F5 Big IP opens up vulnerability during configuration

Security issues in F5 devices potentially make hundreds of thousands of load balancers into cyber-attack entry-points

F5 Networks’ popular load balancing software BIG-IP harbours a vulnerability that could let malicious code seep in using just an online form.

"Load balancing is an important web management process that keeps many internet services ticking," said F-Secure, which spotted the vulnerability. "Without it, banks, governments, and other organisations providing online services to large numbers of people would struggle to keep their websites running."

Senior security consultant Christoffer Jerkeby discovered the vulnerability during a routine assessment. More than 300,000 organisations including state bodies and large banks use BIG-IP, broadening the scope of potential damages. 

"Further research found that, following a successful exploit, an adversary could turn the compromised device back against the organisation or even individuals using the affected services," it added.

F-Secure said it is yet to receive reports of misuse of this vulnerability.

"F5 is a popular technology and we found over 300,000 (potentially vulnerable) devices on-line. Google and services like Shodan can be used to search for these also. The F5 website also contains information on their clients and sectors," Dave Hartley, technical director at F-Secure, told SC Media UK.

He refrained from revealing the names of the vulnerable F5 clients, saying: "These companies may be at risk of compromise."

Curiously, BIG-IP itself is immune to the vulnerability. It creeps up only when an organisation mis-configures BIG-IP’s iRules, the procedures written in tool command language (Tcl) to direct incoming web traffic toward the correct web server.

"These iRules are created using the tool command language (Tcl). Certain coding practices that may seem perfectly functional and practical to an organisation can allow an attacker to inject arbitrary Tcl commands, which could be executed in the security context of the target Tcl script," the report said.

In some implementations of BIG-IP, injecting a malicious code can be as easy as filling an online form, F-Secure revealed.

The threat posed is way bigger than a simple data breach, Hartley told SC Media UK. "An attacker can leverage the issue to achieve much more than a data breach alone. If the prerequisites are met, it could result in an adversary taking full control of the F5." 

However, the scope and depth of the attack will vary, depending on the objective of the attacker and the scale of operations of the target organisation, he said.

"The F5 is often used for perimeter security and traffic management and there have been issues identified in similar technologies in the past that have had a severe impact for some -- firewalls, anti-virus, e-mail sandbox technologies etc.," he explained.

Addressing the issue, F5 released a public advisory

"This is not a vulnerability in Tcl, or F5 products, but rather an issue relating to coding practices used when writing Tcl code. As with most programming or scripting languages, it is possible to write code in a way that may create vulnerabilities. This is not something F5 can prevent the user from doing, as the issue does not lend itself to neither deterministic nor heuristic detection that covers all possible cases," it said.

"The Tcl documentation previously cited provides more comprehensive recommendations. However, the simple answer is that expressions in Tcl should always be braced," it added.

Find this article useful?

Get more great articles like this in your inbox every lunchtime

Video and interviews