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CVE-2023-35311

From Exploited flaws, HARD’s guide to the flaws being used in attacks · Checked
This article is about a flaw in Microsoft Outlook. For every flaw being used in attacks, see Exploited flaws; for Microsoft’s, see Microsoft flaws being used in attacks.

CVE-2023-35311 is a security flaw in Microsoft Outlook, part of Microsoft Office. Attackers can get past a security protection in Microsoft Outlook. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 11 Jul 2023.[1]

Microsoft Outlook is in things households have, such as phones, computers, browsers and everyday apps, so the fix matters at home as much as at work. Fixed in version https://aka.ms/OfficeSecurityReleases, 16.0.5404.1000 and 15.0.5571.1000.

Description

CISA calls it a security feature bypass in Microsoft Outlook. In its words: “Microsoft Outlook contains a security feature bypass vulnerability that allows an attacker to bypass the Microsoft Outlook Security Notice prompt.”[1]

The underlying weakness is CWE-367, in MITRE’s list of common software weaknesses.[2]

It scores 8.8 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by the organisation that published the CVE using CVSS 3.1.[2]

Affected versions

NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]

ProductFixed in
Microsoft 365 Apps for Enterprisehttps://aka.ms/OfficeSecurityReleases
Microsoft Office LTSC 2021https://aka.ms/OfficeSecurityReleases
Microsoft Office 2019https://aka.ms/OfficeSecurityReleases
Microsoft Outlook 201616.0.5404.1000
Microsoft Outlook 201315.0.5571.1000
Microsoft Outlook 2013 Service Pack 115.0.5571.1000

Every device with Microsoft Outlook needs the update.

Exploitation

CISA added it to its catalog of known exploited vulnerabilities on 11 Jul 2023, within a day of it being made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 1 Aug 2023 to fix it.[1]

FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 16%. That’s in the top 4% of the flaws it scores.[4]

CISA assesses it as not automatable: each attack takes some work.

CISA doesn’t say who was attacked, or how.

Fix

Microsoft has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions or discontinue use of the product if updates are unavailable.”[1]

  1. Update Microsoft Outlook now.On Windows: Settings, Windows Update, Check for updates, then restart. Office updates come the same way, or from any Office app: File, Account, Update Options. Then turn on automatic updates, so the next fix arrives on its own.
  2. Check you’re on https://aka.ms/OfficeSecurityReleases or later.Fixed in version https://aka.ms/OfficeSecurityReleases, 16.0.5404.1000 and 15.0.5571.1000. Anything older is what attackers are going after.
  3. If there’s no update, stop using it.CISA’s advice when a flaw being used in attacks has no fix is to stop using the product until it does.

Timeline

DateEvent
The flaw is made public in the National Vulnerability Database, with a severity score of 8.8.[2]
CISA adds it to its catalog of flaws known to be used in attacks.[1]
CISA’s deadline for US government agencies to fix it.[1]
FIRST puts the odds of it being used in attacks in the next 30 days at 16%.[4]

See also

Other Microsoft flaws

  • CVE-2026-96951, Microsoft Windows: command injection ·
  • CVE-2026-65660, Microsoft SharePoint: code injection ·
  • CVE-2026-85880, Microsoft Windows: heap-based buffer overflow ·
  • CVE-2026-81963, Microsoft Windows: link following ·
  • CVE-2019-1068, Microsoft SQL Server: remote code execution ·

Similar flaws

  • CVE-2025-38352, Linux Kernel: time-of-check time-of-use (TOCTOU) race condition ·
  • CVE-2025-22224, VMware ESXi and Workstation: TOCTOU race condition ·
  • CVE-2022-48618, Apple devices: memory corruption ·

Added the same day

  • CVE-2023-36874, Microsoft Windows: error reporting service privilege escalation ·
  • CVE-2023-32049, Microsoft Windows: defender smartScreen security feature bypass ·
  • CVE-2023-32046, Microsoft Windows: MSHTML platform privilege escalation ·
  • CVE-2022-31199, Netwrix Auditor: insecure object deserialization ·

Every Microsoft flaw on CISA’s list · Everything added in July 2023

References

  1. ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Outlook Security Feature Bypass Vulnerability”. Known Exploited Vulnerabilities Catalog, added 11 Jul 2023.
  2. ^ National Institute of Standards and Technology. “CVE-2023-35311 Detail”. National Vulnerability Database, published 11 Jul 2023.
  3. ^ Microsoft. Advisories and fixes: msrc.microsoft.com, msrc.microsoft.com.
  4. ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.155, percentile 96.8, 31 Oct 2026.
  5. CVE Program. “CVE-2023-35311”. The official CVE record.

This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.