Microsoft Outlook Improper Input Validation
CVE-2024-21413 is a critical-severity improper input validation vulnerability in Microsoft 365 Apps for Enterprise from Microsoft, with a CVSS 3.1 base score of 9.8 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 https://aka.ms/OfficeSecurityReleases as the first without the flaw, so updating Microsoft 365 Apps for Enterprise to that release or later removes the exposure. CISA added it to the Known Exploited Vulnerabilities catalogue on February 6, 2025, which means attacks have been observed in the wild.
Attackers are already exploiting this vulnerability, according to CISA.
What to do
- Update Microsoft 365 Apps for Enterprise to https://aka.ms/OfficeSecurityReleases or a later version.
- Update Microsoft Office 2016 to 16.0.5435.1001 or a later version.
- Update Microsoft Office 2019 to https://aka.ms/OfficeSecurityReleases or a later version.
- CISA's required action: Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.
- 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 365 Apps for Enterprise | 16.0.1 up to, but not including, https://aka.ms/OfficeSecurityReleases | https://aka.ms/OfficeSecurityReleases |
| Microsoft | Microsoft Office 2016 | 16.0.0 up to, but not including, 16.0.5435.1001 | 16.0.5435.1001 |
| Microsoft | Microsoft Office 2019 | 19.0.0 up to, but not including, https://aka.ms/OfficeSecurityReleases | https://aka.ms/OfficeSecurityReleases |
| Microsoft | Microsoft Office LTSC 2021 | 16.0.1 up to, but not including, https://aka.ms/OfficeSecurityReleases | https://aka.ms/OfficeSecurityReleases |
| Microsoft | 365 Apps | Not specified in the record | Not stated |
| Microsoft | Office 2016 | Not specified in the record | Not stated |
| Microsoft | Office 2019 | Not specified in the record | Not stated |
| Microsoft | Office Long Term Servicing Channel | 2021 | 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
- 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:N/UI:N/S:U/C:H/I:H/A:H. Scored by CNA (microsoft.com).
Official description
Microsoft Outlook Remote Code Execution Vulnerability
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Improper Input Validation
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
How developers prevent this class of flaw
- Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build recognizers for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of allowing parser code to be scattered throughout the program, where it could be subject to errors or inconsistencies that create weaknesses.
- Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).
- Understand all the potential areas where untrusted inputs can enter the product, including but not limited to: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any external systems that provide data to the application. Remember that such inputs may be obtained indirectly through API calls.
Timeline
- CVE record published.
- Added to CISA's Known Exploited Vulnerabilities catalogue.
- CISA published its SSVC assessment (exploitation: active, automatable: yes, technical impact: total).
- Deadline for US federal agencies to remediate, set by CISA.
- Record last changed materially (score, affected products, status or references).
References
Patches and release notes
Third-party and government advisories
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2024-21413US Government Resource
Technical write-ups (may include proof of concept)
Other references
Other vulnerabilities in Microsoft 365 Apps for Enterprise
- 8.8 High severity
- 6.5 Medium severity
Recent Improper Input Validation vulnerabilities
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 8.1 High severity
- 8.6 High severity
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