说,要说服足够多的独立运营商作出这一改变将很困难。
