CVE-2026-1664
📋 TL;DR
This CVE describes an Insecure Direct Object Reference vulnerability in Cloudflare Agents SDK's email routing function. Attackers can spoof email headers to redirect inbound mail to arbitrary Durable Object instances, potentially accessing unauthorized data or functionality. Users of the Cloudflare Agents SDK with email routing enabled are affected.
💻 Affected Systems
- Cloudflare Agents SDK
⚠️ Manual Verification Required
This CVE does not have specific version information in our database, so automatic vulnerability detection cannot determine if your system is affected.
Why? The CVE database entry doesn't specify which versions are vulnerable (no version ranges provided by the vendor/NVD).
🔒 Custom verification scripts are available for registered users. Sign up free to download automated test scripts.
- Review the CVE details at NVD
- Check vendor security advisories for your specific version
- Test if the vulnerability is exploitable in your environment
- Consider updating to the latest version as a precaution
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of email routing logic allowing attackers to intercept, redirect, or manipulate all inbound email traffic to unauthorized Durable Object instances, potentially leading to data exfiltration or service disruption.
Likely Case
Targeted attacks where attackers redirect specific inbound emails to malicious or unauthorized endpoints, potentially accessing sensitive information or disrupting business workflows.
If Mitigated
Proper validation and cryptographic verification prevent header spoofing, ensuring emails only route to intended, authorized Durable Object instances.
🎯 Exploit Status
Exploitation requires sending emails with spoofed headers to vulnerable endpoints.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: agents@0.3.7
Vendor Advisory: https://github.com/cloudflare/agents
Restart Required: Yes
Instructions:
1. Update package.json to specify agents@0.3.7 or higher
2. Run npm update agents
3. Restart all services using the Agents SDK
4. Verify the update with npm list agents
🔧 Temporary Workarounds
Disable Header-Based Email Routing
allTemporarily disable or remove the createHeaderBasedEmailResolver() function until patching is possible.
Comment out or remove createHeaderBasedEmailResolver() calls in your code
🧯 If You Can't Patch
- Implement additional validation layer to verify email header integrity before routing
- Monitor email routing logs for suspicious header patterns or unexpected destinations
🔍 How to Verify
Check if Vulnerable:
Check if your code uses createHeaderBasedEmailResolver() and verify the agents SDK version is below 0.3.7Check Version:
npm list agentsVerify Fix Applied:
Confirm agents SDK version is 0.3.7 or higher and test email routing with various header inputs to ensure proper validation📡 Detection & Monitoring
Log Indicators:
- Unusual email routing patterns
- Emails routed to unexpected Durable Object instances
- Suspicious Message-ID or References header patterns
Network Indicators:
- Unexpected email traffic to non-standard endpoints
- Email routing to unauthorized namespaces
SIEM Query:
Search for email routing events where Message-ID or References headers contain unexpected patterns or route to unauthorized agentName/agentId values