CVE-2026-55145 Overview
CVE-2026-55145 is a command injection vulnerability affecting Microsoft Outlook Copilot. The flaw stems from improper neutralization of special elements used in a command [CWE-77]. An authorized attacker can exploit this weakness over a network to perform tampering actions against affected systems. Exploitation requires low privileges and user interaction, limiting the scope but not eliminating the risk in enterprise environments where Copilot is widely deployed. Microsoft published the advisory on July 14, 2026, and no public exploit code or in-the-wild exploitation has been reported.
Critical Impact
An authenticated attacker can inject crafted commands into Outlook Copilot to tamper with data confidentiality and integrity over a network, without requiring elevated privileges.
Affected Products
- Microsoft Outlook Copilot (see the Microsoft Security Update CVE-2026-55145 advisory for specific build coverage)
Discovery Timeline
- 2026-07-14 - CVE-2026-55145 published to the National Vulnerability Database (NVD)
- 2026-07-14 - Last updated in NVD database
Technical Details for CVE-2026-55145
Vulnerability Analysis
The vulnerability resides in how Outlook Copilot processes input containing special characters used to delimit or construct commands. When Copilot receives attacker-controlled content, insufficient sanitization allows injected command syntax to be interpreted rather than treated as literal data. The resulting command execution occurs within the Copilot processing context, enabling tampering with information that Copilot handles on behalf of the user.
The impact profile emphasizes confidentiality exposure and limited integrity tampering, with no direct availability impact. Because Copilot integrates with mailbox content, meeting data, and other productivity signals, tampering can influence how downstream automated actions and summaries are generated. Successful exploitation requires an authenticated session and a user interaction step, such as opening a crafted message or invoking a Copilot action against attacker-supplied content.
Root Cause
The root cause is improper neutralization of special elements used in a command, classified under [CWE-77]. Input passed to Copilot's command handling logic is not consistently validated or escaped before being incorporated into command constructs. This allows an attacker to break out of intended data contexts and introduce additional command directives.
Attack Vector
The attack is network-based. An authenticated attacker delivers crafted content, typically via email or shared collaboration objects, that a target user then processes through Outlook Copilot. When Copilot parses the malicious content during a summarization, reply generation, or automation request, the injected commands are executed in the user's context. No exploit code is currently publicly available and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.
No verified public exploit code exists. Refer to the Microsoft Security Update CVE-2026-55145 advisory for authoritative technical details.
Detection Methods for CVE-2026-55145
Indicators of Compromise
- Inbound email or shared content containing unusual command-like syntax, prompt injection markers, or embedded directives targeting Copilot processing.
- Anomalous Copilot activity logs showing unexpected actions triggered shortly after processing external content.
- Unusual outbound requests or data access patterns from Outlook client processes following Copilot invocation on attacker-supplied content.
Detection Strategies
- Inspect Microsoft 365 audit logs for Copilot interactions that correlate with newly received external messages.
- Apply mail hygiene rules that flag messages containing structured payloads or prompt-injection style content aimed at AI assistants.
- Correlate identity and endpoint telemetry to spot tampering actions performed shortly after Copilot processes untrusted input.
Monitoring Recommendations
- Enable and retain Microsoft Purview audit logs covering Copilot activity, mailbox access, and delegated automation actions.
- Baseline normal Copilot usage patterns per user so deviations linked to crafted content stand out.
- Monitor for changes to mailbox rules, forwarding configurations, or shared content that follow suspicious Copilot sessions.
How to Mitigate CVE-2026-55145
Immediate Actions Required
- Apply the Microsoft security update referenced in the Microsoft Security Update CVE-2026-55145 advisory as soon as it is available in your update channel.
- Review Copilot access assignments and remove entitlements from accounts that do not require the feature.
- Enforce phishing-resistant multi-factor authentication (MFA) to reduce the pool of attackers able to satisfy the authenticated precondition.
Patch Information
Microsoft has published guidance and update details in the Microsoft Security Update CVE-2026-55145 advisory. Administrators should validate that affected Outlook and Copilot components are updated to the fixed build in their tenant and endpoints.
Workarounds
- Restrict Copilot processing of external or untrusted mail content through tenant policy until patching is complete.
- Configure mail flow rules to quarantine messages containing content patterns known to abuse AI assistant instructions.
- Educate users to avoid invoking Copilot actions on unexpected messages from external senders.
# Example: Disable Microsoft 365 Copilot for a pilot group via PowerShell
# until affected components are patched.
Connect-MgGraph -Scopes "Policy.ReadWrite.Authorization"
# Remove Copilot license assignment for a specific user
Set-MgUserLicense -UserId user@example.com `
-RemoveLicenses @("<CopilotSkuId>") `
-AddLicenses @()
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
