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SMF 1.0.7 and lower plus 1.1rc2 and lower - IP spoofing vulnerability/IP ban evasion vulnerability

2007.02.17
Credit: Jessica Hope
Risk: Low
Local: No
Remote: Yes
CWE: N/A


CVSS Base Score: 7.5/10
Impact Subscore: 6.4/10
Exploitability Subscore: 10/10
Exploit range: Remote
Attack complexity: Low
Authentication: No required
Confidentiality impact: Partial
Integrity impact: Partial
Availability impact: Partial

====================================================================== Advisory : SMF 1.0.7 and lower plus 1.1rc2 and lower - IP spoofing vulnerability/IP ban evasion vulnerability Release Date : June 02, 2006 Application : SMF Version : SMF 1.0.7 and previous versions, SMF1.1rc2 and lower Platform : PHP Vendor URL : http://www.simplemachines.org/ Author : Jessica Hope (jessicasaulhope (at) googlemail (dot) com [email concealed]) ====================================================================== Overview The IP detection section of SMF's code allows for someone to spoof the X-Forwarded-For header. SMF trusts this value over the IP address reported in general. This allows an attacker to login and post using IP's that are not theirs, making it impossible for the Administrator of the SMF forum to ban the user. ====================================================================== Discussion There's code in QueryString.php that starts: elseif (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) { // If there are commas, get the last one.. probably. if (strpos($_SERVER['HTTP_X_FORWARDED_FOR'], ',') !== false) { $ips = array_reverse(explode(', ', $_SERVER['HTTP_X_FORWARDED_FOR'])); // Go through each IP... foreach ($ips as $i => $ip) { // Make sure it's in a valid range... if (preg_match('~^((0|10|172\.16|192\.168|255|127\.0)\.|unknown)~', $ip) != 0) continue; // Otherwise, we've got an IP! $_SERVER['REMOTE_ADDR'] = trim($ip); break; } } // Otherwise just use the only one. elseif (preg_match('~^((0|10|172\.16|192\.168|255|127\.0)\.|unknown)~', $_SERVER['HTTP_X_FORWARDED_FOR']) == 0) $_SERVER['REMOTE_ADDR'] = $_SERVER['HTTP_X_FORWARDED_FOR']; } This code is used to obtain the users IP address. However, if X-Forwarded-For HTTP header has been provided, it will take the last IP address from the X-Forwared-For header and blindly trust it to be the real IP address. The problem is that the X-Forwarded-For HTTP header is easily forgable via a number of methods. For example, if the X-Forwarded-For header was set: X-Forwarded-For: 1.2.3.4 the SMF application trusts 1.2.3.4 to be their IP address, and will reflect this fact when the user does anything that SMF logs, such as posting to the forum.This makes it possible for a user to set the X-Forwareded-For IP to that of another user in an attempt to masquerade as them. It also would require the SMF administrator to track down the users real IP via httpd server logs, assuming this is possible, which in some cases it is not. This would also assume the SMF administrator knows the IP presented to them isn't real. On top of this, there's code in Security.php that starts: // Check if we have a valid IP address. if (preg_match('/^(\d{1,3})\.(\d{1,3})\.(\d{1,3})\.(\d{1,3})$/', $user_info['ip'], $ip_parts) == 1) { $ban_query[] = "(ban_type = 'ip_ban' AND ($ip_parts[1] BETWEEN ip_low1 AND ip_high1) AND ($ip_parts[2] BETWEEN ip_low2 AND ip_high2) AND ($ip_parts[3] BETWEEN ip_low3 AND ip_high3) AND ($ip_parts[4] BETWEEN ip_low4 AND ip_high4))"; // IP was valid, maybe there's also a hostname... if (empty($modSettings['disableHostnameLookup'])) { $hostname = @gethostbyaddr($user_info['ip']); if (strlen($hostname) > 0) $ban_query[] = "(ban_type = 'hostname_ban' AND ('$hostname' LIKE hostname))"; } } This code indicates that a user could bypass bans on their IP by setting the X-Forwarded-For header, seeing as SMF blindly trusts the X-Forwarded-For IP to be real. ====================================================================== Solution The offical response from SMF was: 'Trusting any server variable that carries the real IP address would make no sense. A hacker would be able to manipulate all headers being received by PHP. SMF tries to give the best estimate of the user's real IP address, but on the internet the IP address authenticity can never be relied upon fully. ' - Hendrik Jan Visser - Lead Developer Simple Machines Plus, when pressed about it being a vulnerability, they had this to say: 'I thought we had replied that none of these represent a security issue? Could you please state again how these could be exploited?' - Grudge ( Matthew Wolf - Developer Simple Machines ). This issue was first reported on the 14th Apr 2006, the first reply was on the 23rd Apr 2006. The follow ups were on the 10th May 2006 and 24th May 2006, with the final reply of not being an issue sent on the 24th May 2006. Thus there is no offical solution. The solution that I will advise is to remove the code quoted from QueryString.php so that SMF doesn't trust the X-Forwarded-For variable. This will also make it possible to properly ban someone by IP address. ====================================================================== Credit This issue is to be credited to Jessica Hope ( jessicasaulhope (at) googlemail (dot) com [email concealed] )


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