Hackers Exploiting ProxyLogon and ProxyShell Flaws in Spam Campaigns
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-2021-34473 +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 article506 words · extracted from thehackernews.com · click to collapse
Ravie LakshmananNov 22, 2021
Threat actors are exploiting ProxyLogon and ProxyShell exploits in unpatched Microsoft Exchange Servers as part of an ongoing spam campaign that leverages stolen email chains to bypass security software and deploy malware on vulnerable systems.
The findings come from Trend Micro following an investigation into a number of intrusions in the Middle East that culminated in the distribution of a never-before-seen loader dubbed SQUIRRELWAFFLE. First publicly documented by Cisco Talos, the attacks are believed to have commenced in mid-September 2021 via laced Microsoft Office documents.
"It is known for sending its malicious emails as replies to pre-existing email chains, a tactic that lowers a victim's guard against malicious activities," researchers Mohamed Fahmy, Sherif Magdy, Abdelrhman Sharshar said in a report published last week. "To be able to pull this off, we believe it involved the use of a chain of both ProxyLogon and ProxyShell exploits."
ProxyLogon and ProxyShell refer to a collection of flaws in Microsoft Exchange Servers that could enable a threat actor to elevate privileges and remotely execute arbitrary code, effectively granting the ability to take control of the vulnerable machines. While the ProxyLogon flaws were addressed in March, the ProxyShell bugs were patched in a series of updates released in May and July.
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| DLL infection flow |
Trend Micro said it observed the use of public exploits for CVE-2021-26855 (ProxyLogon), CVE-2021-34473, and CVE-2021-34523 (ProxyShell) on three of the Exchange servers that were compromised in different intrusions, using the access to hijack legitimate email threads and send malicious spam messages as replies, thereby increasing the likelihood that unsuspecting recipients will open the emails.
"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 researchers said, adding the attackers behind the operation did not carry out lateral movement or install additional malware so as to stay under the radar and avoid triggering any alerts.
The attack chain involves rogue email messages containing a link that, when clicked, drops a Microsoft Excel or Word file. Opening the document, in turn, prompts the recipient to enable macros, ultimately leading to the download and execution of the SQUIRRELWAFFLE malware loader, which acts as a medium to fetch final-stage payloads such as Cobalt Strike and Qbot.
The development marks a new escalation in phishing campaigns where a threat actor has breached corporate Microsoft Exchange email servers to gain unauthorized access to their internal mail systems and distribute malicious emails in an attempt to infect users with malware.
"SQUIRRELWAFFLE campaigns should make users wary of the different tactics used to mask malicious emails and files," the researchers concluded. "Emails that come from trusted contacts may not be enough of an indicator that whatever link or file included in the email is safe."
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Text extracted automatically; images, tables and formatting may be missing. Original: https://thehackernews.com/2021/11/hackers-exploiting-proxylogon-and.html