ProxyShell: Attackers Actively Scanning for Vulnerable Microsoft Exchange Servers (CVE-2021-34473)

Three vulnerabilities from DEVCORE researcher Orange Tsai could be chained to achieve unauthenticated remote code execution. Attackers are searching for vulnerable instances to exploit.

Background

Last week at the Black Hat USA and DEF CON security conferences, DEVCORE researcher Orange Tsai presented a talk titled “ProxyLogon is Just the Tip of the Iceberg: A New Attack Surface on Microsoft Exchange Server!” In his Black Hat presentation, he walked through three vulnerabilities in Microsoft Exchange Server:

Source: Tenable, August 2021

*Please note: Tenable’s Vulnerability Priority Rating (VPR) scores are calculated nightly. This blog post was published on August 9 and reflects VPR at that time.

Orange Tsai is a prolific researcher who has found many high-severity vulnerabilities in a wide range of products. Most relevant is CVE-2021-26855, aka ProxyLogon, which Tsai reported to Microsoft in January (Volexity and Microsoft Threat Intelligence Center also received credit for discovering this vulnerability). Despite this, ProxyLogon was exploited as a zero-day by the threat group HAFNIUM and other advanced persistent threat actors. Even after Microsoft issued an out-of-band patch for ProxyLogon, it continues to be exploited by threat actors for various types of attacks from cryptomining and creating botnets to ransomware.

Analysis

CVE-2021-34473 is a remote code execution vulnerability and the highest rated, receiving a CVSSv3 score of 9.1. CVE-2021-34523 and CVE-2021-31207 were both initially rated as “Exploitation Less Likely” according to Microsoft’s Exploitability Index because of their independent features, but when chained together, they have significant value to attackers. By chaining these vulnerabilities, an attacker could execute arbitrary commands on vulnerable Exchange servers on port 443. Two of the three ProxyShell vulnerabilities, CVE-2021-34473 and CVE-34523, were patched as part of the April 2021 Patch Tuesday release, though Microsoft says they were “inadvertently omitted” from that security update guide. CVE-2021-31207 was patched in May.

Attackers are actively scanning for Exchange Servers vulnerable to ProxyShell

On August 6, security researcher Kevin Beaumont reported attempts to exploit this vulnerability chain in the wild.

My Exchange honeypot has somebody dropping files and executing commands. https://t.co/pMVsl7y4iy

— Kevin Beaumont (@GossiTheDog) August 6, 2021

Over the following days, several Computer Security Incident Response Teams issued alerts about attackers scanning for vulnerable Microsoft Exchange Servers. Because of how widely exploited the ProxyLogon and other Exchange Server vulnerabilities have been so far this year, we recommend organizations patch immediately. Attackers are already finding vulnerable servers to exploit and it may be prudent to initiate incident response procedures if you know you have unpatched servers on your network.

Proof of concept

After Tsai’s presentations last week, two other researchers published their reproduction of Tsai’s work which included more technical details on how to exploit the vulnerability chain. One of the researchers, Jang, also published a proof-of-concept for ProxyLogon earlier this year.

Vendor response

Microsoft has patched all of these vulnerabilities in its April and May Patch Tuesday releases. CVE-2021-34473 and CVE-2021-34523 were patched in April 2021 but Microsoft did not publish advisories until July.

Identifying affected systems

A list of Tenable plugins to identify these vulnerabilities can be found here.

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