Advisory Text :  

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Luigi Auriemma

Application: yaSSL
http://www.yassl.com
Versions: <= 1.7.5
Platforms: Windows and *nix
Bugs: A] buffer-overflow in ProcessOldClientHello
B] buffer-overflow in "input_buffer& operator>>"
C] invalid memory access in HASHwithTransform::Update
Exploitation: remote
Date: 04 Jan 2008
Author: Luigi Auriemma
e-mail: aluigi (at) autistici (dot) org [email concealed]
web: aluigi.org

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1) Introduction
2) Bugs
3) The Code
4) Fix

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

yaSSL is an open source SSL library mainly used in MySQL and in other
projects.
On MySQL, if SSL support is enabled, is possible to use this
vulnerability for pre-authentication code execution.

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

-------------------------------------------
A] buffer-overflow in ProcessOldClientHello
-------------------------------------------

The buffer which contains the data received by the client in the Hello
packet has the following structure (from yassl_imp.hpp):

class ClientHello : public HandShakeBase {
ProtocolVersion client_version_;
Random random_;
uint8 id_len_; // session id
length
opaque session_id_[ID_LEN];
uint16 suite_len_; // cipher suite
length
opaque cipher_suites_[MAX_SUITE_SZ];
uint8 comp_len_; // compression
length
CompressionMethod compression_methods_;
...

where ID_LEN is 32 elements long, MAX_SUITE_SZ 64 and RAN_LEN (Random)
is 32.
The ProcessOldClientHello function called when an old version of the
Hello packet is received doesn't have the needed checks for limiting
the amount of data which will fill these 3 fields leading to a
buffer-overflow vulnerability exploitable for executing code remotely.

From handshake.cpp:

void ProcessOldClientHello(input_buffer& input, SSL& ssl)
...
ClientHello ch;
...
for (uint16 i = 0; i < ch.suite_len_; i += 3) {
byte first = input[AUTO];
if (first) // sslv2 type
input.read(len, SUITE_LEN); // skip
else {
input.read(&ch.cipher_suites_[j], SUITE_LEN);
j += SUITE_LEN;
}
}
ch.suite_len_ = j;

if (ch.id_len_)
input.read(ch.session_id_, ch.id_len_);

if (randomLen < RAN_LEN)
memset(ch.random_, 0, RAN_LEN - randomLen);
input.read(&ch.random_[RAN_LEN - randomLen], randomLen);
...

------------------------------------------------
B] buffer-overflow in "input_buffer& operator>>"
------------------------------------------------

Another buffer-overflow is located in the function used for handling
the normal Hello packet but in this case doesn't seem possible (or
easily possible) to exploit this vulnerability for executing code.

From yassl_imp.cpp:

input_buffer& operator>>(input_buffer& input, ClientHello& hello)
...
hello.id_len_ = input[AUTO];
if (hello.id_len_) input.read(hello.session_id_, ID_LEN);

// Suites
byte tmp[2];
tmp[0] = input[AUTO];
tmp[1] = input[AUTO];
ato16(tmp, hello.suite_len_);
input.read(hello.cipher_suites_, hello.suite_len_);
...

-----------------------------------------------------
C] invalid memory access in HASHwithTransform::Update
-----------------------------------------------------

The usage of a too big size value in the Hello packet leads to a crash
of the library through the reading of data outside the memory
containing the incoming packet.

From hash.cpp:

void HASHwithTransform::Update(const byte* data, word32 len)
{
// do block size increments
word32 blockSz = getBlockSize();
byte* local = reinterpret_cast<byte*>(buffer_);

while (len) {
word32 add = min(len, blockSz - buffLen_);
memcpy(&local[buffLen_], data, add);
...

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===========
3) The Code
===========

http://aluigi.org/poc/yasslick.zip

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======
4) Fix
======

A mail has been sent to the developers yesterday during my tests only
to inform them of the vulnerabilities.

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---
Luigi Auriemma
http://aluigi.org