Microsoft Exchange Server Remote Code Execution
CVE-2021-42321 is a high-severity vulnerability in Microsoft Exchange Server 2016 Cumulative Update 21 from Microsoft, with a CVSS 3.1 base score of 8.8 out of 10. It can be exploited remotely over the network by an attacker with a low-privileged account, without any action from a user. The record lists version 15.01.2308.020 as the first without the flaw, so updating Microsoft Exchange Server 2016 Cumulative Update 21 to that release or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on November 17, 2021, which means attacks have been observed in the wild, including in ransomware campaigns.
Attackers are already exploiting this vulnerability, according to CISA.
What to do
- Update Microsoft Exchange Server 2016 Cumulative Update 21 to 15.01.2308.020 or a later version.
- Update Microsoft Exchange Server 2016 Cumulative Update 22 to 15.01.2375.017 or a later version.
- Update Microsoft Exchange Server 2019 Cumulative Update 10 to 15.02.0792.019 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.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 21 | 15.01.0 up to, but not including, 15.01.2308.020 | 15.01.2308.020 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 22 | 15.0.0 up to, but not including, 15.01.2375.017 | 15.01.2375.017 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 10 | 15.02.0 up to, but not including, 15.02.0792.019 | 15.02.0792.019 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 11 | 15.02.0 up to, but not including, 15.02.0986.014 | 15.02.0986.014 |
| Microsoft | Exchange Server | 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
- LowA basic user account is enough.
- 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
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 8.8 | High | CNA (microsoft.com) |
| CVSS 3.1 | 8.8 | High | NVD |
| CVSS 2.0 | 6.5 | Medium | NVD |
Official description
Microsoft Exchange Server Remote Code Execution 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: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Advisories
Third-party and government advisories
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-42321US Government Resource
Technical write-ups (may include proof of concept)
- packetstormsecurity.com/files/166153/Microsoft-Exchange-Server-Remote-Code-Execution.htmlExploit, Third Party Advisory, VDB Entry
- packetstormsecurity.com/files/168131/Microsoft-Exchange-Server-ChainedSerializationBinder-Remote-Code-Executi…Exploit, Third Party Advisory, VDB Entry
Other vulnerabilities in Microsoft Exchange Server 2016 Cumulative Update 21
- 7.2 High severity
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