Microsoft Exchange Server Security Feature Bypass
CVE-2021-31207 is a medium-severity unrestricted upload of file with dangerous type vulnerability in Microsoft Exchange Server 2013 Cumulative Update 23 from Microsoft, with a CVSS 3.1 base score of 6.6 out of 10. It can be exploited remotely over the network by an attacker with administrator privileges, without any action from a user. The record lists version 15.00.1497.018 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 November 3, 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 2013 Cumulative Update 23 to 15.00.1497.018 or a later version.
- Update Microsoft Exchange Server 2019 Cumulative Update 9 to 15.02.0858.012 or a later version.
- Update Microsoft Exchange Server 2016 Cumulative Update 20 to 15.01.2242.010 or a later version.
- CISA's required action: Apply updates per vendor instructions.
- Read the advisory on portal.msrc.microsoft.com for workarounds and exact fixed builds.
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, 15.00.1497.018 | 15.00.1497.018 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 9 | 15.02.0 up to, but not including, 15.02.0858.012 | 15.02.0858.012 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 20 | 15.01.0 up to, but not including, 15.01.2242.010 | 15.01.2242.010 |
| Microsoft | Microsoft Exchange Server 2016 Cumulative Update 19 | 15.01.0 up to, but not including, 15.01.2176.014 | 15.01.2176.014 |
| Microsoft | Microsoft Exchange Server 2019 Cumulative Update 8 | 15.02.0 up to, but not including, 15.02.0792.015 | 15.02.0792.015 |
| 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
- HighSuccess depends on conditions outside the attacker's control.
- Privileges required
- HighAdministrator-level access is required first.
- 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:H/PR:H/UI:N/S:U/C:H/I:H/A:H. Scored by NVD.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 6.6 | Medium | CNA (microsoft.com) |
| CVSS 3.1 | 6.6 | Medium | NVD |
| CVSS 2.0 | 6.5 | Medium | NVD |
Official description
Microsoft Exchange Server Security Feature Bypass Vulnerability
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Unrestricted Upload of File with Dangerous Type
The product allows the upload or transfer of dangerous file types that are automatically processed within its environment.
How developers prevent this class of flaw
- Generate a new, unique filename for an uploaded file instead of using the user-supplied filename, so that no external input is used at all.
- When the set of acceptable objects, such as filenames or URLs, is limited or known, create a mapping from a set of fixed input values (such as numeric IDs) to the actual filenames or URLs, and reject all other inputs.
- Consider storing the uploaded files outside of the web document root entirely. Then, use other mechanisms to deliver the files dynamically.
More Unrestricted Upload of File with Dangerous Type vulnerabilities (CWE-434)
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
Third-party and government advisories
- zerodayinitiative.com/advisories/ZDI-21-819/Third Party Advisory, VDB Entry
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31207US Government Resource
Technical write-ups (may include proof of concept)
- packetstormsecurity.com/files/163895/Microsoft-Exchange-ProxyShell-Remote-Code-Execution.htmlExploit, Third Party Advisory, VDB Entry
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
Recent Unrestricted Upload of File with Dangerous Type vulnerabilities
- 7.2 High severity
- 8.8 High severity
Drupal core Un-restricted Upload of File Exploited
- 7.2 High severity
- 9.8 Critical severity
- 8.8 High severity
- 6.5 Medium severity
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