Microsoft Outlook Improper Input Validation Vulnerability
CVE-2024-21413 is a security vulnerability in Microsoft Office Outlook, added to CISA KEV on February 6, 2025. Patch priority: Patch this week.
What it is
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.
VulnCheck tracked exploitation 268 days before CISA's KEV listing. VulnCheck's broader catalog of in-the-wild exploitation evidence picked this up before it landed on CISA's formal list. Per-CVE fact about this entry; not a population claim about lead time in general.
Who's affected
Affects anyone using Microsoft 365 or Office to compose, store, or send email, documents, or spreadsheets. In a small practice, that's typically where client communications, engagement letters, and case notes live — credential compromise here means an attacker reads everything that platform stores.
What to do
Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.
If you don't have someone in-house to verify the patch deployed across every endpoint — or you're not sure whether you're affected — that's exactly the kind of triage we do. Book a free 20-minute triage call.
Insurance readiness
Your insurer will ask if this is patched.
Unpatched entries on CISA's Known Exploited Vulnerabilities list are exactly what cyber-insurance carrier questionnaires probe for, and knowing which controls they check is how you keep a renewal from stalling.
Get the free carrier questionnaire →Severity
CVSS base score: 9.8 — CRITICAL
Weakness classification: CWE-20
Microsoft fix
- Click to Run (Security Update)
- KB5002537 (Security Update)
- KB5002467 (Security Update)
- KB5002522 (Security Update)
- KB5002519 (Security Update)
Microsoft Exploitability Index (assessed at update release): Exploitation Unlikely
This CVE is on CISA's Known Exploited Vulnerabilities catalog. Microsoft's index reflects their release-time forecast, not current observed activity, and is not automatically revised upward when exploitation appears later.
Vendor workaround
No vendor workaround published. Patching is the only fix.
CISA due date
Federal deadline: February 27, 2025. Federal agencies must complete the required action by this date. For private SMBs the deadline is advisory — but treat it as a strong recommendation, especially if you handle regulated data (HIPAA, GLBA, ABA model rules).
EPSS
EPSS score: 94.7% — higher than 100% of all scored CVEs.
EPSS estimates the probability of exploitation activity in the next 30 days. Every entry on this site is already confirmed exploited, so read it as relative urgency among the things you still have open, not as permission to wait.
Source
This entry comes from CISA's Known Exploited Vulnerabilities catalog, public US-Government data we pull daily. The plain-language framing and the what-to-do guidance above are ours. View the original entry on cisa.gov.