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Authorization Bypass (IDOR)Weakness · CWE-639

CVE-2026-1664

MEDIUM · 6.9 CVSS v4.0 Published 2026-02-03
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
78/100
Remediation priority · High
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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]

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The `createHeaderBasedEmailResolver()` function in Cloudflare Agents SDK parses Message-ID and References headers to derive agentName and agentId for routing inbound mail to Durable Object instances. Because these headers are not cryptographically verified or validated for origin, an attacker controlling these headers can spoof them to redirect email to arbitrary agent namespaces, achieving IDOR.

MitigationUpgrade to [email protected] and refactor the resolver to implement cryptographic verification or strict origin validation for Message-ID and References headers before using them for routing decisions.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
Authentication
X
User interaction
None
Scope
X

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify if Cloudflare Agents SDK is installed
    Check your package.json, package-lock.json, or node_modules for the 'agents' package from Cloudflare
    Affected if The agents package is present in dependencies
  2. Check agents package version
    Run 'npm list agents' or check package.json version field for the agents dependency
    Affected if Version is lower than 0.3.7 or unspecified (any version below 0.3.7)
  3. Identify use of createHeaderBasedEmailResolver
    Search codebase for 'createHeaderBasedEmailResolver' function calls in your source files
    Affected if This function is imported and used in your code
  4. Verify email routing configuration
    Check your Cloudflare Workers configuration or wrangler.toml for email routing setup that uses the header-based resolver
    Affected if Email routing to Durable Objects is configured using header-based routing
  5. Check for cryptographic validation code
    Search for Message-ID or References header validation logic in your email handling code
    Affected if No validation or verification of these headers exists before routing decisions

You are affected if you use the agents package version below 0.3.7 and utilize createHeaderBasedEmailResolver for routing inbound email to Durable Object instances without additional origin validation.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade to [email protected] and refactor the resolver to implement cryptographic verification or strict origin validation for Message-ID and References headers before using them for routing decisions.

Recommended fix Moderate confidence

[email protected]

  1. Update the agents-sdk dependency in your project to version 0.3.7 (e.g., npm install [email protected] or yarn add [email protected])
  2. Verify the package was installed correctly by checking the installed version (e.g., npm list agents)
  3. Rebuild and redeploy your Cloudflare Workers that use the createHeaderBasedEmailResolver() function
  4. Test that email routing still functions correctly with the updated SDK

Generated from the published advisory — verify against the referenced sources before acting.

Have this fixed Scoped from the published advisory
  • Consultation6.0 h
  • Implementation12.0 h
  • Testing6.0 h
  • Review / QA3.0 h
27.0 hours of engineering $4,800
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-1664 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data