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X41 D-Sec GmbH Security Advisory: X41-2017-006

Multiple Vulnerabilities in PSFTPd Windows FTP Server
=====================================================

Overview
--------
Confirmed Affected Versions: 10.0.4 Build 729
Confirmed Patched Versions: None
Vendor: Sergei Pleis Softwareentwicklung
Vendor URL: http://www.psftp.de/ftp-server/
Vector: Network
Credit: X41 D-Sec GmbH, Eric Sesterhenn, Markus Vervier
Status: Public
Advisory-URL: https://www.x41-dsec.de/lab/advisories/x41-2017-006-psftpd/


Summary and Impact
------------------
Several issues have been identified, which allow attackers to hide
information in log files, recover passwords and crash the whole server.

It uses neither ASLR nor DEP to make exploitation harder.


Product Description
-------------------
From the vendor page, roughly translated:
PSFTPd is a userfriendly, functional and robust FTP server software with
support for FTP, FTPS and SFTP.



Use after free
==============
Severity Rating: High
Vector: Network
CVE: CVE-2017-15271
CWE: 416
CVSS Score: 7.5
CVSS Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H


Summary and Impact
------------------
An invalid memory access issue could be triggered remotely in the SFTP
component of PSFTPd. This issue could be triggered prior authentication.
The PSFTPd server did not automatically restart, which enabled attackers
to perform a very effective DoS attack against this service. By sending
the following SSH identification / version string to the server, a NULL
pointer dereference could be triggered:

$ cat tmp.14
SSH-2.0-BBBBBBBB
CCCCCCCCCCCC

$ cat tmp.14 | socat - TCP:192.168.122.50:22

The issue appears to be a race condition in the window message handling,
performing the cleanup for invalid connections. Upon further
investigation X41 D-Sec GmbH could confirm that the accessed memory was
already freed.

X41 D-Sec GmbH enabled the memory debugging functionality page heap for
the psftpd_svc.exe exeutable using the command agflags.exe /p /disable
psftpd_svc.exe /fulla. When observing the crash in the WinDBG 19
debugging tool, it could be confirmed that access to an already freed
page was taking place.



Log Injection
=============
Severity Rating: Medium
Vector: Network
CVE: CVE-2017-15270
CWE: 117
CVSS Score: 5.3
CVSS Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N


Summary and Impact
------------------
The PSFTPd server does not properly escape data before writing it into a
Comma Separated Values (CSV) file. This can be used by attackers to hide
data in the Graphical User Interface (GUI) view and create arbitrary
entries to a certain extent.
Special characters as '"', ',' and '\r' are not escaped and can be used
to add new entries to the log.


Workarounds
-----------
None



Passwords stored in Plain Text
==============================
Severity Rating: Low
Vector: Local
CVE: CVE-2017-15272
CWE: 312
CVSS Score: 3.3
CVSS Vector: CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N


Summary and Impact
------------------
The PSFTPd server stores its configuration inside the PSFTPd.dat. This
file is a Microsoft Access Database and can be extracted by using the
command "mdb-export PSFTPd.dat USERS" from mdbtools
(https://github.com/brianb/mdbtools). The application sets the encrypt
flag with the password "ITsILLEGAL", but this is not required to extract
the data.

The users password is shown in clear text, since it is not stored securely.


Workarounds
-----------
Use the Active Directory connector for your users.



FTP Bounce Scan
===============
Severity Rating: Medium
Vector: Network
CVE: CVE-2017-15269
CWE: 441
CVSS Score: 5.0
CVSS Vector: CVSS:3.0/AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:N/A:N


Summary and Impact
------------------
The PSFTPd server does not prevent FTP bounce scans by default.
These can be performed using "nmap -b" and allow to perform scans via
the FTP server.


Workarounds
-----------
It is possible to prevent FTP bounce scans by setting: Kontrollmanager >
 Domain > Sicherheit > Register "FTP Bounce and FXP"




Workarounds
-----------
None



About X41 D-Sec GmbH
--------------------
X41 D-Sec is a provider of application security services. We focus on
application code reviews, design review and security testing. X41 D-Sec
GmbH was founded in 2015 by Markus Vervier. We support customers in
various industries such as finance, software development and public
institutions.



Timeline
--------
2017-08-31  Issues found
2017-09-18  Vendor contacted
2017-09-19  Vendor reply
2017-10-11  CVE IDs requested
2017-10-11  CVE IDs assigned
2017-11-06  Vendor informed us, that apparently a fixed version was
    released. We cannot confirm, since we do not have
    access.
2017-11-07  Public release