CVE-2026-21521: Copilot Information Disclosure Vulnerability

CVE-2026-21521 Overview

CVE-2026-21521 is an information disclosure vulnerability affecting Microsoft Copilot caused by improper neutralization of escape, meta, or control sequences. This flaw allows an unauthorized attacker to disclose sensitive information over a network by exploiting how the application processes special character sequences.

Critical Impact

Unauthorized attackers can exploit this vulnerability to access and exfiltrate sensitive information from Copilot without requiring any privileges, potentially exposing confidential data across organizational boundaries.

Affected Products

• Microsoft Copilot

Discovery Timeline

• January 22, 2026 - CVE-2026-21521 published to NVD
• January 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-21521

Vulnerability Analysis

This vulnerability stems from CWE-150: Improper Neutralization of Escape, Meta, or Control Sequences. The flaw exists in how Microsoft Copilot handles special character sequences in user input or data streams. When escape sequences, meta characters, or control sequences are not properly sanitized or neutralized, an attacker can craft malicious input that manipulates the application's behavior to disclose information that should otherwise be protected.

The vulnerability requires user interaction to exploit, meaning an attacker would typically need to convince a user to interact with malicious content or follow a specially crafted link. Once triggered, the attack can cross security boundaries (indicated by the changed scope), potentially allowing information from protected contexts to be disclosed to the attacker.

Root Cause

The root cause is insufficient input validation and sanitization of escape sequences and control characters within Copilot's processing logic. When special character sequences such as ANSI escape codes, terminal control sequences, or application-specific meta characters are passed through without proper neutralization, they can alter the intended behavior of the application. This allows attackers to inject sequences that cause the application to reveal information it would not normally expose.

Attack Vector

The attack is conducted over a network with no privileges required. An attacker crafts input containing malicious escape or control sequences designed to exploit the parsing vulnerabilities in Copilot. The attack requires user interaction—likely through social engineering to get the victim to process attacker-controlled content through Copilot.

The malicious sequences bypass normal security boundaries, causing confidential information to be disclosed. The changed scope indicator means that the vulnerable component and the impacted component are different, suggesting that Copilot processes input in a way that affects other security domains, amplifying the potential impact of information disclosure.

Detection Methods for CVE-2026-21521

Indicators of Compromise

• Unusual escape sequence patterns or control characters in Copilot input logs or request data
• Unexpected data exfiltration attempts from Copilot services to external destinations
• Anomalous user session behavior involving specially crafted content processing

Detection Strategies

• Monitor Copilot service logs for suspicious input patterns containing escape sequences, ANSI codes, or unusual control characters
• Implement network traffic analysis to detect potential data exfiltration following Copilot interactions
• Deploy endpoint detection rules to identify malicious content being passed to Copilot applications

Monitoring Recommendations

• Enable verbose logging for Copilot services to capture input processing details
• Configure alerts for unusual information access patterns or cross-boundary data flows
• Monitor for social engineering attempts that may precede exploitation of this vulnerability

How to Mitigate CVE-2026-21521

Immediate Actions Required

• Apply the latest security updates from Microsoft as soon as they become available
• Review and restrict Copilot access to sensitive data where possible until patching is complete
• Educate users about potential social engineering attacks that may attempt to exploit this vulnerability
• Monitor Copilot usage for suspicious activity patterns

Patch Information

Microsoft has published security guidance for this vulnerability. Organizations should consult the Microsoft Security Update Guide for official patch information and apply updates according to their patch management processes. Given the network-based attack vector and potential for information disclosure, patching should be prioritized.

Workarounds

• Implement strict input validation at network boundaries to filter potentially malicious escape sequences before they reach Copilot
• Consider temporarily limiting Copilot functionality for processing external or untrusted content until patches are deployed
• Apply network segmentation to limit the potential impact of information disclosure from Copilot services
• Enable additional security controls and monitoring on Copilot-related services to detect exploitation attempts