CVE-2020-2035: PAN-OS: URL filtering policy is not enforced on TLS handshakes for decrypted HTTPS sessions
First published: Wed Aug 12 2020(Updated: )
When SSL/TLS Forward Proxy Decryption mode has been configured to decrypt the web transactions, the PAN-OS URL filtering feature inspects the HTTP Host and URL path headers for policy enforcement on the decrypted HTTPS web transactions but does not consider Server Name Indication (SNI) field within the TLS Client Hello handshake. This allows a compromised host in a protected network to evade any security policy that uses URL filtering on a firewall configured with SSL Decryption in the Forward Proxy mode. A malicious actor can then use this technique to evade detection of communication on the TLS handshake phase between a compromised host and a remote malicious server. This technique does not increase the risk of a host being compromised in the network. It does not impact the confidentiality or availability of a firewall. This is considered to have a low impact on the integrity of the firewall because the firewall fails to enforce a policy on certain traffic that should have been blocked. This issue does not impact the URL filtering policy enforcement on clear text or encrypted web transactions. This technique can be used only after a malicious actor has compromised a host in the protected network and the TLS/SSL Decryption feature is enabled for the traffic that the attacker controls. Palo Alto Networks is not aware of any malware that uses this technique to exfiltrate data. This issue is applicable to all current versions of PAN-OS. This issue does not impact Panorama or WF-500 appliances.
Credit: psirt@paloaltonetworks.com
| Affected Software | Affected Version | How to fix |
|---|---|---|
| Paloaltonetworks Pan-os |
Remedy
Palo Alto Networks is currently working to improve our inspection engines by adding a URL filtering policy check on both the TLS SNI field and the HTTP Host and URL headers for decrypted HTTPS transactions. Apply any of the workarounds to completely mitigate the risk of evasion. There are currently no PAN-OS updates available for this issue.
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Reference Links
Frequently Asked Questions
What is CVE-2020-2035?
CVE-2020-2035 is a vulnerability that affects Palo Alto Networks PAN-OS firewall software.
How does CVE-2020-2035 impact SSL/TLS Forward Proxy Decryption mode?
CVE-2020-2035 allows attackers to bypass the PAN-OS URL filtering feature in SSL/TLS Forward Proxy Decryption mode.
What is the severity of CVE-2020-2035?
CVE-2020-2035 has a low severity rating.
How can I fix CVE-2020-2035?
To fix CVE-2020-2035, update your Palo Alto Networks PAN-OS software to the latest version.
Where can I find more information about CVE-2020-2035?
More information about CVE-2020-2035 can be found on the Palo Alto Networks security advisory page and the mnemonic blog.
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