Crooks compromise Microsoft Exchange servers to hijack internal email chains
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2021-26855 | Unauthenticated SSRF/RCE in Microsoft Exchange Server (ProxyLogon) CVE-2021-26855 is a server-side request forgery flaw (CWE-918) in Microsoft Exchange Server that allows an unauthenticated remote attacker to send specially crafted HTTP requests and have the Exchange server process them as itself, disclosing sensitive session information. When chained with sibling Exchange flaws (the 'ProxyLogon' chain), it yields authentication bypass and arbitrary file write, escalating to full remote code execution with SYSTEM-level privileges on the on-premises Exchange server. Any organization running an affected on-premises Exchange server reachable over HTTP/HTTPS (typically outbound webmail) is exposed; Exchange Online was not affected. Exploitation is confirmed in the wild at large scale: the flaw was mass-exploited beginning in early 2021 (notably by the HAFNIUM group), is on the CISA KEV with documented ransomware use, and has a maximum EPSS score of 100% (100th percentile), despite no public PoC listing. Do: Apply the vendor's March 2021 Exchange security updates (or later cumulative updates) immediately, per the CISA required action; until patched, limit Exchange (ECP/OWA) exposure to the internet via firewall/VPN rules. Hunt for compromise: review IIS logs for unrecognized authenticated activity against FrontEnd HttpProxy endpoints, and check for malicious files or webshells under inetpub\wwwroot\aspnet_client, given the known ransomware use. | 9.1 | 100% | KEV ransomware PoC ×4 |
| masshundreds of thousands of on-premises deployments; tens of thousands of internet-exposed Exchange servers | |
| 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 article423 words · extracted from securityaffairs.com · click to collapse
Pierluigi Paganini
November 21, 2021
A malware campaign aimed at Microsoft Exchange servers exploits ProxyShell and ProxyLogon issues and uses stolen internal reply-chain emails.
A malware campaign aimed at Microsoft Exchange servers exploits ProxyShell and ProxyLogon issues and uses stolen internal reply-chain emails to avoid detection.
The campaign was uncovered by TrendMicro researchers that detailed the technique used to trick victims opening the malicious email used as the attack vector.
The attacks were orchestrated by Squirrelwaffle, a threat actor known for sending malicious spam as replies to existing email chains.
The investigation into three incidents revealed that attackers used exploits for CVE-2021-26855 (ProxyLogon), CVE-2021-34473, and CVE-2021-34523 (ProxyShell).
Once compromised the Exchange servers, threat actors use the access to reply to the company’s internal emails in reply-chain attacks containing links to weaponized documents. Sending the messages from the organizations allow the attackers to bypass detection.
“In the same intrusion, we analyzed the email headers for the received malicious emails, the mail path was internal (between the three internal exchange servers’ mailboxes), indicating that the emails did not originate from an external sender, open mail relay, or any message transfer agent (MTA).” reads the analysis published by Trend Micro. “Delivering the malicious spam using this technique to reach all the internal domain users will decrease the possibility of detecting or stopping the attack, as the mail getaways will not be able to filter or quarantine any of these internal emails.”
The emails originate from the same internal network, appear to be a continuation of a previous discussion between two employees.
The attacker did not use tools for lateral movement or execute malware on the Exchange servers to avoid detection.
The emails use weaponized Office documents or include a link to them. Upon enabling the content, malicious macros are executing to download and install the malware, such as Qbot, Cobalt Strike, and SquirrelWaffle.
The excel sheets used in this campaign contain malicious Excel 4.0 macros used to download and execute the malicious DLL.
Experts recommend securing their Microsoft Exchange servers by installing security updates published by Microsoft.
“As mentioned earlier, by exploiting ProxyLogon and ProxyShell attackers were able to bypass the usual checks that would have stopped the spread of malicious email.” concludes the analysis. “It is important to ensure that patches for Microsoft Exchange Server vulnerabilities, specifically ProxyShell and ProxyLogon (CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207) have already been applied.”
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