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CVE-2024-21413

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

CVE-2024-21413 is a security flaw in Microsoft Office Outlook, part of Microsoft Office. Attackers can run their own code on Microsoft Office Outlook. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 6 Feb 2025.[1]

Microsoft Office 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 and 16.0.5435.1001.

Description

CISA calls it an outlook improper input validation in Microsoft Office Outlook. In its words: “Microsoft Outlook contains an improper input validation vulnerability that allows for remote code execution. Successful exploitation of this vulnerability would allow an attacker to bypass the Office Protected View and open in editing mode rather than protected mode.”[1]

The underlying weakness is improper input validation (CWE-20), in MITRE’s list of common software weaknesses.[2]

It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, 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 201616.0.5435.1001
Microsoft Office 2019https://aka.ms/OfficeSecurityReleases
Microsoft Office LTSC 2021https://aka.ms/OfficeSecurityReleases

Every device with Microsoft Office Outlook needs the update.

Exploitation

CISA added it to its catalog of known exploited vulnerabilities on 6 Feb 2025, 359 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 27 Feb 2025 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 95%. That’s in the top 1% of the flaws it scores.[4]

CISA assesses it as automatable: attackers can run it against many targets at once.

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

Fix

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

  1. Update Microsoft Office 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 and 16.0.5435.1001. 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 9.8.[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 95%.[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 ·

Other improper input validation flaws

  • CVE-2026-93952, Arista VeloCloud Orchestrator: improper input validation ·
  • CVE-2026-12569, PTC Windchill and FlexPLM: improper input validation ·
  • CVE-2026-34910, Ubiquiti UniFi OS: improper input validation ·
  • CVE-2026-6973, Ivanti Endpoint Manager Mobile (EPMM): improper input validation ·
  • CVE-2026-34197, Apache ActiveMQ: improper input validation ·

Added the same day

Every Microsoft flaw on CISA’s list · Everything added in February 2025

References

  1. ^ Cybersecurity and Infrastructure Security Agency. “Microsoft Outlook Improper Input Validation Vulnerability”. Known Exploited Vulnerabilities Catalog, added 6 Feb 2025.
  2. ^ National Institute of Standards and Technology. “CVE-2024-21413 Detail”. National Vulnerability Database, published 13 Feb 2024.
  3. ^ Microsoft. Advisories and fixes: msrc.microsoft.com, vicarius.io.
  4. ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.947, percentile 99.9, 31 Oct 2026.
  5. CVE Program. “CVE-2024-21413”. 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.