Advisory EM04/2002: Multiple MySQL vulnerabilities



                           e-matters GmbH
www.e-matters.de

-= Security Advisory =-



Advisory: Multiple MySQL vulnerabilities
Release Date: 2002/12/12
Last Modified: 2002/12/13
Author: Stefan Esser [s.esser@hardened-php.net]

Application: MySQL <= 3.23.53a, <= 4.0.5a
Severity: Several vulnerabilities within (lib)MySQL could
allow (remote) compromise of client and/or server.
Risk: Medium to critical
Vendor Status: Vendor released MySQL 3.23.54
Reference: http://security.e-matters.de/advisories/042002.html



Overview:

We have discovered two flaws within the MySQL server that can be used
by any MySQL user to crash the server. Furthermore one of the flaws can
be used to bypass the MySQL password check or to execute arbitrary code
with the privileges of the user running mysqld.

We have also discovered an arbitrary size heap overflow within the mysql
client library and another vulnerability that allows to write '\0' to any
memory address. Both flaws could allow DOS attacks against or arbitrary
code execution within anything linked against libmysqlclient.


Details:

While auditing the MySQL sourcetree we discovered several bugs within
the MySQL client and server that are listed below:


+++ SERVER +++ COM_TABLE_DUMP - Signed Integer Vulnerability

When handling the COM_TABLE_DUMP package MySQL < 4.x takes two chars
from the packet, casts them directly to unsigned integers and uses
them as length parameters for memcpy. Obviously negative values within
the chars will turn into very big unsigned numbers. Because this is a
heap to heap copy operation and there is no memory allocating function
within the SIGSEGV handler we strongly believe this bug can only be used
for denial of service attacks. Depending on the packet mysqld will
directly crash or hang in an endless loop of segmentation faults.
This was tested against Windows, Linux and FreeBSD systems.


+++ SERVER +++ COM_CHANGE_USER - Password Length Vulnerability

In February 2000 Robert van der Meulen discovered a flaw within the
main password authentication system of MySQL: The MySQL challenge
response algorithm creates an expected response with exactly the
length of the response provided by the client. So if the client sends
only a one char response MySQL will check only one byte. But this
means it is possible to give the correct response with only 32 tries
(because the charset is only 32 chars big). When this bug was fixed
in 2000 the MySQL authors simply added a check in the server that the
response must be 8 chars long. However they forgot to add this check
to the COM_CHANGE_USER command, too. So it is still possible for an
attacker with a valid mysql-account to compromise the other accounts
that are allowed to login from the same host. For a local user this
means he can break into the mysql root account and so compromise all
databases. This is especially dangerous in a shared environment or if
the root user is allowed to login from other hosts than localhost.
While the attacker can supply a one byte response to break into the
other accounts he can also send an oversized one. If the response is
longer than 16 chars the internal created expected answer overflows
a stack buffer. If the response is long enough it is possible to
overwrite the saved instruction pointer with bytes that are generated
by the random number generator of the password verification algorithm.
While this sounds hard or impossible to exploit, we successfully
exploited this bug on our linux machines. Due to the fact that mysql
restarts on crash you have unlimited tries. Because of the limited
set of characters generated by the random number generator we strongly
believe that this bug is not exploitable on Windows, because it
is not possible to overwrite the instruction pointer with valid
controllable addresses.


+++ CLIENT +++ libmysqlclient read_rows Overflow

When the MySQL client library receives answer rows from the server it
wants to copy the answers into another buffer. Therefore it loops
through the returned fields and copies them to the other location.
This is done without actually checking if the stored field sizes are
within the destination buffer boundaries. Additionally there is also a
terminating '\0' added to the end of all fields without checking for
enough space within the destination buffer. Due to the fact that this
bug gets already triggered by a simple SELECT query anything that is
linked against libmysql is potentially vulnerable. Due to the nature
of this bug it is trivial to use it as denial of service attack against
the client applications (A negative fieldsize will do the job). If it
possible to use this overflow to execute code on the client system
is different from application to application. It depends mainly on the
fact if malloc() overflows are exploitable on that particular system
and if the application allows enough control over the heap structure
by triggering different execution paths.


+++ CLIENT +++ libmysqlclient read_one_row Byte Overwrittes

When the MySQL client library fetchs one row from the MySQL server it
loops through the fields to remember pointers to the field values.
The field sizes are trusted and not checked against out of boundary
conditions. After remembering the pointer the previous field gets
zero terminated. A malformed packet can supply any field size and so
overwrite some arbitrary memory address with a '\0'. An invalid address
will of course crash the client. Because the address that is written to
is arbitrary (maybe hard to supply because it must be supplied as delta)
all clients that make use of fetching the answer row by row are most
probably vulnerable to arbitrary code execution exploits.


Finally it must be mentioned that an attacker can of course use a
combination of the described attacks to break into a system or to get
access to privileges he normaly does not own. f.e. it is possible for
a local user to crash the server with the COM_TABLE_DUMP bug (if he
cannot takeover the root account with the COM_CHANGE_USER bug) and
then bind a fake server to the MySQL port 3306. And with a fake server
he can exploit the libmysqlclient overflow. Another scenario would be
an attacker that tries to exploit his favourite mod_scripting language
to takeover the webserver by connecting to an external fake server...


Proof of Concept:

e-matters is not going to release an exploit for these vulnerabilities
to the public.


Vendor Response:

03. December 2002 - Vendor was contacted via email.
04. December 2002 - Vendor informs me that bugs are fixed and that
they started building new packages.
12. December 2002 - Vendor has released MySQL 3.23.54 which fixes these
vulnerabilites.


Recommendation:

We suggest anyone using MySQL to upgrade to a new or patched version
as soon as possible.


GPG-Key:

http://security.e-matters.de/gpg_key.asc

pub 1024D/75E7AAD6 2002-02-26 e-matters GmbH - Securityteam
Key fingerprint = 43DD 843C FAB9 832A E5AB CAEB 81F2 8110 75E7 AAD6


Copyright 2002 Stefan Esser. All rights reserved.
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