JUNOS (Juniper) Flaw Exposes Core Routers to Kernel Crash

A report has been received from Juniper at 4:25pm under bulletin PSN-2010-01-623 that a crafted malformed TCP field option in the TCP header of a packet will cause the JUNOS kernel to core (crash). In other words the kernel on the network device (gateway router) will crash and reboot if a packet containing this crafted option is received on a listening TCP port. The JUNOS firewall filter is unable to filter a TCP packet with this issue. Juniper claims this issue as exploit was identified during investigation of a vendor interoperability issue.

There is talk that backbone Internet providers have been quickly patching this issue since yesterday night.

TCP Header Option Space

“Options occupy space at the end of the TCP header. All options are included in the checksum. An option may begin on any byte boundary. The TCP header must be padded with zeros to make the header length a multiple of 32 bits.”

(Source: http://www.networksorcery.com/enp/protocol/tcp.htm)

[caption id=“attachment_2819” align=“alignnone” width=“300” caption=“The TCP Header”]The TCP Header[/caption] Source: http://www.software-engineer-training.com/wp-content/uploads/2007/12/tcp_header.png

The Kernel

At a high level, the kernel in an operating system serves as the bridge between applications and the actual data processing of the hardware the OS is running on. The kernel manages system resources and abstracts resources that applications must access.

[caption id=“attachment_2837” align=“alignnone” width=“300” caption=“Basic Kernel Representation”]Basic Kernel Representation[/caption]

Affected Devices

It is basically all of them save the more recent version. If you’ve installed a device with a JUNOS release version released later then 1/28/09, this issue is already corrected. Apparently the original issue and its correction did not conceive of this problem as a security vulnerability, and thus the criticality of applying the patch was not initially understood until this week.

Please note the versions below were removed from the bulletin today, 01/07/09. This is likely because, as Matt pointed out below, these are end of life versions of the OS (meaning likely still vulnerable if you happen to be running them, but out of scope for Juniper because from their standpoint these should already have been upgraded).

Juniper’s Advice

Juniper references best common practice (BCP) 38, a methodology for reducing the amount of bad packets being forwarded by network devices (basically prohibiting packets where the originator can’t effectively be identified), as a possible mitigating control.

However there is no completely effective workaround available other then upgrading the OS.


Juniper responded to the Register as follows:

“that the bulletin was one of seven security advisories the company issued under a policy designed to prevent members of the public at large from getting details of the vulnerabilities.”

“Because of Juniper’s ‘Entitled Disclosure Policy,’ only our customers and partners are allowed access to the details of the Security Advisory,”

  • Juniper spokeswoman

Interesting approach, and probably would be better received if vulnerabilities only affected those entitled. Unfortunately the networks that run high end Juniper equipment serve a great many end users, and thus in this case the general public would probably like some informed background. At the point the media is contacting you, it is safe to say the “cat is out of the bag”. And this is the response from a company that is a strong player in the information security appliance space?

The flip side is that the Juniper response to this issue from a technical perspective has appeared to be at first glance fairly comprehensive, a PR opportunity if managed correctly.

And yes, this is the same firm that feels this way when it is they who are discussing the vulnerability of someone else’s product:

Juniper believes that Jack’s research (on ATM vulnerabilities) is important to be presented in a public forum in order to advance the state of security,

We agree with the second Juniper: more education, especially after the problem has been corrected, is better.


More information will be posted as it becomes available. This was a serious issue which appears to have been averted through a coordinated response. Essentially, given the core equipment (big Telco routers) running “Big Iron” type Juniper network devices, portions of the Internet could have gone black with a successful implementation of this exploit. Routers at this level are not patched like your local Windows OS, it takes something important to justify an outage. As previously noted, even though the code problem itself was identified last year, it appears that the problem was not identified as a mechanism for creating a remote exploit until now, raising the criticality of patching the issue severely.