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Websites, apps vulnerable to low-bandwidth, bot-free takedown, say researchers

Gregg Keizer | Dec. 30, 2011
Hackers armed with a single machine and a minimal broadband connection can cripple Web servers, researchers said Wednesday. Microsoft today shipped an emergency update to fix the flaw.

Hackers armed with a single machine and a minimal broadband connection can cripple Web servers, researchers disclosed Wednesday, putting uncounted websites and Web apps at risk from denial-of-service attacks.

In a security advisory issued the same day, Microsoft, whose ASP .Net programming language is one of several affected by the flaw, promised to patch the vulnerability and offered customers ways to protect their servers until it releases an update.

In a follow-up message, Microsoft announced it was shipping an "out-of-band," or emergency update today. The update was released at 1 p.m. ET. Designated MS11-100 , it also fixed three other bugs in ASP .Net, one tagged "critical." None of those three had been disclosed publicly prior to today.

The problem that caused a stir in the security community exists in many of the Web's most popular application and site programming languages, including ASP .Net, the open-source PHP and Ruby, Oracle's Java and Google's V8 JavaScript, according to two German researchers, Alexander Klink and Julian Walde.

Klink and Walde, who presented their findings at the Chaos Communication Congress (CCC) conference in Berlin on Wednesday, traced the flaw to those languages' -- and others' -- handling of hash tables, a programming structure used to quickly store and retrieve data.

Unless a language randomizes hash functions or takes into account "hash collisions" -- when multiple data generates the same hash -- attackers can calculate the data that will trigger large numbers of collisions, then send that data as a simple HTTP request. Because each collision chews up processing cycles on the targeted server, a hacker using relatively small attack packets could consume all the processing power of even well-equipped servers, effectively knocking them offline.

Microsoft confirmed that a single 100K specially-crafted HTTP request sent to a server running ASP .Net would consume 100% of one CPU core for 90-110 seconds.

"An attacker could potentially repeatedly issue such requests, causing performance to degrade significantly enough to cause a denial of service condition for even multi-core servers or clusters of servers," company engineers Suha Can and Jonathan Ness said in a post to the Security Research & Defense blog yesterday.

Klink and Walde estimated that packets as small as 6K would keep a single-core processor busy on a Java server.

The implications are significant for Web apps and sites that run on those servers.

"An attacker with little resources can effectively take out a site fairly easily," said Andrew Storms, director of security operations at nCircle Security, today. "No botnet required to create havoc here."

Microsoft's rush to patch the flaw in ASP .Net hinted at the seriousness of the bug.

"Microsoft will be the one to watch and see if they go out of band and if so, when," Storms said Wednesday night, before Microsoft announced today's patch. "If they do, I sense it will be soon."


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