Forum Discussion

Dave_Pisarek's avatar
Dave_Pisarek
Icon for Cirrus rankCirrus
May 19, 2021

XFF and sleep

Recently I was asked about mitigating the below XFF header:   X-Forwarded-For: (select(0)from(select(sleep(5)))v)/*'+(select(0)from(select(sleep(5)))v)+'"+(select(0)from(select(sleep(5)))v)+"*/   ...
ASM Advanced WAF
BIG-IP
security
JRahm's avatar
JRahm
Icon for Admin rankAdmin

If you don't have ASM/Advanced WAF and would like to do this in an iRule, you could do a check to make sure the value(s) are at least numeric and then act accordingly on finding something else. For example:

% set x (select(0)from(select(sleep(5)))v)/*'+(select(0)from(select(sleep(5)))v)+'"+(select(0)from(select(sleep(5)))v)+"*/
(select(0)from(select(sleep(5)))v)/*'+(select(0)from(select(sleep(5)))v)+'"+(select(0)from(select(sleep(5)))v)+"*/
% foreach ip $x {
  if { [string is integer [lindex [split $ip "."] 0]] } {
    puts "header ok"
  } else { puts "header not ok" }
 
 
}
header not ok
% set x 1.2.3.4
1.2.3.4
% foreach ip $x {
  if { [string is integer [lindex [split $ip "."] 0]] } {
    puts "header ok"
  } else { puts "header not ok" }
 
 
}
header ok

Note that this is very rudimentary to show what's possible. If you want to verify actual valid IP addresses in the header, far more logic is required.

Daniel_Wolf's avatar
Daniel_Wolf
Icon for MVP rankMVP
May 26, 2021

I took a different approach with an iRule. You can verify it the XFF header contains a valid IP address:

when HTTP_REQUEST {
  if { [scan [HTTP::header "X-Forwarded-For"] %d.%d.%d.%d a b c d] == 4 && 0 <= $a && $a <= 255 && 0 <= $b && $b <= 255 && 0 <= $c && $c <= 255 && 0 <= $d && $d <= 255}  {
    #log local0. "Valid XFF Header: [HTTP::header "X-Forwarded-For"]"
  } else {
    reject
    #log local0. "Invalid XFF Header: [HTTP::header "X-Forwarded-For"]"
  }
}

From my log:

May 26 08:15:16 awaf.mydomain.de info tmm1[11286]: Rule /Common/rule_verify_xff <HTTP_REQUEST>: Invalid XFF Header: (select(0)from(select(sleep(5)))v)/*'+(select(0)from(select(sleep(5)))v)+'"+(select(0)from(select(sleep(5)))v)+"*/

May 26 08:15:57 awaf.mydomain.de info tmm3[11286]: Rule /Common/rule_verify_xff <HTTP_REQUEST>: Invalid XFF Header: 10.200.200.1000

May 26 08:16:06 awaf.mydomain.de info tmm2[11286]: Rule /Common/rule_verify_xff <HTTP_REQUEST>: Invalid XFF Header: 10.200.200.256

May 26 08:16:26 awaf.mydomain.de info tmm2[11286]: Rule /Common/rule_verify_xff <HTTP_REQUEST>: Valid XFF Header: 10.200.200.254

  • JRahm's avatar
    JRahm
    Icon for Admin rankAdmin
    May 26, 2021

    It's true that scan as written will validate a specific IP address, but it will not handle the case where an IP address is in the string with other characters, or in a list of IP addresses (as is sometimes the case with XFF), or finally, a valid IP address in a superset range where, in the case of network 10.10.10.0/23, the IP 10.10.10.255 is completely valid because the broadcast is 10.10.10.11.255. (Update...actually, we just need to validate that it is a possible IP address, not that it is valid in the context of a client network, so ignore this last point. And with that, just adding a foreach loop to your logic to handle multiple IP addresses might be good enough)

     

    That's why I stayed rudimentary, because the logic is a project since we don't have the full breadth of Tcl libraries available in iRules that have already solved this problem.

     

    Also, ASM/AdvWAF is the way to go here, if you have it. And if you don't, why not? :)