Newsletters unauthenticated API key type-juggling bypass

Newsletters before 4.16 uses loose equality in wpMailAPI::api_init() to compare a client-supplied api_key with the configured key. When the optional API is enabled, an unauthenticated request to /wp-admin/admin-ajax.php?action=newsletters_api can supply a non-string JSON value that PHP considers equal under type juggling, then choose api_method and api_data. Exposed methods include subscriber_add, subscriber_delete, send_email and send_newsletter, allowing subscriber-record changes and email or newsletter sending without the real key. The exact bypass value is not disclosed, and any configured API host allowlist remains an additional prerequisite. Version 4.16 requires non-empty strings and validates them with hash_equals().

CVE / CNA score 4.8 CVSS 3.1 · 134c704f-9b21-4f2e-91b3-4a467353bcc0

NVD score Pending NVD has not published its own CVSS assessment.

Component: Newsletters
Plugin slug: newsletters-lite
Affected: < 4.16
Safe version: 4.16
Published: Aug 08, 2026
Weakness: CWE-287 — Improper Authentication

This CVE was published Aug 08, 2026 and is one of 37 known issues for this plugin.

What to do

Update, patch or deactivate.

Update to 4.16 or later. Until updated, disable the optional Newsletters API or restrict it to trusted source addresses. Rotate the API key and review subscriber changes and outbound mail activity after updating.

A safe version is available, so updating to that version or later is the preferred remediation. If an immediate update is not practical, consider a targeted application patch or temporarily restricting the affected functionality.

Deactivate only when warranted by your risk profile, or when advised by your hosting provider in the limited circumstances where the vulnerability cannot otherwise be mitigated. If you’re unsure which action is appropriate, contact Fused or your hosting provider for guidance.

Source record

Technical description

The Newsletters WordPress plugin before 4.16 does not strictly compare its API authentication key, allowing unauthenticated attackers to bypass the API authentication via type juggling and perform privileged actions such as modifying subscriber records and sending emails, when the optional API has been enabled.

CVE / CNA vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L

References

Primary and upstream sources

Data attribution: This product uses the NVD API but is not endorsed or certified by the NVD.

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