Attackers Heavily Targeting VPN Vulnerabilities

Attacks on virtual private networks, like those this week targeting a trio of known vulnerabilities in Pulse Secure appliances, have intensified in recent months along with the increase in remote and hybrid work environments since the outbreak of COVID-19.

The trend requires organizations to patch VPN and other externally facing devices with the highest priority, says a new report from Digital Shadows.

The report, based on an analysis of vulnerability activity in first quarter of 2021, highlights other threats as well, including increased targeting of remote code execution (RCE) vulnerabilities such as one affecting Oracle WebLogic (CVE-2020-14882) and widespread attacks targeting the ProxyLogon flaws in Microsoft Exchange Server.

"[VPNs] continue to be targeted by a plethora of threat groups, which will almost certainly continue for the remainder of 2021," says Chris Morgan, senior cyber-threat intelligence analyst at Digital Shadows. "VPN devices, in addition to other remote access software, are often prioritized as a useful entry point that can provide threat groups with a stable foothold onto target networks."

The threat intelligence firm's analysis of vulnerability activity in the first quarter of this year shows cyber adversaries are actively targeting VPN vulnerabilities, more so than most other attack avenues, to break into enterprise networks. VPN accesses were among the top three access types listed for sale on cybercriminal forums last quarter, Digital Shadows says.

According to the firm, attackers targeted vulnerabilities in a range of VPN appliances, including one in the Fortinet FortiGate VPN (CVE-2018-13379) and an older, previously patched flaw in Pulse Connect Secure VPN (CVE-2019-11510). Both the Fortinet and Pulse VPN appliances were the subject of a joint advisory last week from the National Security Agency (NSA), FBI, and the Cyber Security & Infrastructure Security Agency (CISA). The advisory warned US organizations of Russia's Foreign Intelligence Services (SVR) — the actor behind the SolarWinds attack — actively targeting the VPN flaws and flaws in three other products.

"Easily identifiable public-facing infrastructure will always garner significant attention from advanced actors," Morgan says, pointing to the attacks that targeted Pulse Secure VPNs this week. The attacks — by multiple threat groups, including one believed to have links to the Chinese government — have affected several organizations within the US defense industrial base and other sectors. Researchers are currently tracking as many as 12 separate malware families targeting vulnerabilities in Pulse Secure VPNs. Patches have been available for some time for all three of the vulnerabilities in Pulse Secure VPNs that are being attacked.

Thousands of Attacks
Meanwhile, other significant threat activity that Digital Shadows observed last quarter included heavy targeting of RCE flaws and a barrage of attacks aimed at ProxyLogon, a set of four critical vulnerabilities in Exchange Server, which Microsoft disclosed in March.

"Tens of thousands of companies worldwide were impacted by exploiting and chaining of the four zero-day vulnerabilities," Morgan says. "Our observation of this particular set of bugs includes a diverse set of threat groups, including both nation-state and cybercriminal actors."

The sheer scope of the attack activity highlighted both the ease with which the now-patched vulnerabilities could be exploited and the multiple potential courses of action available to an attacker after successful exploitation, he says.

A major concern related to the attacks was the strategy by one hacking group to deploy malicious Web shells on compromised Exchange Server systems so they could maintain a persistent presence on them. Concerns over the Web shells on US systems were so high that a court authorized the FBI to remove the shells from systems on which they have been deployed, including those belonging to private companies.

"While active exploitation of the bugs will likely subside in the aftermath of companies updating their servers, there is a distinct possibility that advanced groups could have created other avenues of approach and entry points onto targeted networks," Morgan warns. Last week, CISA updated its original guidance around the flaws, which suggests that Exchange Servers are still being compromised via these bugs even though a vast majority of vulnerable systems have been patched, he says.

Digital Shadows' first-quarter threat analysis shows that RCE flaws were the most commonly exploited flaws, just as they were in the fourth quarter of 2020. Twenty-three percent of attacks involved RCE exploits in the first quarter. The most likely reason for attackers targeting this class of vulnerabilities, according to Digital Shadows, is that they enable a wide range of malicious activities.