Attack Chain

1. An attacker identifies an M365 Copilot instance exposed to network traffic.
2. The attacker crafts a malicious input containing special elements designed to exploit the improper neutralization vulnerability (CVE-2026-26129).
3. The crafted input is sent to the M365 Copilot instance via a network request.
4. M365 Copilot fails to properly neutralize the special elements within the input.
5. The special elements are processed, leading to unintended data access.
6. Sensitive information is extracted by the attacker.
7. The extracted information is transmitted over the network to a location controlled by the attacker.
8. The attacker gains unauthorized access to the disclosed information.

Impact

Successful exploitation of CVE-2026-26129 can lead to the unauthorized disclosure of sensitive information accessible by M365 Copilot. The impact could range from exposure of internal documents and communications to the revelation of proprietary data and confidential customer information. The number of potential victims is dependent on the scope of M365 Copilot deployments and the sensitivity of the data it handles. A successful attack could result in significant reputational damage, financial losses, and legal liabilities for affected organizations.

Recommendation

• Apply the security update provided by Microsoft to address CVE-2026-26129 as soon as possible, referencing the advisory at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-26129.
• Deploy the Sigma rule "Detect M365 Copilot Special Element Injection Attempt" to identify potential exploitation attempts based on specific special characters in network requests.
• Monitor network traffic for suspicious patterns indicative of information disclosure, focusing on communications originating from M365 Copilot instances.
• Review and strengthen input validation and output encoding mechanisms within M365 Copilot to prevent future instances of improper neutralization, referencing CWE-138.