CVE-2026-21521
Microsoft 365 Word Copilot
Raw vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:NSummary
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 Content Injection (T1659); ranked at the 41th 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; in the Privacy and Disclosure risk domain.
The strongest mitigations our analysis identified map to SI-10 (Information Input Validation) — see the control section below for these in your framework.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
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.
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-4498
Vulnerability Data
Improper neutralization of escape, meta, or control sequences in Copilot allows an unauthorized attacker to disclose information over a network.
- CWE(s)
AI Security AnalysisAI
- AI Category
- Enterprise AI Assistants
- Risk Domain
- Privacy and Disclosure
- OWASP Top 10 for LLMs 2025
- None mapped
- Classification Reason
- Matched keywords: copilot
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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V1.2.10V1.3.12
Mitigating Controls (NIST 800-53 r5) AI
SI-10 requires validation/sanitization of inputs that directly stops escape/meta/control sequences from reaching downstream components.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Secure SDLC practices directly require input neutralization and escaping to prevent this class of flaw.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Security testing can detect the weakness but does not itself implement neutralization.
Secure development lifecycle mandates input validation and output encoding that directly neutralizes escape/meta sequences.
Application security requirements explicitly call for controls against injection and malformed input sequences.
Secure architecture principles reduce attack surface but do not prescribe specific neutralization techniques.
Secure coding standards require proper escaping and neutralization of control characters before downstream processing.