Microsoft Exchange Server Information Disclosure
CVE-2021-33766 is a high-severity vulnerability in Microsoft Exchange Server 2013 Cumulative Update 23 from Microsoft, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists version 15.00.1497.015 as the first without the flaw, so updating Microsoft Exchange Server 2013 Cumulative Update 23 to that release or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on January 18, 2022, which means attacks have been observed in the wild.
Attackers are already exploiting this vulnerability, according to CISA.
What to do
- Update Microsoft Exchange Server 2013 Cumulative Update 23 to 15.00.1497.015 or a later version.
- Update Microsoft Exchange Server 2016 Cumulative Update 19 to 15.01.2176.012 or a later version.
- Update Microsoft Exchange Server 2016 Cumulative Update 20 to 15.01.2242.008 or a later version.
- CISA's required action: Apply updates per vendor instructions.
- Read the advisory on msrc.microsoft.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Microsoft | Microsoft Exchange Server 2013 Cumulative Update 23 | 15.00.0 up to, but not including, 15.00.1497.015 | 15.00.1497.015 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 19 | 15.01.0 up to, but not including, 15.01.2176.012 | 15.01.2176.012 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 20 | 15.01.0 up to, but not including, 15.01.2242.008 | 15.01.2242.008 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 8 | 15.02.0 up to, but not including, 15.02.0792.013 | 15.02.0792.013 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 9 | 15.02.0 up to, but not including, 15.02.0858.010 | 15.02.0858.010 |
| Microsoft | Exchange Server | 2013; 2016; 2019 | Not stated |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- NetworkCan be exploited remotely over a network, including the internet.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- NoneNo account or login is needed.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- NoneNothing can be modified.
- Availability
- NoneAvailability is not affected.
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 7.3 | High | CNA (microsoft.com) |
| CVSS 3.1 | 7.5 | High | NVD |
| CVSS 2.0 | 5.0 | Medium | NVD |
Official description
Microsoft Exchange Server Information Disclosure Vulnerability
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
Timeline
- CVE record published.
- Added to CISA's Known Exploited Vulnerabilities catalogue.
- Deadline for US federal agencies to remediate, set by CISA.
- CISA published its SSVC assessment (exploitation: active, automatable: yes, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Advisories
Third-party and government advisories
- zerodayinitiative.com/advisories/ZDI-21-798/Third Party Advisory, VDB Entry
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-33766US Government Resource
Other vulnerabilities in Microsoft Exchange Server 2013 Cumulative Update 23
- 8.8 High severity
- 9.8 Critical severity
- 8.0 High severity
- 8.8 High severity
- 9.8 Critical severity
- 9.8 Critical severity
Spotted an error? Tell us.