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CVE-2026-12583 – Unauthenticated PHP Object Injection Leading to RCE – Newsletters WordPress Plug…

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Summary

CVE-2026-12583 is a high-severity PHP object deserialization vulnerability in the Newsletters WordPress plugin (by Tribulant Software), affecting all versions prior to 4.15. Unauthenticated remote attackers can inject a malicious PHP object through the plugin’s publicly accessible subscription form and, via a property-oriented gadget chain bundled within the plugin itself, write arbitrary files and execute code on the server. The vulnerability carries a CVSS v3.1 score of 8.1 (High).

Technical details

  • Root cause: The plugin fails to validate or sanitize serialized data submitted through custom text fields in its public subscriber registration form before deserializing it — a classic CWE-502 (Deserialization of Untrusted Data) flaw.
  • Trigger conditions: An attacker submits a crafted serialized PHP payload through an unprotected custom subscriber field on a subscribable mailing list. Exploitation is triggered when a newsletter campaign uses a personalization token referencing that custom field and the campaign is rendered by the plugin.
  • Attack vector: Fully remote and unauthenticated — the subscription form is publicly accessible to any anonymous visitor, requiring no WordPress account or victim interaction.
  • Gadget chain: The exploit leverages the GuzzleHttpCookieFileCookieJar destructor from the Guzzle HTTP library bundled with the plugin, making the attack entirely self-contained with no external dependencies.
  • Impact: Successful exploitation achieves arbitrary file write on the web server, which can be used to deploy web shells or other malicious payloads, resulting in full remote code execution (RCE) with the privileges of the web server process.
  • Context: This vulnerability represents an incomplete remediation of CVE-2025-67911, a prior deserialization flaw in the same plugin, as confirmed by the plugin’s own version 4.15 changelog.

Affected software

  • Newsletters WordPress Plugin (newsletters-lite) by Tribulant Software — all versions prior to 4.15

Severity

  • CVSS v3.1 Base Score: 8.1 (High)
  • Vector String: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Update the Newsletters plugin to version 4.15 or later. Version 4.15 addresses this vulnerability by preventing PHP object instantiation when rendering serialized subscriber custom-field values during newsletter personalization.
  • If patching is not immediately feasible: Disable or remove custom text fields from all public subscriber subscription forms, and consider temporarily deactivating the plugin until the update can be applied. Restricting public access to subscription form endpoints at the network or WAF layer can reduce exposure.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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