CVE-2021-31196
Microsoft Exchange Server Information Disclosure Vulnerability
Description
CVE-2021-31196 is a high-severity remote code execution vulnerability (CVSS 7.2) in Microsoft Exchange Server affecting versions 2013, 2016, and 2019. This vulnerability allows an authenticated attacker with high privileges to execute arbitrary code on the Exchange server. It has been added to CISA's Known Exploited Vulnerabilities (KEV) catalog in 2024, confirming active exploitation in the wild years after its initial disclosure in 2021. The EPSS score of 3.34% (87.1st percentile) indicates a significantly elevated exploitation probability, and the continued targeting of Exchange Server vulnerabilities by threat actors makes timely patching essential.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:HOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| microsoft | exchange server | 2013; 2016; 2019 |
Multiple CVSS Assessments
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-31196(Patch, Vendor Advisory)
- https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31196(Patch, Vendor Advisory)
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31196(US Government Resource)
Weakness Type
Remote Code Execution in Microsoft Exchange Server
This vulnerability enables remote code execution on Microsoft Exchange Server, one of the most widely deployed enterprise email platforms. Remote code execution (RCE) vulnerabilities in Exchange Server are particularly dangerous because Exchange servers typically hold elevated privileges within Active Directory environments, process sensitive email communications, and are internet-facing by design. The RCE class of vulnerability allows attackers to execute arbitrary commands or code on the target system, potentially leading to complete server compromise. Exchange Server has been a frequent target of sophisticated threat actors, as demonstrated by the ProxyLogon and ProxyShell attack campaigns that exploited similar RCE vulnerabilities in 2021.
Impact Analysis
The CVSS score of 7.2 (High) reflects a serious threat that requires authenticated high-privilege access to exploit. Confidentiality impact is rated High, meaning successful exploitation could expose all email communications, attachments, contact lists, calendars, and other sensitive data stored on the Exchange server. Integrity impact is also High, allowing attackers to modify emails, create forwarding rules, manipulate mailbox data, or plant backdoors on the server. Availability impact is High as well, since remote code execution could be used to disrupt email services for the entire organization.
The business risk of an Exchange Server compromise extends far beyond email disruption. Exchange servers often hold domain-joined machine accounts with elevated Active Directory privileges, making them a prime pivot point for lateral movement across the enterprise network. Attackers who gain code execution on an Exchange server can potentially harvest credentials, access other internal systems, and establish persistent access to the corporate environment. The EPSS score of 3.34% (87.1st percentile) indicates that this vulnerability is significantly more likely to be exploited than the average CVE. The fact that CISA added this 2021 vulnerability to the KEV catalog in 2024 demonstrates that threat actors continue to target unpatched Exchange servers years after fixes were released. The ransomware association is unknown, though Exchange Server vulnerabilities have historically been leveraged in ransomware campaigns.
Exploit Maturity
CVE-2021-31196 has been confirmed as actively exploited, evidenced by its inclusion in CISA's Known Exploited Vulnerabilities catalog with a remediation deadline of September 11, 2024. Notably, this vulnerability was originally disclosed and patched in July 2021, but its addition to the KEV catalog three years later indicates that exploitation persists due to organizations failing to apply patches. The EPSS score of 3.34% (87.1st percentile) confirms the elevated exploitation risk. Microsoft has published a security advisory with detailed patch information. The continued targeting of Exchange Server vulnerabilities by advanced persistent threat groups, ransomware operators, and initial access brokers means that any unpatched Exchange server is at significant risk. While the ransomware association for this specific CVE is listed as unknown, the Exchange Server attack surface has been extensively leveraged in numerous high-profile ransomware incidents.
Remediation
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Apply Microsoft security updates immediately — Install the cumulative update and security patch from the Microsoft Security Response Center advisory. Ensure that Exchange Server 2013, 2016, or 2019 is updated to the latest available cumulative update and security update.
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Verify patch installation — Use the Exchange Server Health Checker script (available from Microsoft on GitHub) to confirm that all security updates have been properly applied and that no known vulnerabilities remain unpatched.
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Audit Exchange Server access and accounts — Review all accounts with administrative access to Exchange Server. Remove unnecessary elevated privileges, enforce multi-factor authentication for Exchange management access, and disable any unused service accounts.
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Scan for indicators of compromise — Run the Microsoft Safety Scanner and EOMT (Exchange On-premises Mitigation Tool) to detect signs of prior exploitation. Check for web shells in IIS directories, unusual scheduled tasks, suspicious mailbox forwarding rules, and unauthorized administrative accounts.
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Consider migration to Exchange Online — For organizations still running on-premises Exchange, evaluate migration to Microsoft 365 Exchange Online. Cloud-hosted Exchange receives automatic security updates, reducing the patching burden and closing the window of exposure for server-level vulnerabilities.
Technical Details
The CVSS v3.1 vector string CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H details the attack requirements and impact. Attack Vector (Network) means the vulnerability is remotely exploitable, typically through Exchange's web services. Attack Complexity (Low) indicates that no special conditions are needed beyond meeting the authentication requirement. Privileges Required (High) means the attacker needs elevated Exchange administrative credentials. User Interaction (None) confirms the attack can be carried out without any victim action. Scope (Unchanged) keeps the impact within the Exchange server context, though in practice, compromising an Exchange server often provides a path to broader Active Directory compromise. All three impact metrics are rated High.
Microsoft Exchange Server remote code execution vulnerabilities typically exploit weaknesses in the server's web services, PowerShell remoting interfaces, or backend processing components. While Microsoft's advisory provides limited technical details for this specific CVE, Exchange Server RCE vulnerabilities generally follow patterns where an authenticated attacker sends specially crafted requests to the Exchange server's web interfaces (such as Exchange Web Services, OWA, or the Exchange Control Panel) that are processed in an insecure manner, leading to deserialization attacks, command injection, or server-side request forgery that ultimately enables arbitrary code execution. The requirement for high-privilege authentication reduces the immediately exploitable attack surface but does not prevent exploitation by attackers who have obtained Exchange administrative credentials through phishing, credential stuffing, or prior compromise of other systems in the organization.
Frequently Asked Questions
What is CVE-2021-31196?
CVE-2021-31196 is a high-severity remote code execution vulnerability in Microsoft Exchange Server 2013, 2016, and 2019. It allows an authenticated attacker with administrative privileges to execute arbitrary code on the server, potentially leading to complete compromise of the email infrastructure.
Why was this 2021 vulnerability added to the KEV catalog in 2024?
CISA adds vulnerabilities to the KEV catalog when there is evidence of active exploitation. The 2024 addition indicates that threat actors continue to exploit unpatched Exchange servers, targeting organizations that have not applied the security updates released in 2021. This highlights the persistent risk of delayed patching for internet-facing infrastructure.
Does this vulnerability require authentication to exploit?
Yes, exploitation requires high-level administrative privileges on the Exchange server. However, attackers frequently obtain these credentials through phishing, credential theft, or by compromising other vulnerabilities first. The authentication requirement should not be relied upon as a sole defense.
What should organizations running on-premises Exchange do?
Apply all available security updates immediately, verify patch status using Microsoft's Health Checker tool, audit administrative accounts, and scan for indicators of prior compromise. For long-term risk reduction, consider migrating to Exchange Online where security updates are applied automatically by Microsoft.
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