Date: Fri, 26 Nov 2010 12:04:22 +0100
From: George Hedfors <George.Hedfors@...ercomgroup.com>
To: "full-disclosure@...ts.grok.org.uk" <full-disclosure@...ts.grok.org.uk>,
	"submissions@...ketstormsecurity.org" <submissions@...ketstormsecurity.org>
Cc: "psirt \(mailer list\)" <psirt@...co.com>
Subject: [CSESA-2010-8] Cisco Clientless SSL VPN Weak URL
 encoding and dangerous default access policy

------------------------------------------------------------------ -- -
Cybercom Sweden AB Security Advisory CSESA-2010-8     info@...ercom.com
http://newsroom.cybercom.com/                            George Hedfors
                                                      November 26, 2010
------------------------------------------------------------------ -- -

Vendor: Cisco Inc.
Product: Cisco ASA 5500 Clientless SSL VPN
Vulnerability: Weak URL encoding and dangerous default access policy
Problem type: remote
CVE id(s): N/A

Cisco Clientless SSL VPN (Secure Desktop) can be misconfigured when
disabling the portal toolbar. The Portal toolbar is independent from
filtering the actual browser requests.

This means that all URL's and plugins are by default allowed even if
the administrator only chooses to publish a few bookmarks to key
systems where users should have access. This may lead to the
possibility of giving unintended access to other systems behind the
ASA.

The URL is transliterated to permit encoding of the user URL's. This
URL is then transmitted inside an already established TLS session.
The URL encoding is however easily broken and altered in order to
specify alternative URL's that may be of interest. 

Plugins for Telnet, SSH and remote desktop are also accessible using
static URLs that also are accessible unless they are disabled.

For SSH:
https://vpn.victim.com/+CSCO+0075676763663A2F2F2E637968747661662E++/ssh,telnet/index.html?target=telnet://x.y.z.w:22?csco_lang=en

For Telnet:
https://vpn.victim.com/+CSCO+0075676763663A2F2F2E637968747661662E++/ssh,telnet/index.html?target=telnet://x.y.z.w:22?csco_lang=en

For RDP:
https://vpn.victim.com/+CSCO+0075676763663A2F2F2E637968747661662E++/rdp/index.html?target=rdp://x.y.z.w/?geometry=1280x800&FullScreen=true&csco_lang=en

The URL obfuscation is done using the good old Caesar cipher, first
used around the year 56AD (according to Wikipedia) with an
overlaying HEX encoding.

Obfuscation example:
https://vpn.victim.com/+CSCO+00756767633A2F2F7A6E76792E69767067767A2E70627A2F726B70756E617472++/
^ ^ ^
uggc://znvy.ivpgvz.pbz/rkpunatr
^ ^ ^
http://mail.victim.com/exchange

Vendor recommendation:
Configure Assign the web ACL to any policies (group policies, dynamic
access policies, or both) that you have configured for clientless
access.
Please follow the guidelines posted in:
ASA Configuration Guide: Configuring Clientless SSL VPN
http://www.cisco.com/en/US/docs/security/asa/asa83/configuration/guide/webvpn.html

Vulnerability report timeline:
8, October 2010 - Initial vulnerability submission
8, October 2010 - Vendor response
4, November 2010 - Vulnerability redefined
17, November 2010 - Vendor provides workaround and suggested mitigations
26, November 2010 - Advisory and workaround release

Cisco bug IDs:
CSCtk08440    ASDM: When portal toolbar is removed, ASDM should point to
WebACL config
CSCtk08633    ASA doc "url-entry" needs to include reference to WebACL

References:
http://www.cisco.com/en/US/docs/security/asdm/6_2/user/guide/vpn_web.html#wp1072626
http://en.wikipedia.org/wiki/Caesar_cipher

Attached URL encode/decode tool (MIME):
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dGQgLSBkZWNvZGUgaGV4IHRvIHN0clxuXG4iOwoJZXhpdCAtMTsKfQo=

------------------------------------------------------------------ -- -

George Hedfors
IT- & Information Security Consultant

Cybercom Sweden East AB
Lindhagensgatan 126   Box 30154   SE-104 25 Stockholm
Phone +46 8 726 75 00   Fax +46 8 19 33 22

PGP: 0x0A13FDB8/79A9 D843 B792 1EA3 B8C6 
                F792 D480 84DE 0A13 FDB8


Download attachment "PGP.sig" of type "application/pgp-signature" (236 bytes)

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