CVE-2021-26855: Microsoft Exchange Server Remote Code Execution Vulnerability

What it is

Microsoft Exchange Server Remote Code Execution Vulnerability

Exploitation evidence

First exploit evidence seen: 2021-03-02 (earliest date across the sources below).

Exploited in the wild

• CISA KEV: listed since 2021-11-03. Source

Public exploit code

Metasploit

• auxiliary/gather/exchange_proxylogon_collector: Microsoft Exchange ProxyLogon Collector (disclosure 2021-03-02)
• auxiliary/scanner/http/exchange_proxylogon: Microsoft Exchange ProxyLogon Scanner (disclosure 2021-03-02)
• exploit/windows/http/exchange_proxylogon_rce: Microsoft Exchange ProxyLogon RCE (disclosure 2021-03-02)

Exploit-DB

• EDB-49637: Microsoft Exchange 2019 - Server-Side Request Forgery (Proxylogon) (PoC) (2021-03-11)
• EDB-49663: Microsoft Exchange 2019 - Server-Side Request Forgery (2021-03-14)
• EDB-49879: Microsoft Exchange 2019 - Unauthenticated Email Download (2021-05-18)
• EDB-49895: Microsoft Exchange 2019 - Unauthenticated Email Download (Metasploit) (2021-05-21)

GitHub proof-of-concept repositories

53 public repositories from 51 accounts, first seen 2021-03-03. Most starred: Flangvik/SharpProxyLogon (247 stars), view repository.

Verify before running: public PoCs can be malicious. Read the code and only run it in an isolated lab. A PoC means exploit code is public, not that this CVE is exploited in the wild.

Sources: CISA KEV · Rapid7 Metasploit Framework module metadata · Exploit-DB (OffSec) · nomi-sec/PoC-in-GitHub.

How to fix it

CISA required action

Apply updates per vendor instructions. (source: CISA KEV)

Fixed versions

NVD does not list a fixed version for this CVE. Use the vendor advisory below.

Derived from the NVD CPE configuration (versionEndExcluding / versionEndIncluding). Always confirm with the vendor advisory.

Vendor advisories and patches

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26855 (Patch, Vendor Advisory)
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26855 (Patch, Vendor Advisory)

MITRE ATT&CK mitigations for this chain

• M1021 Restrict Web-Based Content (for T1133) : Restrict all traffic to and from public Tor nodes.
• M1030 Network Segmentation (for T1133) : Deny direct remote access to internal systems through the use of network proxies, gateways, and firewalls.
• M1042 Disable or Remove Feature or Program (for T1133, T1505.003) : Disable or block remotely available services that may be unnecessary.
• M1035 Limit Access to Resource Over Network (for T1133) : Limit access to remote services through centrally managed concentrators such as VPNs and other managed remote access systems.
• M1032 Multi-factor Authentication (for T1133) : Use strong two-factor or multi-factor authentication for remote service accounts to mitigate an adversary's ability to leverage stolen credentials, but be aware of [Multi-Factor Authentication Interception](https://attack.mitre.org/techniques/T1111) techniques for some two-factor authentication implementations.
• M1018 User Account Management (for T1505.003) : Enforce the principle of least privilege by limiting privileges of user accounts so only authorized accounts can modify the web directory.
• M1057 Data Loss Prevention (for T1005) : Data loss prevention can restrict access to sensitive data and detect sensitive data that is unencrypted.
• M1037 Filter Network Traffic (for T1090) : Traffic to known anonymity networks and C2 infrastructure can be blocked through the use of network allow and block lists. It should be noted that this kind of blocking may be circumvented by other techniques like [Domain Fronting](https://attack.mitre.org/techniques/T1090/004).
• M1031 Network Intrusion Prevention (for T1090) : Network intrusion detection and prevention systems that use network signatures to identify traffic for specific adversary malware can be used to mitigate activity at the network level. Signatures are often for unique indicators within protocols and may be based on the specific C2 protocol used by a particular adversary
• M1020 SSL/TLS Inspection (for T1090) : If it is possible to inspect HTTPS traffic, the captures can be analyzed for connections that appear to be domain fronting.

Detection Generic guidance

• Initial Access: Alert on unusual requests to the affected service (odd paths, oversized payloads, exploit strings) in WAF, reverse-proxy and application logs.
• Persistence: Look for new files in web roots, new scheduled tasks/services, and new accounts created after the exposure window.
• Collection: Watch for bulk reads of databases, file shares or mailboxes by the affected service.
• Command and Control: Alert on outbound connections from the server to new or rare destinations.

Response checklist Generic guidance

1. Inventory: find every instance of the affected product and version (include internet-facing and forgotten ones).
2. Patch or mitigate per the vendor advisory; if you cannot patch, remove internet exposure or apply the vendor workaround.
3. Assume compromise if the system was exposed before patching: hunt for the indicators in the advisory and review logs back to the disclosure date.
4. Rotate credentials and secrets stored on or used by the affected system if compromise cannot be ruled out.