Juniper Networks, one of the leading manufacturers of high-end networking equipment, has issued a security advisory concerning a critical vulnerability, which affects all versions of its operating system. The flaw allows attackers to crash routers by simply sending a specially crafted TCP packet that cannot be easily filtered.

According to the security bulletin, identified as PSN-2010-01-623, the issue consists of a malformed field option in the header of a TCP packet. Upon receiving such a packet on any listening port, the JUNOS kernel will crash and the device will be forced to reboot.

The vulnerability affects all networking devices running versions of the JUNOS operating system starting with 3.x onward, with the exception of the ones built after January 28, 2009. The company was aware of the issue and addressed it almost a year ago, but its security implications were not discovered until just recently.

"Because of Juniper's 'Entitled Disclosure Policy,' only our customers and partners are allowed access to the details of the Security Advisory," a Juniper spokesperson commented for The Register. According to Praetorian Security Group, a managed security provider, affected Internet providers have been hard at work to deploy patches since Tuesday night.

It is worth noting that Juniper's high-performance routers are used by large ISPs and Internet backbones that service hundreds of thousands, if not millions, of users worldwide. Considering that Juniper's own firewall is unable to filter out these malicious packets, it is understandable why the company would want to keep details out of the public domain for as long as possible.

Temporary mitigation can be achieved by following the guidelines described in the BCP38 (Best Current Practices) document of The Internet Engineering Task Force (IETF). This refers to techniques such as ingress filtering, for defeating Denial of Service (DoS) attacks.