Zero-day vulnerability in Palo Alto’s PAN-OS Firewall allows hackers to bypass restrictions

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Potential hackers can bypass Palo Alto Network’s PAN-OS Firewall with this zero-day vulnerability

Palo Alto’s PAN-OS Firewall is vulnerable to a zero-day which could let hackers bypass all the restrictions and take control of the host server. The exploitation of this zero-day flaw would allow threat actors to evade security controls in a PAN-OS protected network to evade any security policy that uses URL filtering on a firewall configured with SSL Decryption in the Forward Proxy mode.

PANOS is the software that runs all Palo Alto Networks next-generation firewalls. The zero-day vulnerability has been issued a unique identifier, CVE-2020-2035 with a CVSS score of 3/10.

Systems and servers running any version of the Palo Alto’s PAN-OS Firewall are vulnerable with this flaw. 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.

Any potential hacker can then use this technique to evade detection of communication on the TLS handshake phase between a compromised host and a remote malicious server. However, there are certain preconditions for the flaw to be exploited remotely.

  • A host in the network is already compromised by a malicious actor.
  • The PAN-OS configuration must have the SSL/TLS Decryption Forward Proxy feature enabled for the specific traffic that the attacker controls.
  • There is no other security device in the chain that performs URL Filtering or can block access to malicious URLs or IP-addresses on the encrypted SSL/TLS traffic.

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 says that they have not found any evidence of the vulnerability exploited in the wild nor any they are aware of any malware that uses this technique to exfiltrate data.

Palo Alto Networks has given a workaround to mitigate this zero-day vulnerability here. They have also recommended clients to use an endpoint protection solution such as the Cortex XDR agent to significantly reduce the potential risk of hosts being compromised.

There are currently no PAN-OS updates available for this issue.

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