Home page logo

fulldisclosure logo Full Disclosure mailing list archives

Apple Airport Wireless Products: Promiscuous FTP PORT Allowed in FTP Proxy Provides Security Bypass
From: Sabahattin Gucukoglu <mail () sabahattin-gucukoglu com>
Date: Thu, 4 Mar 2010 17:19:15 +0000

The FTP proxy used in Apple's Airport Express, Airport Extreme, Time Capsule and possibly elsewhere doesn't check the 
client provided address and port given by the FTP PORT command against the IP address of the connecting client, or 
against the use of privileged ports.  (The FTP PORT command is used by a FTP client to tell an FTP  server which 
address and data port to initiate the data connection on.)  The FTP proxy is used to provide assistance to clients 
operating in NAT environments served by the Apple products.  FTP servers running behind a NAT with this assistance can 
have addresses in the command channel rewritten for them so that external clients can reach them when operating in 
passive mode.  The ALG operates as a proxy server, assuming responsibility for connections to the FTP server, and must 
therefore also handle and modify rewriting of the PORT command.  It looks like it might be ftp-proxy from PF.

The effect of this problem is to allow anybody with access to the FTP port forwarded on the exterior side of an Apple 
Airport product that offers NAT to internal clients, which for a publicly-accessible FTP server is the big bad world, 
to induce an FTP server operating behind a NAT to send data to arbitrary addresses and ports.  This is true even if the 
FTP server is configured to operate more securely, since it sees connections from the NAT's exterior interface, not the 
connecting client.  This is useful for bouncing anonymous port scans off the victim NAT, or if data is available or can 
be written to and then read from the FTP server, potentially for anonymous attacks, spam, news floods, and other such 
badness.  Any trust relationship and/or security implied or assumed by a NAT is also gone, since the PORT command can 
also specify private addresses, inside the NAT, for victimisation.  Best of all, the gateway itself makes no log entry 
concerning FTP connections that have been run through the proxy.

Workarounds: do not use FTP; do not trigger the use of the ALG (FTP proxy) by explicitly using ports other than 21 on 
the inbound port mapping.  If you can't do those things, you can avoid the worst effects of this attack by disabling 
FTP uploads that can later be downloaded by anonymous users.

Apple likes to keep secrets for the protection of its customers.  Since the reasonable release of this advisory removes 
that protection, confidential information vouchsafed to me can be safely disclosed with no ill effects.  Apple has a 
fix, and according to its last seemingly automatic template message, they are still testing it and do not know 
precisely when it will be released.  This is confidential information.  DO NOT DISCLOSE!

Advisory history:

Apple were notified on 4 Dec 2009, and responded promptly.  They were given 60 days initially.

Apple contacted me on 7 January 2010 to ask who to give credit to.  Personal attribution.

On 18 Jan I contacted Apple, advising that they'd passed the six weeks milestone.

On 25 January I contacted Apple, advising that they'd passed the 7 weeks milestone.  They volunteered confidential 

On 4 Feb, I urged Apple to tell me when a fix was to be issued, approximately.  They'd had their two months, and 
release cycles happen, but I wanted news within a fortnight.  Didn't they understand that their customers were at easy 
risk, and that keeping it quiet didn't change that?  By today - that is, by about 3 months - they would certainly be 
beyond reconciliation.  They volunteered confidential information.

On 4 March, I got bored of waiting, and made this announcement.  The fix is not out; apply workarounds, or trust to the 
fates and the security of your network.


Attachment: smime.p7s

Full-Disclosure - We believe in it.
Charter: http://lists.grok.org.uk/full-disclosure-charter.html
Hosted and sponsored by Secunia - http://secunia.com/

  By Date           By Thread  

Current thread:
  • Apple Airport Wireless Products: Promiscuous FTP PORT Allowed in FTP Proxy Provides Security Bypass Sabahattin Gucukoglu (Mar 04)
[ Nmap | Sec Tools | Mailing Lists | Site News | About/Contact | Advertising | Privacy ]