VulnerabilityPublished 11 Jul 2023Updated 21 Oct 2025

CVE-2023-35311: Microsoft 365 Apps for Enterprise · Time-of-check Time-of-use (TOCTOU) Race Condition

Affects Microsoft 365 Apps for EnterpriseSource CVE Database V5

Exploited in the wild · KEV due 1 Aug 2023 · public exploit available · 15% EPSS : apply the vendor's fix or mitigations to Microsoft 365 Apps for Enterprise now.

Data as of 26 Oct 2026, 12:29 UTC

CVSS 3.1 base score

8.8High

How severe the flaw is on the 0–10 CVSS scale.

KEV deadline: 1 Aug 2023 US federal agencies must patch by this date; a sensible deadline for everyone else.
EPSS: 15.5% chance of exploitation in 30 days – higher than 96% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C

Base metrics

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: Required
Scope: Unchanged
Confidentiality: High
Integrity: High
Availability: High

Temporal metrics

Exploit Code Maturity: Functional
Remediation Level: Official Fix
Report Confidence: Confirmed

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).
Exploit Code Maturity: Functional: Functional exploit code is available. The code works in most situations where the vulnerability exists.
Remediation Level: Official Fix: A complete vendor solution is available. Either the vendor has issued an official patch, or an upgrade is available.
Report Confidence: Confirmed: Detailed reports exist, or functional reproduction is possible (functional exploits may provide this). Source code is available to independently verify the assertions of the research, or the author or vendor of the affected code has confirmed the presence of the vulnerability.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

t = 0 is the CVE’s publication on cve.org

Disclosed: evidence found.
CVE published 11 Jul 2023
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: evidence found.
CVSS exploit maturity: proof of concept public
Exploited in the wildExploited: evidence found.
Listed in CISA KEV the same day as disclosure
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

CVE ID reserved
by Microsoft Corporation
14 Jun 202323:09 UTC, 27 days before disclosure
CVE published
on cve.org
11 Jul 202317:03 UTC, t = 0
Added to CISA KEV (opens in a new tab)
exploitation confirmed
11 Jul 2023, the same day as disclosure
KEV remediation due
US federal deadline
1 Aug 2023, 21 days after disclosure

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), CVSS vector, CISA KEV (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Apply updates per vendor instructions or discontinue use of the product if updates are unavailable.

Affected software

Product	Affected	Fixed in
Microsoft Microsoft 365 Apps for Enterprise	Affected =16.0.1	Fixed in–

Description

CVE-2023-35311 is a high-severity Time-of-check Time-of-use (TOCTOU) race condition vulnerability in Microsoft 365 Apps for Enterprise, specifically affecting Microsoft Outlook. This flaw allows an attacker to bypass security features, potentially leading to full compromise of confidentiality, integrity, and availability of affected systems. The vulnerability requires no privileges but does require user interaction, and it can be exploited remotely over the network. Although no known exploits are currently in the wild, the high CVSS score of 8.8 indicates a significant risk. European organizations using Microsoft 365 Apps for Enterprise version 16.0.1 are at risk, especially those with large deployments of Outlook. Mitigation involves applying patches once available, restricting user interaction with untrusted content, and monitoring for suspicious activity. Countries with high Microsoft 365 adoption and strategic targets, such as Germany, France, the UK, and the Netherlands, are most likely to be affected.

Impact

For European organizations, this vulnerability poses a substantial risk due to the widespread use of Microsoft 365 Apps for Enterprise, particularly Outlook, across public and private sectors. Exploitation could lead to unauthorized access to sensitive communications, data leakage, and disruption of business operations. Given the high confidentiality, integrity, and availability impacts, critical infrastructure, government agencies, financial institutions, and large enterprises are at heightened risk. The requirement for user interaction means phishing or social engineering campaigns could be used to trigger exploitation. The potential for remote exploitation without privileges increases the attack surface, making it easier for threat actors to target organizations. This could result in significant financial losses, reputational damage, and regulatory penalties under GDPR if personal data is compromised. The absence of known exploits currently provides a window for mitigation but also underscores the urgency for organizations to prepare defenses.

Mitigation

1. Monitor Microsoft security advisories closely and apply official patches or updates immediately once released for Microsoft 365 Apps for Enterprise, especially Outlook version 16.0.1. 2. Implement strict email filtering and anti-phishing controls to reduce the likelihood of malicious emails reaching end users. 3. Educate users on the risks of interacting with unsolicited or suspicious emails and attachments to minimize the chance of triggering the vulnerability. 4. Employ application control policies to restrict execution of untrusted code or macros within Outlook. 5. Use endpoint detection and response (EDR) solutions to monitor for anomalous behaviors indicative of exploitation attempts. 6. Restrict network access and apply segmentation to limit the spread of potential compromise. 7. Conduct regular vulnerability assessments and penetration testing focused on Microsoft 365 environments to identify and remediate weaknesses. 8. Prepare incident response plans specifically addressing potential exploitation scenarios involving Outlook and Microsoft 365 Apps.

Summary

CVE-2023-35311 is a security feature bypass vulnerability classified under CWE-367 (Time-of-check Time-of-use (TOCTOU) race condition) affecting Microsoft 365 Apps for Enterprise, specifically version 16.0.1 of Microsoft Outlook. The vulnerability arises from a race condition where the state of a resource is checked and then used in a manner that allows an attacker to manipulate the timing between these operations, leading to a bypass of security controls. This can enable an attacker to execute unauthorized actions or escalate privileges by exploiting the window between the check and use phases. The vulnerability is remotely exploitable over the network without requiring privileges but does require user interaction, such as opening a malicious email or attachment. The CVSS v3.1 score of 8.8 reflects the critical impact on confidentiality, integrity, and availability, with low attack complexity and no privileges required. Although no public exploits are currently known, the flaw's nature and impact make it a significant threat. The lack of available patches at the time of publication necessitates proactive mitigation strategies. This vulnerability could be leveraged to bypass Outlook's security features, potentially allowing attackers to execute arbitrary code, access sensitive information, or disrupt services within enterprise environments.

CWE-367Time-of-check Time-of-use (TOCTOU) Race Condition (opens in a new tab)
Base weaknessLikelihood of exploit: Medium
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.
Implementation
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.
Implementation
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.
Architecture and Design
Limit the interleaving of operations on files from multiple processes.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V1.11.3 (opens in a new tab)
V11.1.6 (opens in a new tab)
V1.11.2 (opens in a new tab)
Attack patterns
Leveraging Race Conditions via Symbolic Links (opens in a new tab)
Leveraging Time-of-Check and Time-of-Use (TOCTOU) Race Conditions (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

Timeline

What to do

Affected software

Description

AI analysisMachine-generated, updated 28 Oct 2025, 23:51 UTC

WeaknessesCWE-367

Technical details

References

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