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/

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 “gflags.exe /p /disable psftpd_svc.exe /full”. 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"

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

Author: Eric Sesterhenn
Date: November 10, 2017