Conti ransomware gang targets Microsoft Exchange servers with ProxyShell exploits
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2021-31207 | Security feature bypass in Microsoft Exchange Server (ProxyShell) enables webshell RCE CVE-2021-31207 is a security feature bypass in Microsoft Exchange Server, classified as an unrestricted file-write issue (CWE-434) that lets an attacker bypass intended restrictions and write files of their choosing to the server. It is the final bug in the ProxyShell chain: chained with the autodiscover SSRF and PowerShell backend elevation flaws, it allows an unauthenticated attacker to reach the Exchange PowerShell API, write arbitrary files such as an ASPX webshell, and execute code with SYSTEM privileges. A successful attacker gains full control of the on-premises Exchange server, including mailbox access, stolen credentials, and a foothold for lateral movement; the bug has been used to deploy ransomware and keyloggers. On-premises deployments of Microsoft Exchange Server (2013, 2016, and 2019 per vendor advisories) are affected, while Exchange Online/cloud mailboxes are not. Exploitation is essentially certain and ongoing: the flaw is in CISA's KEV (added 2021-11-03) with known ransomware use, public ProxyShell PoC/exploit code is available, and EPSS puts the 30-day exploitation probability at 99.8%. Do: Apply Microsoft's July 2021 security updates for Exchange Server 2013, 2016, and 2019 (or any later security/cumulative update) per vendor instructions, and restrict untrusted access to Exchange's autodiscover and PowerShell endpoints. Hunt for ASPX webshells under the Exchange FrontEnd directories and review IIS logs for autodiscover.json requests chaining PowerShell, since many servers were compromised before patching. Given known ransomware use and KEV listing, treat any server that was unpatched or internet-exposed during the exploitation window as potentially compromised. | 6.6 | 100% | KEV ransomware PoC |
| massHundreds of thousands of on-premises Exchange servers (public scans around the July 2021 ProxyShell disclosure showed roughly 400,000-600,000 internet-exposed… | |
| CVE-2021-34473 +1 in the same advisory: …34523 | SSRF (CWE-918) in Microsoft Exchange Server Enabling RCE (ProxyShell) Microsoft Exchange Server contains a server-side request forgery (SSRF, CWE-918) in the Autodiscover service that unauthenticated remote attackers can reach over HTTP. This flaw is the first stage of the widely documented 'ProxyShell' chain, in which the SSRF is combined with privilege escalation and an arbitrary file write in the Exchange PowerShell backend to achieve unauthenticated remote code execution on the server. An attacker who successfully exploits it gains the ability to run code on the Exchange server, and ransomware operators (e.g., Cuba ransomware) leveraged this chain to deploy payloads. Organizations running on-premises Microsoft Exchange Server (2013, 2016, and 2019 per the vendor's advisories) are affected; hosted Exchange Online is a separately managed cloud service. Exploitation is confirmed in the wild: CISA added the flaw to the KEV catalog on 2021-11-03 with known ransomware use, and EPSS assigns it a 100% probability of exploitation within 30 days (100th percentile). Do: Apply Microsoft's July 2021 (or later) Exchange Server security updates, which include the ProxyShell fixes, to all on-premises servers. If patching is delayed, restrict internet access to the Autodiscover/EWS frontend endpoints and consider the URL Rewrite mitigation Microsoft published for ProxyShell. Given the KEV listing and known ransomware use, treat internet-facing, unpatched Exchange servers as potentially compromised and hunt for webshells, unusual Exchange processes, and suspicious account or mailbox activity. | 9.1 group max | 100% | KEV ransomware PoC |
| masshundreds of thousands of on-prem Exchange servers; public internet scans at the time of disclosure showed roughly 400,000+ internet-exposed Exchange instances,… |
Full article624 words · extracted from securityaffairs.com · click to collapse
Pierluigi Paganini
September 03, 2021
The Conti ransomware operators are targeting Microsoft Exchange servers leveraging recently disclosed ProxyShell vulnerability exploits.
The Conti ransomware gang is targeting Microsoft Exchange servers leveraging exploits with recently disclosed ProxyShell vulnerabilities.
ProxyShell is the name of three vulnerabilities that could be chained by an unauthenticated remote attacker to gain code execution on Microsoft Exchange servers.
The three vulnerabilities used in ProxyShell attacks are:
- CVE-2021-34473 – Pre-auth Path Confusion leads to ACL Bypass (Patched in April by KB5001779)
- CVE-2021-34523 – Elevation of Privilege on Exchange PowerShell Backend (Patched in April by KB5001779)
- CVE-2021-31207 – Post-auth Arbitrary-File-Write leads to RCE (Patched in May by KB5003435)
The vulnerabilities are exploited remotely through Microsoft Exchange’s Client Access Service (CAS) running on port 443 in IIS.
The vulnerabilities were discovered by security Researcher Tsai orange from Devcore, the issues were awarded $ 200,000 during the April 2021 Pwn2Own hacking contest.
Last week, while researchers from Sophos providing incident response support to a customer discovered that the threat actors breached the network exploiting Microsoft Exchange ProxyShell vulnerabilities.
Conti Ransomware operators, like other threat actors, are attempting to target organizations using Exchange Server that have yet to update their installs.
Once gained access to the network they first dropped web shells to execute commands and compromise the server, then manually deployed the ransomware to infect the larger number of systems as possible on the network.
“In the case of one of the group of ProxyShell-based attacks observed by Sophos, the Conti affiliates managed to gain access to the target’s network and set up a remote web shell in under a minute. Three minutes later, they installed a second, backup web shell. Within 30 minutes they had generated a complete list of the network’s computers, domain controllers, and domain administrators. Just four hours later, the Conti affiliates had obtained the credentials of domain administrator accounts and began executing commands.” reads the analysis published by Sophos. “Within 48 hours of gaining that initial access, the attackers had exfiltrated about 1 Terabyte of data. After five days had passed, they deployed the Conti ransomware to every machine on the network, specifically targeting individual network shares on each computer.”
Experts noticed that the ransomware gang installed less than seven backdoors on the target network, a couple of web shells, Cobalt Strike, and AnyDesk, Atera, Splashtop and Remote Utilities commercial remote access tools.
“Using ProxyShell, the attackers created a new mailbox for “administrator,” and then assigned new roles to that mailbox using Microsoft Exchange “cmdlets”—including rights to remotely execute PowerShell commands.” continues the report. “In another recent Conti ProxyShell attack, the attacker created a mailbox referencing Evil Corp, the organization behind Dridex (as well as a fictional company the television show Mr. Robot) , as part of the ProxyShell attack.”
Once gained access to the target network, the ransomware operators upload stolen data to the MEGA file sharing server. Only after five days, the group started encrypting the devices on the network launching the attack from an unprotected server.
The threat actors launched batch files that repeatedly invoked the ransomware executable “x64.exe2, experts noticed that in each iteration it targeted specific drives on every Windows system on the network by their default file sharing names (C$, D$, etc.):
start C:\x64.exe -m -net -size 10 -nomutex -p \\[computer Active Directory name]\C$
The Conti ransomware operators were able to exfiltrate 1 TB of data in only 48 hours.
Unfortunately, multiple threat actors already leverage ProxyShell vulnerabilities in attacks aimed at organizations worldwide, for this reason, experts recommend admins install cumulative updates on their servers.
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(SecurityAffairs – hacking, Conti Ransomware)
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Text extracted automatically; images, tables and formatting may be missing. Original: https://securityaffairs.com/121815/cyber-crime/conti-ransomware-gang-proxyshell.html