An anonymous reader quotes a report from Ars Technica : In 2008, researcher Dan Kaminsky revealed one of the more severe Internet security threats ever: a weakness in the domain name system that made it possible for attackers to send users en masse to imposter sites instead of the real ones belonging to Google, Bank of America, or anyone else. With industrywide coordination, thousands of DNS providers around the world installed a fix that averted this doomsday scenario. Now, Kaminsky’s DNS cache poisoning attack is back. Researchers on Wednesday presented a new technique that can once again cause DNS resolvers to return maliciously spoofed IP addresses instead of the site that rightfully corresponds to a domain name. On Wednesday, researchers from Tsinghua University and the University of California, Riverside presented a technique that, once again, makes cache poisoning feasible. Their method exploits a side channel that identifies the port number used in a lookup request. Once the attackers know the number, they once again stand a high chance of successfully guessing the transaction ID. The side channel in this case is the rate limit for ICMP, the abbreviation for the Internet Control Message Protocol. To conserve bandwidth and computing resources, servers will respond to only a set number of requests from other servers. After that, servers will provide no response at all. Until recently, Linux always set this limit to 1,000 per second. To exploit this side channel, the new spoofing technique floods a DNS resolver with a high number of responses that are spoofed so they appear to come from the name server of the domain they want to impersonate. Each response is sent over a different port. When an attacker sends a response over the wrong port, the server will send a response that the port is unreachable, which drains the global rate limit by one. When the attacker sends a request over the right port, the server will give no response at all, which doesn’t change the rate limit counter. If the attacker probes 1,000 different ports with spoofed responses in one second and all of them are closed, the entire rate limit will be drained completely. If, on the other hand, one out of the 1,000 ports is open, then the limit will be drained to 999. Subsequently, the attacker can use its own non-spoofed IP address to measure the remaining rate limit. And if the server responds with one ICMP message, the attacker knows one of the previously probed 1,000 ports must be open and can further narrow down to the exact port number. Linux kernel developers responded by introducing a change that causes the rate limit to randomly fluctuate between 500 and 2,000 per second, preventing the new technique from working. Cloudflare also introduced a fix where its DNS service will fall back to TCP, “which is much more difficult to spoof,” reports Ars. The researchers’ press release is available here.

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