CVE-2021-22893: Zero-Day Vulnerability in Pulse Connect Secure Exploited in the Wild

Threat actors are leveraging a zero-day vulnerability in Pulse Connect Secure, for which there is no immediate patch scheduled for release.

Background

On April 20, Pulse Secure, which was acquired by Ivanti last year, published an out-of-cycle security advisory (SA44784) regarding a zero-day vulnerability in the Pulse Connect Secure SSL VPN appliance. In addition to the advisory, Pulse Secure also published a blog post detailing observed exploit behavior related to the zero-day as well others linked to previously disclosed vulnerabilities in its Pulse Connect Secure solution.

Analysis

CVE-2021-22893 is a critical authentication bypass vulnerability in Pulse Connect Secure. While no specific details about the flaw are available yet, it is likely that a remote, unauthenticated attacker could exploit this vulnerability by sending a specially crafted HTTP request to a vulnerable device. Successful exploitation of this vulnerability would grant an attacker the ability to execute arbitrary code on the Pulse Connect Secure Gateway. The vulnerability has been assigned a CVSSv3 score of 10.0, underscoring its severity.

Attackers leveraging multiple Pulse Connect Secure vulnerabilities

The Pulse Secure blog post notes that the bulk of attacker-related activity is centered around the following three previously known vulnerabilities.

*Please note: Tenable’s Vulnerability Priority Rating (VPR) scores are calculated nightly. This blog post was published on April 8 and reflects VPR at that time.

One of the three flaws, CVE-2019-11510, is a pre-authentication vulnerability in Pulse Connect Secure that has been exploited in the wild since August 2019, including by ransomware groups and foreign threat actors. Attackers have also leveraged CVE-2019-11510 as part of a vulnerability chain to gain initial access before pivoting into using CVE-2020-1472, also known as Zerologon, to gain domain admin access over an organization’s Active Directory infrastructure. CVE-2019-11510 is also one of the Top 5 vulnerabilities we highlighted in Tenable’s 2020 Threat Landscape Retrospective report because of its ease of exploitation and continued preference amongst a variety of attackers long after patches were made available for it.

The other two vulnerabilities, CVE-2020-8243 and CVE-2020-8260, are post-authentication vulnerabilities that require an attacker to have established administrator access to the vulnerable Pulse Connect Secure device. Based on the authentication requirement for these vulnerabilities, they are likely to be used in combination with CVE-2019-11510 and CVE-2021-22893 as part of a chained attack.

Researchers at NCCGroup published technical advisories in October 2020 for both flaws. For CVE-2020-8243, the researchers detail how an attacker could exploit the flaw to gain arbitrary code execution privileges on the underlying operating system by injecting a backdoored template file. For CVE-2020-8260, the researchers detail how an attacker could exploit the flaw to perform an overwrite of arbitrary files, resulting in remote code execution.

Defense, government and financial organizations targeted

According to an article in Reuters, Pulse Connect Secure vulnerabilities including CVE-2021-22893 have been used to target U.S. government, defense and financial organizations. Researchers are attributing these attacks to China-linked threat actors.

Implanting malware and harvesting credentials

In a blog post from FireEye's Mandiant division, researchers identified at least 12 malware families linked to all four of these Pulse Connect Secure SSL VPN vulnerabilities, which they’ve been tracking from August 2020 through March 2021.

According to FireEye, some of the threat actors they’ve identified are harvesting account credentials in order to perform lateral movement within compromised organizations environments. They have also observed threat actors deploying modified Pulse Connect Secure files and scripts in order to maintain persistence.

Proof of concept

At the time this blog post was published, there were no proof-of-concept (PoC) exploit scripts available for CVE-2021-22893. However, there are at least 14 PoCs for CVE-2019-11510, a working PoC for CVE-2020-8243, and PoC details for CVE-2020-8260.

Solution

As of April 20, Pulse Secure has not yet released patches to address CVE-2021-22893, though the projected timeframe for availability is sometime in May 2021. However, Pulse Secure did release a temporary workaround that can be implemented to mitigate attempts to exploit the zero-day.

The temporary workaround requires disabling two features within the Pulse Connect Secure appliances: Windows File Share Browser and Pulse Secure Collaboration. Because the workaround details may be updated in the future, please refer to the Pulse Secure advisory for more information.

Identifying affected systems

A list of Tenable plugins to identify CVE-2021-22893 will appear here as they’re released. For the remaining Pulse Connect Secure vulnerabilities, please refer to the table below.

Get more information

• SA44784: Pulse Secure Out-of-Cycle Advisory for CVE-2021-22893
• Pulse Secure Blog Post: "Pulse Connect Secure Security Update"
• FireEye/Mandiant Blog Post: APT Activity Linked to Pulse Connect Secure Flaws
• NCCGroup Blog Post for CVE-2020-8243
• NCCGroup Blog Post for CVE-2020-8260

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