CVE-2026-1664 Insecure Direct Object Reference (IDOR) via Header-Based Email Routing
Attackers can route inbound emails to arbitrary agent instances by spoofing headers, potentially accessing or modifying data in unauthorized Durable Objects.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-1664?
CVE-2026-1664 is a medium-severity missing authorization vulnerability in agents, published on 3 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Summary
An Insecure Direct Object Reference has been found to exist in `createHeaderBasedEmailResolver()` function within the Cloudflare Agents SDK. The issue occurs because the `Message-ID` and `References` headers are parsed to derive the target agentName and agentId without proper validation or origin checks, allowing an external attacker with control of these headers to route inbound mail to arbitrary Durable Object instances and namespaces .
Root cause
The `createHeaderBasedEmailResolver()` function lacks cryptographic verification or origin validation for the headers used in the routing logic, effectively allowing external input to dictate internal object routing.
Impact
Insecure Direct Object Reference (IDOR) in email routing lets an attacker steer inbound mail to arbitrary Agent instances via spoofed Message-ID.
Mitigation:
* PR: https://github.com/cloudflare/agents/blob/main/docs/email.md ] provides the necessary architectural context for coding agents to mitigate the issue by refactoring the resolver to enforce strict identity boundaries. * Agents-sdk users should upgrade to [email protected]
Weakness: CWE-639
Which versions are affected by CVE-2026-1664?
2 open source packages ship an affected version, including agents.
| Package | Affected | Fixed in |
|---|---|---|
agents npm | All versions | 0.3.7 |
agents npm | All versions | 0.3.7 |
How do I fix CVE-2026-1664?
Upgrade agents to 0.3.7 or later.
Upgrade, and check permissions on every request on the server, not only in the interface. More about Missing authorization.
Is CVE-2026-1664 being exploited?
There is no known exploitation of CVE-2026-1664 in the wild and no public exploit code.
Sources
Also known as GHSA-r7x9-8ph7-w8cg.