Juniper Networks yesterday issued a patch for a serious and exploitable flaw in its router operating system that could cause a router to crash altogether.

The critical security alert issued to Juniper customers and partners set off a rare flurry of router-patching last night, security experts say. "It's not atypical to not apply low-criticality patches to core routers. Now that this has been classified as 'critical,' people seem to be reacting," says Daniel Kennedy, a partner with Praetorian Security Group, which has been studying the vulnerability and says it heard many ISPs were already applying the patches last night.

Organizations don't routinely patch routers; they typically don't have the resources to do so, nor is patching a router as high a priority as patching a server, for instance. Some avoid patching because a major breach is less likely than the possibility of an unintentional outage from applying a router update.

And many organizations incorrectly assume their customized router configurations aren't as big of a target. But security experts -- such as Felix "FX" Lindner, who has conducted groundbreaking research on Cisco router vulnerabilities -- have demonstrated that exploiting routers isn't as hard as was once thought.

Dan Kaminksy, director of penetration testing for IOActive, says this Juniper vulnerability should be patched--and fast: "A remote kernel panic on most versions of the second most popular routing platform on the Internet is significant. IT admins should take this threat to their infrastructure seriously, and patch immediately," Kaminsky says.

There has been no sign of an active exploit for the Juniper router vulnerability yet, but the attack works like this: When the router receives a TCP header with a crafted options field, JUNOS, Juniper's router operating system kernel, crashes and the OS reboots. JUNOS' firewall filter is unable to filter the packet, either. Juniper's JUNOS versions 3, 4, 5, 6, 7, 8, and 9 series routers are all affected by the flaw, which basically results in a denial-of-service attack.

"So JUNOS versions released after January are reported to crash upon receiving a TCP packet that ACL filtering will not stop. Per Juniper, there is no real effective workaround, just application of the provided patch," Kennedy says.

Juniper had fixed the problem about a year ago, he says, but apparently discovered recently that the flaw was exploitable while working on an interoperability issue.

The advisory, which was among seven posted and available only to Juniper customers and partners, included a patch and workarounds, according to Barry Greene, director of Juniper's security incident response team. "These are all scheduled security advisories as part of a monthly schedule (similar to Microsoft's Patch Tuesday). All have fixed code available and feasible workarounds which can be used and deployed immediately by our customers," Greene said in a statement.

Meanwhile, Praetorian today tested all 256 instances of the TCP option field in the header, Kennedy says, and found the one that causes the router to crash. "The exploit is thus confirmed," he says.

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