Secure Remote Access to an Internal Web Server

Secure Remote Access to an Internal Web Secure Remote Access to an Internal Web ServerServer

Christian Gilmore, David Kormann, and Aviel D. Rubin

ATT Labs - Research

“The security policy usually amounts to total trust of all insiders and total mistrust of outsiders, where the firewall defines the

boundary.”

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OutlineOutline

• Requirements vs. the current architecture constrains.• Proposed solution.• Security assessment of the proposed solution.• Conclusion.• Questions.

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RequirementsRequirements

• Access to the internal web server from outside of the firewall boundary.

• Proposed solution should not involve – changes to the firewall configuration on the network or...– changes to the firewall policies

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EnvironmentEnvironment

• Firewall– “[Inside user] …can establish TCP connection to hosts

outside the firewall on any port, while inbound connections are tightly restricted.”

– “[Firewall] … tears down inactive connections every 15 min.”• DWT (dumb Web Terminal)

– “We strive to treat the DWT as “untrusted” as possible”

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Possible of the shelf solutionsPossible of the shelf solutions

• Telnet or text-based browser such as Lynx– Disadvantage:

• HTML travels in plain text over the network• No support for multimedia

• Tunneling protocols (IPSpec, SSLtelnet).– Disadvantage:

• requires advance access to the remote client browser settings and computer settings.

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ArchitectureArchitecture

Internet

DWT

Proxy

Authentication

Server

Web

Server

Firewall

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ProxyProxy

PushWeb Absent DWTWeb

Server Firewall

Control Connection

Data Connection

Web Request

Web Reply

Web Request

Web Reply

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Authentication and SecurityAuthentication and Security

• User Authentication

– Hash Chaining– User has to re-enter password every 20 minutes

• Connection Confidentiality

– HTTP over SSL - Secure Socket Layer.

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ProxyProxy

PushWeb Absent DWTWeb

Server

Firewall

Control Connection

Data Connection

Web Request

Web Reply

Web Request

Web Reply

SSL Session

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Connection ConfidentialityConnection Confidentiality

• After the user was successfully authenticated the PushWeb establishes the SSL connection to the DWT.

• The SSL on the Server is configured to restrict the set of ciphers supported only to those that provide USA domestic-quality encryption.

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Security AssessmentSecurity Assessment

• Compromise of Absent– DoS attack - not preventable– Eavesdrop on the user session - SSL prevents it.– Replay attack - SSL makes it almost impossible.– Spoofing - user must check SSL certificate.– Obtain root on PushWeb or access the internal web:

• data cannot be moved over control connection• the same effort as from any other outside host

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Security Assessment (Continued)Security Assessment (Continued)

• Compromise of PushWeb– PushWeb has limited access rights on the network– No other services are available from the PushWeb– No user data stored on the PushWeb

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ConclusionConclusion

• The solution achieved its goal:– No changes were required to the network infrastructure– The system provides “...internal Web access from sites such

as terminal rooms and Internet cafes.”

• The system is using well tested protocols - one time password and SSL, but “… protocol composition is a very hard problem and has led to security problems in the past.”

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QuestionsQuestions

• To overcome the firewall policy authors used PushWeb / Absent configuration. Is there any security gain in connecting through Absent machine as oppose to connecting straight through a firewall?

• If there is a gain, than against what type of attacks?

Internet

DWT

SSL

Web

Server

Firewall

PushWeb Absent