The URL Detonation Reputation Problem

URL Detonation Reputation is a security feature designed to protect users from malicious email links. However, it has been flagging legitimate websites and URLs as potential phishing threats, leading to widespread email disruptions.

Key Issues:

• False Positives: Many legitimate URLs are incorrectly identified as malicious, resulting in quarantined or blocked emails.
• Cross-Tenant Impact: The problem affects not only the organisation owning the flagged domain but also other Microsoft 365 tenants trying to send emails containing the affected URLs.
• Lack of Transparency: Users and administrators often struggle to understand why specific URLs are being flagged, as the system provides limited information about the reasons for blocking.
• Persistent Blocks: Even after submitting URLs for review as false positives, many users report that the blocks persist, causing ongoing communication issues.

Impact on Businesses

The overreach of URL Detonation Reputation filtering has led to severe consequences for many organisations:

• Communication Disruptions: Important emails containing legitimate URLs are being quarantined, leading to delays and missed communications.
• Customer Complaints: Businesses are facing complaints from customers who are not receiving their emails, potentially damaging relationships and reputations.
• Productivity Loss: IT teams spend significant time troubleshooting and attempting to resolve these issues, diverting resources from other critical tasks.

Potential Solutions

While there is no one-size-fits-all solution, here are some steps that affected organisations can take:

1. Submit False Positives

You can use the admin submission process to report false positives to Microsoft. This can help improve the filtering system's accuracy over time.

2. Utilize Tenant Allow/Block List

To create a temporary override signal, add affected URLs to the Tenant Allow/Block List (TABL). This can help bypass the filters while Microsoft reviews the submission.

3. Engage Microsoft Support

Open a support ticket with Microsoft to escalate the issue. Persistence may be necessary, as resolution times can vary.

4. Implement Mail Flow Rules

As a temporary measure, create mail flow rules to bypass Spam Confidence Level (SCL) filtering for specific URLs. However, this should be done cautiously and only for trusted domains.

5. Review and Optimize Email Authentication

Ensure that SPF, DKIM, and DMARC are correctly configured for your domain. While this may not directly solve URL reputation issues, it can improve overall email deliverability.

6. Consider Advanced Delivery Options

For legitimate phishing simulations or similar scenarios, use the Advanced Delivery options in Microsoft 365 Defender to whitelist specific IPs and domains.

Conclusion

While Microsoft's URL Detonation Reputation filtering aims to protect users from phishing attacks, its current implementation is causing significant challenges for many organisations. Microsoft must improve its accuracy as the system evolves and provide more transparency to affected users. In the meantime, organisations must remain vigilant, proactively address false positives, and work closely with Microsoft support to resolve persistent issues.

Citations:

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