BigIP iRule Tcl code injection
Description
F5 BIG-IP products include iRule functionality that allows administrators to write custom traffic management logic using Tool Command Language (Tcl). When iRule code or other Tcl scripts on BIG-IP systems are written without proper input sanitization, attackers can inject malicious Tcl commands into user-controlled data. These injected commands are then executed by the Tcl interpreter with the privileges of the BIG-IP system. This vulnerability affects iRules, Local Traffic Policies, iCall scripts, and any custom Tcl code running on BIG-IP appliances.
Remediation
Always enclose Tcl expressions in curly braces <strong>{ }</strong> to prevent command substitution and injection attacks. Braces ensure that expressions are evaluated literally without interpreting variable or command substitutions from user input.<br/><br/>Review all iRules, Local Traffic Policies, and custom Tcl scripts for unbraced expressions, particularly in conditional statements and string comparisons.<br/><br/><strong>Vulnerable code example:</strong><pre>if $myVar eq "String" { # User input in $myVar can inject commands }</pre><strong>Secure code example:</strong><pre>if {$myVar eq "String"} { # Expression is evaluated safely without substitution }</pre>Additionally, validate and sanitize all user-supplied input before using it in Tcl expressions. Avoid using the <strong>eval</strong> and <strong>subst</strong> commands with untrusted data. Consult F5 Knowledge Article K15650046 for comprehensive guidance on secure Tcl coding practices for BIG-IP systems.