Mass Exploitation of Exchange Server ProxShell Bugs
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 article252 words · extracted from infosecurity-magazine.com · click to collapse
Tens of thousands of global Microsoft Exchange servers could be at risk after threat actors began exploiting three so-called “ProxyShell” vulnerabilities.
The three bugs were discovered in the April Pwn2Own competition and patched by Microsoft in April and May. However, the tech giant only assigned CVEs to them in July, complicating efforts by some sysadmins to check if their systems were vulnerable.
In the meantime, threat actors managed to take publicly available information on the vulnerabilities and craft exploits for the three bugs.
Now the Cybersecurity and Infrastructure Security Agency (CISA) has urged vulnerable organizations to patch the flaws.
“Malicious cyber actors are actively exploiting the following ProxyShell vulnerabilities: CVE-2021-34473, CVE-2021-34523, and CVE-2021-31207. An attacker exploiting these vulnerabilities could execute arbitrary code on a vulnerable machine,” it said.
“CISA strongly urges organizations to identify vulnerable systems on their networks and immediately apply Microsoft’s Security Update from May 2021 — which remediates all three ProxyShell vulnerabilities—to protect against these attacks.”
Security experts have warned that threat actors actively scan for vulnerable servers to install web shells on, enabling further malicious activity. The situation calls to mind the four zero-day ProxyLogon bugs patched in March, which were exploited far and wide.
Huntress Lab said it had seen over 140 web shells installed across 1900+ unpatched servers in just 48 hours last week.
The bugs are apparently also being used in conjunction with the recently revealed PetitPotam vulnerability to deliver LockFile ransomware.
Symantec explained the threat in an updated blog post yesterday.
Text extracted automatically; images, tables and formatting may be missing. Original: https://www.infosecurity-magazine.com/news/mass-exploitation-exchange-server/