Patching vulnerable enterprise VPNs from Pulse Secure is not enough to keep out malicious actors who have already exploited a vulnerability, the U.S. Cybersecurity and Infrastructure Security Agency (CISA) warns.
Last year, security researchers disclosed many flaws in VPN products from Fortinet, Palo Alto Networks and Pulse Secure, revealing that attackers could target them to infiltrate corporate networks, steal sensitive data, or even eavesdrop on communications.
A total of 10 vulnerabilities were reported to Pulse Secure in March last year, and patches for them were released on April 24, 2019. The most severe of these issues, which was assigned CVE-2019-11510 (CVSS score 10), can be abused by an unauthenticated, remote attacker to execute arbitrary code.
In August last year, over 14,500 vulnerable VPN servers were identified globally and the National Security Agency (NSA) issued an alert in October to warn that threat actors were actively targeting the vulnerability. As of January 2020, cybercriminals were exploiting the bug to deliver ransomware.
In August last year, Pulse Secure said that a majority of customers had installed the fixes released in April, but CISA now says that patching alone might not be enough to ensure the security of affected systems.
The vulnerability, CISA says, is being targeted with crafted requests for “files that allow for Credential Dumping plaintext passwords from the VPN appliance.”
In an update to an alert issued in January 2020, CISA says that the attackers might still have access to enterprise networks that were previously compromised via the Pulse Secure vulnerability, if administrators did not change credentials after applying the available patches.
“Although Pulse Secure released patches for CVE-2019-11510 in April 2019, CISA has observed incidents where compromised Active Directory credentials were used months after the victim organization patched their VPN appliance,” the agency says.
Cybercriminals were observed connecting to compromised environments using Tor and virtual private servers (VPSs) to avoid detection.
After gaining access to the networks, threat actors would create scheduled tasks, install remote access Trojans, or abuse tools such as LogMeIn and TeamViewer to achieve persistence. They would also harvest files for exfiltration, and execute ransomware, CISA says.
One attacker was observed attempting to sell the stolen credentials following numerous unsuccessful attempts to connect to an environment to install ransomware. The same threat actor was also spotted targeting hospitals and U.S. government entities with ransomware.
CISA also developed detection methods to help organizations find out whether their patched appliances were hit by attackers targeting CVE-2019-11510, as they may still be at risk for exploitation from pre-patch compromise.
Network administrators could detect past attacks by looking at unauthenticated log requests, checking logs for exploit attempts and lateral movement activity, reviewing logs for unauthorized sessions, and using CISA’s IOC detection tool to identify artifacts associated with the exploitation of CVE-2019-11510.
“CISA strongly urges organizations that have not yet done so to upgrade their Pulse Secure VPN to the corresponding patches for CVE-2019-11510. If—after applying the detection measures in this alert—organizations detect evidence of CVE-2019-11510 exploitation, CISA recommends changing passwords for all Active Directory accounts, including administrators and services accounts,” CISA concludes.
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