External risk intelligence

Microsoft Exchange Server Remote Code Execution Vulnerability.

CVE advisoryKnown Exploit

CVE-2021-26857

Microsoft Exchange Server has a vulnerability that permits unauthorized code execution. This could lead to system compromise and data loss for affected organizations. The business risk is significant, requiring prompt attention to mitigate potential impacts on operations and data security.

5Halo Surface Signal

Deserialization

Microsoft Exchange Server

2010201320162019

External exposure likelihood

Halo Surface Signal score for CVE-2021-26857

Microsoft Exchange Server is an enterprise email and collaboration platform designed to be exposed to the internet to facilitate remote access for mail clients, web-based email (OWA), and synchronization services. Its role as an internet-facing gateway or edge service makes it a quintessential example of an internet-accessible system in standard business deployments.

Horizon Alert

Summary of the vulnerability and why it matters

Microsoft Exchange Server contains a flaw that allows for unauthorized code execution. This vulnerability, related to deserialization of untrusted data, can lead to the execution of arbitrary code on affected systems. Such an event could compromise the confidentiality, integrity, and availability of business data and systems.

Vulnerable component: Microsoft Exchange Server
Core weakness: Deserialization of untrusted data
Main business impact: System compromise and data loss

Attack Path

How an attacker could exploit the issue

Microsoft Exchange Server can be targeted through an internal exposure, allowing an attacker with initial access to leverage this vulnerability. Exploitation can lead to significant compromise, impacting system integrity and confidentiality. Organizations using affected versions of Exchange Server face potential risks to their data and operational continuity.

Internal system access required.
Attacker triggers a vulnerability.
Remote code execution and data compromise.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability in Microsoft Exchange Server presents a significant risk due to its potential for remote code execution. Organizations utilizing affected versions of Exchange Server could face severe compromise if this vulnerability is exploited. The complexity of exploitation and the impact on business operations necessitate a high level of attention.

Likely attacker skill level: Low
Required access or conditions: None
Business risk or urgency: High

Priority actions

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability affects Microsoft Exchange Server installations and could allow attackers to execute code remotely. Given its classification as an internal exposure and its presence on the Known Exploited Vulnerabilities catalog, prompt action is recommended to mitigate potential business risk. Organizations should prioritize identifying and securing their Exchange Server assets.

Find all Exchange Server assets.
Reduce exposure or isolate risk.
Fix, verify, and monitor.

Frequently asked questions

What is Microsoft Exchange Server and its primary function?

Microsoft Exchange Server is an enterprise-level platform designed for email and collaboration. It enables organizations to manage email, calendars, contacts, and tasks, often providing users with remote access to these services.

What type of vulnerability is CVE-2021-26857?

CVE-2021-26857 is a remote code execution vulnerability found in Microsoft Exchange Server. The core weakness is identified as insecure deserialization (CWE-502), which can allow an attacker to execute malicious code on the server.

How can CVE-2021-26857 be exploited by attackers?

This vulnerability in the Exchange Unified Messaging service can be exploited by attackers through deserialization of untrusted data. Successful exploitation, often chained with other vulnerabilities like CVE-2021-26855, can lead to arbitrary code execution with SYSTEM-level privileges on the Exchange server, requiring prior authentication or another vulnerability to be exploited first.

What is the relevance of Halo Surface Signal for CVE-2021-26857?

Halo Surface Signal identifies CVE-2021-26857 as 'Very likely' to be exploited because Microsoft Exchange Server is an internet-facing system commonly used for remote access, making it a prime target for exploitation.

What are the recommended actions for mitigating CVE-2021-26857?

Organizations should immediately apply the security updates released by Microsoft for all affected Exchange Server versions. Running the Microsoft Safety Scanner (MSERT) tool is also recommended to detect and remove known threats. Prioritize patching externally facing servers, and investigate for signs of compromise even after patching.

References

Written by Nicholas Merritt

External-surface CVE analysis and Halo Surface Signal prioritization for Halo Threat Intelligence.