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Half-Life Servers Remote Buffer Overflow and Denial of Service Vulnerabilities
03 August 2003
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From: Auriemma Luigi <aluigi(at)pivx.com>

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Applications: Half-Life (http://half-life.sierra.com)
Versions: 1.1.1.0 and previous versions (including all MODs based on the game, such as Counter-Strike and DoD) 3.1.1.1c1 and 4.1.1.1a of the free dedicated server
Platforms: Windows and Linux
Bugs: Remote buffer overflow and Denial of Service
Risk High
Author: Auriemma Luigi
Senior Security Researcher, PivX Solutions, LLC
e-mail: aluigi@pivx.com
web: http://www.pivx.com/luigi/

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1) Introduction
2) Bugs resume
3) Bugs details (long, unuseful and boring)
4) The Code
5) Fix
6) Researcher's Philosophy

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===============
1) Introduction
===============

Valve's Half-Life was released in 1998 but still remains as the worlds most popular FPS game.

The success of the game is largely due to the overwhelming community support, which has spawned a range of MODs for the game - including the popular Counter-Strike MOD and Day Of Defeat.

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==============
2) Bugs resume
==============

There is a buffer overflow in the Half-Life servers.

Both the dedicated server and the game server are vulnerable.

The only limitation in this buffer-overflow is that some bytes can not be used in the shellcode because they are delimiters or otherwise reserved for use by the Half-Life protocol. This puts some minor constraints on the execution of the remote code, but is far from limiting.

Further, there is a Denial of Service vulnerability that completely freezes the server, entering it into an infinite loop.

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==========================================
3) Bugs details (long, useless and boring)
==========================================

---
The following information is long, read only if you want some useless details on this vulnerability or simply if you want too appreciate all the time I have lost debugging this game, it has not been completely lost 8-)
---

The first thing that I want to specify is that some bytes cannot be used to fully exploit the following buffer-overflow, so the code execution could theoretically be limited.

The explanation of the bug is divided into 4 sections used to show the effects of the long string as parameter and value on the graphical game and on the dedicated server:

BUG 1: buffer-overflow
BUG 2: freeze (infinite loop)

With parameter and value I mean:

\name\Test
| |
| value
parameter

The problem happens because Half-Life uses other instructions if the total lenght of the string sent by the client is major than 256 bytes:

cmp ecx, edi
jl 00d3e687

---------------------------
BUG 1: HLDS.EXE (parameter)
---------------------------

First of all, I want to explain the buffer overflow that only occurs within the Half-Life dedicated server.

During my explanation I will refer only to the exact addresses of the Half-Life 1.1.1.0 dedicated server on Windows (hlds.exe from the retail game).

The problem happens when a too long parameter is passed in the packet to join multiplayer matches sent by the client to the server and the following is an example in C language of the UDP packet plus a big parameter (that I have called PAYLOAD):

#define PAYLOAD [268 chars]
#define BOF "\xff\xff\xff\xff" \
/* 1 */ "connect %d" \
/* 2 */ " %s \"" \
"\\prot\\2" \
"\\unique\\-1" \
"\\raw\\00000000000000000000000000000000" \
"\" \"" \
"\\model\\" MODEL \
"\\topcolor\\" TOPCOLOR \
"\\bottomcolor\\" BOTTOMCOLOR \
"\\rate\\9999.000000" \
"\\cl_updaterate\\20" \
"\\cl_lw\\1" \
"\\cl_lc\\1" \
"\\cl_dlmax\\128" \
"\\hud_classautokill\\1" \
"\\name\\" NAME \
/* 3 */ "\\" PAYLOAD "\\value" \
"\"\n"

where:
1) the first "%d" is the protocol version supported by the server
2) "%s" is the challenge key sent by the server previously
3) PAYLOAD is a long string of 268 chars (268 are needed to overwrite the stored EBP and EIP registers in the stack, respectively at offset 260 and 264 of the PAYLOAD string)

The dangerous code is located in the function at address 0xD3E3F0 of SWDS.DLL (address that in memory will become 0x63ce3f0, so if you want to debug it in real-time remember to add 0x5690000...).

This function seems to be something similar to a strcpy() function but it is used ONLY with parameters, and it looks not only for NULL bytes but also for backslashes '\' in the parameters sent by the client.

The problem however is located "exactly" in the loop that starts from address 0xD3E425 to 0xD3E432:

:00D3E425 3C5C cmp al, 5C
:00D3E427 740B je 00D3E434
:00D3E429 8801 mov byte ptr [ecx], al
:00D3E42B 8A4601 mov al, byte ptr [esi+01]
:00D3E42E 41 inc ecx
:00D3E42F 46 inc esi
:00D3E430 3AC3 cmp al, bl
:00D3E432 75F1 jne 00D3E425

As you can see, this loop makes the following things:

1) if the current byte in our string is equal to '\' the loop will be broken
2) it stores the current byte in memory (buffer overflow)
3) it gets the next byte from our string
4) the pointer now points to the next memory position and next byte of the string
5) if the current byte in our string is a NULL byte the loop will be broken (BL in our case is a NULL byte)

Wonderful!
In the meantime no instruction checks if the string/parameter passed by the client is too long for the local buffer, so our Half-Life server is in a very bad situation... In fact the previously stored EIP (that was equal to 0x63ce614) has been fully overwritten by our string.

-------------------------
BUG 1: HL.EXE (parameter)
-------------------------

:01D3E425 3C5C cmp al, 5C
:01D3E427 740B je 01D3E434
:01D3E429 8801 mov byte ptr [ecx], al
:01D3E42B 8A4601 mov al, byte ptr [esi+01]
:01D3E42E 41 inc ecx
:01D3E42F 46 inc esi
:01D3E430 3AC3 cmp al, bl
:01D3E432 75F1 jne 01D3E425

(The executable seems to decode itself in memory at runtime or something similar)

-----------------------
BUG 2: HLDS.EXE (value)
-----------------------

A similar problem happens in the value field, for example inserting the PAYLOAD as value of a normal parameter, like (C language):

"\\parameter\\" PAYLOAD

In the dedicated server (SWDS.DLL) after the vulnerable loop that checks the parameter (0xD3E425 as seen previously) there is another loop that instead checks the value of the parameters. This loop goes from 0xD3E45B to 0xD3E468:

:00D3E45B 3C5C cmp al, 5C
:00D3E45D 740B je 00D3E46A
:00D3E45F 8801 mov byte ptr [ecx], al
:00D3E461 8A4601 mov al, byte ptr [esi+01]
:00D3E464 41 inc ecx
:00D3E465 46 inc esi
:00D3E466 3AC3 cmp al, bl
:00D3E468 75F1 jne 00D3E45B

This loop copies our string/value to another buffer in memory that is located before the buffer used to store the parameter.

The stack of these functions is similar to the following:

[value_buff]...[parameter_buff]...[EBP][EIP]
0x415df94 0x415e094 0x415e19c

Fortunately Half-Life can accept only values minor/equal than 380 chars (parameters limit is minor than 380), so the string to use to exploit the server is limited and cannot reach the position in memory where is stored the EIP value.

Practical resuming:
- The function that checks the value uses a buffer that starts from the memory position: 0x415df94
- The function that checks the parameter uses a buffer that starts from the memory position: 0x415e094
- EIP is stored at position: 0x415e19c

So: 0x415df94 + 0x17c = 0x415E110 (that is 140 bytes minor than the position of the stored EIP in memory)

However the problem is not finished here because a buffer-overflow doesn't exist in the value, but a good Denial of Service does exist.

In fact, the effect in the Half-Life dedicated server is an infinite loop in SWDS.DLL, from the memory address 0x63ce60d (0xD3E60D) to 0x63ce645 (0xD3E645).

This function simply makes an infinite check of the same string given by the user, this is the simple cause of the DoS.

---------------------
BUG 2: HL.EXE (value)
---------------------

The problem is the same in HLDS.EXE

The only things that change are the offsets of the checking function because the vulnerable loop is in HL.EXE and in memory it starts from 0x01d3e60d to 0x01d3e645:

:01D3E60D 57 push edi
:01D3E60E 56 push esi
:01D3E60F E8DCFDFFFF call 01D3E3F0 <-- vulnerable function

...
:01D3E643 84C0 test al, al
:01D3E645 75C6 jnz 01D3E60D

---

NOTE: the same things happen on the Linux dedicated server too. However I have noted a difference between the 3.1.1.1 and the latest versions of the dedicated server where seems that the buffer-overflow in the parameter cause an infinite loop...

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===========
4) The Code
===========

The proof-of-concept exploit is very simple, and acts partly as a DoS and a code execution exploit.

The return address is overwritten with the offset of a function in SWDLL.DLL that displays a message in the console of the dedicated server, after which it crashes.

This approach was chosen to demonstrate actual code execution without endangering the administrator, enabling the admin to easily verify whether the server is vulnerable.

The POC exploit can be used against both the dedicated and the game servers, overwriting the stored address with 0x063c27f5.

It can be compiled on both Windows and Unix and can test both the buffer-overflow in the parameter (code-execution) and in the value (DoS):

http://www.pivx.com/luigi/poc/hlbof-server.zip

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======
5) Fix
======

Valve was notified of this vulnerability on April 14 2003, and replied that they were working to patch these bugs.

Since that last point of contact, Valve and it's representatives have been contacted on multiple occasions for a status update on the patch, without any replies.

---

I have found a little work-around to the problem (just a jump). Practically I avoid that the data sent by the client will be managed by the vulnerable functions if it is longer than 256 bytes. I don't have the source code of the game and I'm not a Valve's programmer so this is just a personal and simple work-around that I have tested which seems not to have any problems. Feel free to try it until an official patch will be provided and please let me know if there are problems:

Half-Life 1.1.1.0 dedicated server (retail game):

http://www.pivx.com/luigi/patches/hlbof-server-1110-fix.zip

Half-Life 4.1.1.1a dedicated server for Windows:

http://www.pivx.com/luigi/patches/hlbof-server-4111a-fix.zip

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==========================
5) Researcher's Philosophy
==========================

Be free.
The researchers' community needs your reversing, your programs, and your research. Never let your passion die and don't stop your work!

Disclosure:
Full and responsible disclosure can lead to a quick fix, and prevent a problem before it gets into the wrong hands.

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====================
About PivX Solutions ====================

PivX Solutions, is a premier network security consultancy offering a myriad of network security services to our clients.

For more information go to http://www.PivX.com

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---
Researcher
http://www.pivx.com/luigi/



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