Microsoft Exchange Server Information Disclosure Vulnerability (CVE-2021-33766)
Published Last modified In CISA KEV since Sources: CVE.org
What is affected, what to do
Microsoft Exchange Server Information Disclosure Vulnerability
The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Exchange Server 2013 Cumulative Update 23 to version 15.00.1497.015, Microsoft Exchange Server 2016 Cumulative Update 19 to version 15.01.2176.012, Microsoft Exchange Server 2016 Cumulative Update 20 to version 15.01.2242.008. Immediate actions, mitigation and detection rules are in the runbook below.
- Affected
- Microsoft Exchange Server 2013 Cumulative Update 23
lt 15.00.0..15.00.1497.015 - Microsoft Exchange Server 2016 Cumulative Update 19
lt 15.01.0..15.01.2176.012 - Microsoft Exchange Server 2016 Cumulative Update 20
lt 15.01.0..15.01.2242.008 - Microsoft Exchange Server 2019 Cumulative Update 8
lt 15.02.0..15.02.0792.013 - Microsoft Exchange Server 2019 Cumulative Update 9
lt 15.02.0..15.02.0858.010
- Microsoft Exchange Server 2013 Cumulative Update 23
- Fixed in
- Microsoft Exchange Server 2013 Cumulative Update 23
15.00.1497.015 - Microsoft Exchange Server 2016 Cumulative Update 19
15.01.2176.012 - Microsoft Exchange Server 2016 Cumulative Update 20
15.01.2242.008 - Microsoft Exchange Server 2019 Cumulative Update 8
15.02.0792.013 - Microsoft Exchange Server 2019 Cumulative Update 9
15.02.0858.010
- Microsoft Exchange Server 2013 Cumulative Update 23
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Metrics
7.3
Priority
XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.
Source: NVD, CVSS v3
7.3
CVSS v3
Severity High on the scale from 0 to 10.
Source: CVE.org
98.2 %
EPSS score
High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
Source: FIRST EPSS, percentile 99.91
Show all metrics
- Severity
- Critical
Priority (0–325)
The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?
Read in glossary- 216.36 Methodology
Exploit maturity
An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.
Read in glossary- actively exploited
XIEM Effective CVSS
XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.
Read in glossary- 7.3 Methodology
CVSS v3
CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.
Read in glossary- 7.3
EPSS score
The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.
Read in glossary- 98.2 %
- High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
EPSS percentile
The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.
Read in glossary- 99.91
- Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
KEV status
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- Actively exploited
- Published
In CISA KEV since
The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.
Read in glossary- CISA assessment (SSVC)
- Exploitation: active · automatable · Technical impact: partial
- Source: CISA Vulnrichment, as of 4 February 2025
Response and mitigation
Why act now?
Prioritisation rationale
This flaw is network-reachable without authentication or user interaction. Public exploit code has been available since 2021, and exploitation-prediction data places it among the CVEs most likely to be exploited. Attackers use it to plant silent forwarding rules in arbitrary mailboxes for persistent mail interception. Executives, finance and procurement are the prime targets, feeding espionage and business email compromise. CISA does not flag ransomware use, but a high rating is still warranted. Score any internet-exposed ECP on an unpatched build as critical, and internal-only servers with restricted ECP access as high.
Runbook step 1
Immediate response (0-24 h)
- Verify the patch level on every Exchange server: Run
Get-Command ExSetup.exe | ForEach-Object { $_.FileVersionInfo }on each server, or use Microsoft'sHealthChecker.ps1from the CSS-Exchange repo.AdminDisplayVersiondoes not reliably reflect Security Updates. Minimum fixed builds: Exchange 2013 CU23 → 15.00.1497.015, Exchange 2016 CU19 → 15.01.2176.012, CU20 → 15.01.2242.008, Exchange 2019 CU8 → 15.02.0792.013, CU9 → 15.02.0858.010. - Install the Security Update: Bring older CUs up to a supported CU first, then apply the matching SU. Look up the exact KB number in Microsoft's advisory for CVE-2021-33766. Exchange 2013 is out of support and belongs on the migration list regardless.
- Restrict external ECP access now: If you cannot patch within hours, limit
/ecpto internal admin subnets at the reverse proxy, at the WAF, or via IIS "IP Address and Domain Restrictions". This bug ("ProxyToken") is triggered by unauthenticated requests to/ecp/over TCP 443. - Start a compromise assessment: Enumerate forwarding rules across all mailboxes:
Get-Mailbox -ResultSize Unlimited | Get-InboxRule | Where-Object { $_.ForwardTo -or $_.RedirectTo -or $_.ForwardAsAttachmentTo } | Select MailboxOwnerId,Name,ForwardTo,RedirectTo. Treat any external recipient with no business justification as an incident. - Remove malicious rules and preserve evidence: Export each rule with its timestamps first, then delete it with
Remove-InboxRule. Notify the affected mailbox owners and your DPO. Exfiltrated mail traffic may trigger GDPR Art. 33 and NIS2 reporting obligations.
- Verify the patch level on every Exchange server: Run
Runbook step 2
Mitigation layers
- Network segmentation: Do not publish ECP alongside OWA. Restrict ECP to the management network or VPN. Any external publishing should go through a pre-authenticating reverse proxy, such as Entra ID Application Proxy or an upstream IdP.
- WAF/IPS rule: Block requests to
/ecp/*that carry a non-emptySecurityToken=cookie. That cookie is what makes the frontend skip authentication. Also deny POSTs to*/RulesEditor/InboxRules.svc/NewObjectfrom external networks. - Configuration hardening against exfiltration: Disable external auto-forwarding with
Set-RemoteDomain Default -AutoForwardEnabled $false. Add a transport rule that rejects auto-forwarded messages (message typeAutoForward) to external domains. This neutralises the attacker's objective even if a rule slips through. - Logging uplift: Enable the
Cookie (cs(Cookie))field in IIS logging for both "Default Web Site" and "Exchange Back End". Make sure mailbox auditing (UpdateInboxRules) is enabled for all mailboxes. Without both, forensic reconstruction is close to impossible. - Lifecycle management: Classify Exchange servers as tier-critical in patch management with a fixed monthly SU window. Plan the move from Exchange 2013/2016/2019 to Exchange Server Subscription Edition.
Runbook step 3
Detection rules
- IIS access logs (frontend): Look for unauthenticated hits on ECP rule endpoints. Sigma shape:
cs-uri-stem|contains: '/ecp/' AND cs-uri-stem|contains: 'InboxRules.svc' AND cs-username: '-'. Sentinel KQL:W3CIISLog | where csUriStem has "/ecp/" and csUriStem has "RulesEditor" and (isempty(csUserName) or csUserName == "-"). - ProxyToken fingerprint: With cookie logging enabled, hunt for
SecurityToken=combined with themsExchEcpCanary=URL parameter and a third-party mailbox in the path (/ecp/<victim>@<domain>/...). A preceding HTTP 500 from the same client usually indicates canary harvesting. - Exchange mailbox audit: Flag
UpdateInboxRulesorNew-InboxRuleevents not originating from the owner's Outlook client, especially withForwardTo,RedirectToorForwardAsAttachmentTopointing to an external address. Query withSearch-MailboxAuditLog -ShowDetails -Operations UpdateInboxRules. - Transport and network telemetry: Review message tracking (
Get-MessageTrackingLog -EventId SEND) for new high-volume external recipients. In Zeek/Suricata, watch for source IPs from VPS or hosting ASNs that hit/ecp/directly with no prior OWA login. - Baseline drift: Diff all inbox rules weekly against the previous snapshot. Raise a SOC alert for any new forwarding rule without a matching change ticket.
- IIS access logs (frontend): Look for unauthenticated hits on ECP rule endpoints. Sigma shape:
Detection rules
No NEOSEC detection rule is available for this vulnerability yet.
Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
ISO/IEC 27001
ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.
Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
Microsoft Exchange Server Information Disclosure Vulnerability
Source: CISA KEV Catalog, NVD (NIST), cvelistv5, CVE.org (MITRE)
Affected products
The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.
| Affected versions | Fixed in | ||
|---|---|---|---|
| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 23 |
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 19 |
|
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| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 20 |
|
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| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 8 |
|
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| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 9 |
|
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| microsoft | exchange_server |
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Vendors and products
- Microsoft Microsoft Exchange Server
- Microsoft Microsoft Exchange Server 2013 Cumulative Update 23
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 19
- Microsoft Microsoft Exchange Server 2016 Cumulative Update 20
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 8
- Microsoft Microsoft Exchange Server 2019 Cumulative Update 9
Public exploit references
Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.
| CISA KEV Catalog | CISA Known Exploited Vulnerability (opens in a new tab)cisa.gov | actively exploited |
References & sources
- portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-33766x_refsource_MISC, x_transferred
- zerodayinitiative.comadvisories/ZDI-21-798x_refsource_MISC, x_transferred
- Microsoft Security Update GuideMicrosoft Exchange Server Information Disclosure VulnerabilityVendor advisory, Patch
- cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33766government-resource
Changes to this CVE
Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).
NVD analysis modifiedNVD
Show the original change
Individual changes of this event as published by NVD Type Action Before After Reference type added —Microsoft Corporation: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-33766 Types: Patch, Vendor AdvisorySource identifier: nvd@nist.gov
Record modifiedMicrosoft
Show 6 original changes
Individual changes of this event as published by NVD Type Action Before After Reference added —https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-33766Reference removed https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766—Reference removed https://www.zerodayinitiative.com/advisories/ZDI-21-798/—Reference type removed https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33766 Types: Patch, Vendor Advisory—Reference type removed https://www.zerodayinitiative.com/advisories/ZDI-21-798/ Types: Third Party Advisory, VDB Entry—Affected versions (publisher) changed Microsoft Exchange Server 2019 Cumulative Update 9, Microsoft Exchange Server 2016 Cumulative Update 20, Microsoft Exchange Server 2013 Cumulative Update 23 (+2)Microsoft Exchange Server 2013 Cumulative Update 23, Microsoft Exchange Server 2016 Cumulative Update 19, Microsoft Exchange Server 2016 Cumulative Update 20 (+2)Source identifier: secure@microsoft.com
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<a href="https://intel.neosec.eu/en/cve/CVE-2021-33766"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-33766.svg?lang=en" alt="CVE-2021-33766 – NEOSEC Intel" height="20"></a>