Cloudflare Agents up to 0.3.6 Message createHeaderBasedEmailResolver Message-ID/References authorization

Summaryinfo

A vulnerability classified as critical has been found in Cloudflare Agents up to 0.3.6. The affected element is the function createHeaderBasedEmailResolver of the component Message Handler. Performing a manipulation of the argument Message-ID/References results in authorization. This vulnerability is known as CVE-2026-1664. Remote exploitation of the attack is possible. No exploit is available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability, which was classified as critical, was found in Cloudflare Agents up to 0.3.6. This affects the function createHeaderBasedEmailResolver of the component Message Handler. The manipulation of the argument Message-ID/References with an unknown input leads to a authorization vulnerability. CWE is classifying the issue as CWE-639. The system's authorization functionality does not prevent one user from gaining access to another user's data or record by modifying the key value identifying the data. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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]

The advisory is shared at github.com. This vulnerability is uniquely identified as CVE-2026-1664 since 01/29/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. Technical details are known, but no exploit is available.

Upgrading to version 0.3.7 eliminates this vulnerability.

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Productinfo

Type

• Cloud Software

Vendor

• Cloudflare

Name

• Agents

Version

• 0.3.0
• 0.3.1
• 0.3.2
• 0.3.3
• 0.3.4
• 0.3.5
• 0.3.6

CPE 2.3info

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CPE 2.2info

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