CVE-2026-21521

High

Published: 22 January 2026

Published

22 January 2026

Modified

02 February 2026

KEV Added

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Patch

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CVSS Score 7.4 CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N

EPSS Score 0.0007 22.3th percentile

Risk Priority 15 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-21521 is a high-severity Improper Neutralization of Escape, Meta, or Control Sequences (CWE-150) vulnerability in Microsoft 365 Word Copilot. Its CVSS base score is 7.4 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 22.3th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

This vulnerability is AI-related — categorised as Enterprise AI Assistants.

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190) and 1 other technique.

Threat & Defense Details

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

T1204.001 Malicious Link Execution

An adversary may rely upon a user clicking a malicious link in order to gain execution.

attack.mitre.org →

Why these techniques?

Improper neutralization (CWE-150) in public-facing Copilot enables remote exploitation via malicious link/input for info disclosure (T1190); requires user interaction consistent with malicious link execution (T1204.001).

Confidence: MEDIUM · MITRE ATT&CK Enterprise v18.1

NVD Description

Improper neutralization of escape, meta, or control sequences in Copilot allows an unauthorized attacker to disclose information over a network.

Deeper analysisAI

CVE-2026-21521 is an improper neutralization of escape, meta, or control sequences vulnerability (CWE-150) affecting Copilot. Published on 2026-01-22, it carries a CVSS v3.1 base score of 7.4 (AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N), indicating high severity due to network accessibility, low attack complexity, and significant confidentiality impact.

An unauthorized attacker with no privileges required can exploit this vulnerability remotely by inducing user interaction, such as clicking a malicious link or input. Successful exploitation changes the scope and enables disclosure of sensitive information over the network, with no impact on integrity or availability.

The Microsoft Security Response Center advisory at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21521 provides details on mitigation and patches.

Details

CWE(s): CWE-150

Affected Products

microsoft

365 word copilot

all versions

AI Security AnalysisAI

AI Category: Enterprise AI Assistants
Risk Domain: N/A
OWASP Top 10 for LLMs 2025: None mapped
Classification Reason: Matched keywords: copilot

CVEs Like This One

CVE-2025-59252Same product: Microsoft 365 Word Copilot

CVE-2026-26149Same vendor: Microsoft

CVE-2026-26133Same vendor: Microsoft

CVE-2026-21520Same vendor: Microsoft

CVE-2026-20947Same vendor: Microsoft

CVE-2026-20856Same vendor: Microsoft

CVE-2025-21385Same vendor: Microsoft

CVE-2025-62549Same vendor: Microsoft

CVE-2025-59287Same vendor: Microsoft

CVE-2025-53766Same vendor: Microsoft

References

https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21521
Vendor Advisory · secure@microsoft.com