Microsoft Outlook Security Feature Bypass
CVE-2023-35311 is a high-severity time-of-check time-of-use (TOCTOU) race condition vulnerability in Microsoft 365 Apps for Enterprise 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, but only if a user takes part, for example by opening a link or file. 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 July 11, 2023, 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 LTSC 2021 to https://aka.ms/OfficeSecurityReleases or a later version.
- Update Microsoft Office 2019 to https://aka.ms/OfficeSecurityReleases or a later version.
- CISA's required action: Apply updates per vendor instructions or discontinue use of the product if updates 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.
- Because the attack needs a user to take part, warn users about unexpected links and files related to this product until it is patched.
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 LTSC 2021 | 16.0.1 up to, but not including, https://aka.ms/OfficeSecurityReleases | https://aka.ms/OfficeSecurityReleases |
| Microsoft | Microsoft Office 2019 | 19.0.0 up to, but not including, https://aka.ms/OfficeSecurityReleases | https://aka.ms/OfficeSecurityReleases |
| Microsoft | Microsoft Outlook 2016 | 16.0.0.0 up to, but not including, 16.0.5404.1000 | 16.0.5404.1000 |
| Microsoft | Microsoft Outlook 2013 | 14.0.0 up to, but not including, 15.0.5571.1000 | 15.0.5571.1000 |
| Microsoft | Microsoft Outlook 2013 Service Pack 1 | 15.0.0.0 up to, but not including, 15.0.5571.1000 | 15.0.5571.1000 |
| Microsoft | 365 Apps | Not specified in the record | Not stated |
| Microsoft | Office 2019 | Not specified in the record | Not stated |
| Microsoft | Office 2021 | Not specified in the record | Not stated |
| Microsoft | Outlook | 2013; 2016 | 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
- NoneNo account or login is needed.
- User interaction
- RequiredA user must take an action such as opening a file or link.
- 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:N/UI:R/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 | 7.5 | High | NVD |
Official description
Microsoft Outlook Security Feature Bypass Vulnerability
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Time-of-check Time-of-use (TOCTOU) Race Condition
The product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check.
How developers prevent this class of flaw
- The most basic advice for TOCTOU vulnerabilities is to not perform a check before the use. This does not resolve the underlying issue of the execution of a function on a resource whose state and identity cannot be assured, but it does help to limit the false sense of security given by the check.
- When the file being altered is owned by the current user and group, set the effective gid and uid to that of the current user and group when executing this statement.
- Limit the interleaving of operations on files from multiple processes.
More Time-of-check Time-of-use (TOCTOU) Race Condition vulnerabilities (CWE-367)
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
- cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2023-35311US Government Resource
Other vulnerabilities in Microsoft 365 Apps for Enterprise
- 8.8 High severity
- 6.5 Medium severity
Recent Time-of-check Time-of-use (TOCTOU) Race Condition vulnerabilities
- 7.0 High severity
- 7.0 High severity
- 7.0 High severity
- 7.0 High severity
- 7.0 High severity
- 7.0 High severity
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