CVE-2026-87741 in ConvertPlus Plugin
Summary
by MITRE • 09/28/2026
The ConvertPlus plugin for WordPress is vulnerable to Deserialization of Untrusted Data in all versions up to, and including, 3.6.3 via the style parameter of the cp_display_preview_modal AJAX action. The vulnerability exists because the action's nonce guard is gated behind an isset() check and fails open when the cp_admin_page_nonce parameter is omitted entirely, no capability check is performed on the callback, and sanitize_text_field() — applied to the $style value before it is concatenated directly into a shortcode string evaluated by do_shortcode() — does not strip shortcode delimiters, allowing an attacker to inject a second, fully attacker-controlled [smile_modal] invocation that causes smile_modal_popup() to pass attacker-supplied base64-decoded bytes to maybe_unserialize() with no allowed_classes restriction. This makes it possible for authenticated attackers, with Subscriber-level access and above, to inject a PHP object. No known POP chain is present in the vulnerable software, which means this vulnerability has no impact unless another plugin or theme containing a POP chain is installed on the site. If a POP chain is present via an additional plugin or theme installed on the target system, it may allow the attacker to perform actions like delete arbitrary files, retrieve sensitive data, or execute code depending on the POP chain present.
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Analysis
by VulDB Data Team • 09/28/2026
The ConvertPlus WordPress plugin contains a critical vulnerability affecting all versions up to and including 3.6.3, specifically within the cp_display_preview_modal AJAX action handler. This flaw stems from improper input validation and insufficient access control mechanisms that allow for deserialization of untrusted data. The core issue lies in how the nonce verification is implemented; rather than strictly enforcing authentication, the code uses an isset() check on the cp_admin_page_nonce parameter which fails open if the parameter is omitted entirely. Furthermore, there is no capability check performed to ensure the user invoking this action possesses administrative privileges or any other required role level. This combination of weak authentication checks and missing authorization controls creates a pathway for authenticated attackers with subscriber-level access and above to interact with sensitive backend functions that should be restricted.
The technical mechanism of exploitation involves manipulating the style parameter passed during the AJAX request. The application applies sanitize_text_field() to this value, which is intended to strip HTML tags but notably does not remove shortcode delimiters such as square brackets. Consequently, an attacker can inject a crafted string containing a second [smile_modal] shortcode invocation that overrides or supplements the original intent of the function. This injected shortcode triggers the smile_modal_popup callback, which processes user-supplied input by decoding base64-encoded bytes and passing them directly to PHP's maybe_unserialize() function without any restriction on allowed classes. This lack of allowlisting for deserialization targets is a severe security anti-pattern that enables Object Injection attacks, allowing an attacker to instantiate arbitrary PHP objects within the application context.
From a classification perspective, this vulnerability aligns with CWE-502 Deserialization of Untrusted Data and CWE-862 Missing Authorization. The failure to verify user capabilities before executing privileged operations also relates to CWE-284 Improper Access Control. In terms of offensive security frameworks like MITRE ATT&CK for Enterprise, this behavior maps to T1059 Command and Scripting Interpreter through the potential execution of arbitrary code via object injection, as well as T1078 Valid Accounts if exploited by lower-privileged users who gain elevated capabilities through the deserialization chain. The vulnerability represents a classic case where input sanitization is mistakenly assumed to be sufficient for security, ignoring the specific risks associated with dynamic evaluation and serialization in PHP environments.
The operational impact of this vulnerability depends heavily on the broader ecosystem of the targeted WordPress installation. While ConvertPlus itself does not contain any known Proof-of-Concept (POP) chains that would allow direct remote code execution or arbitrary file deletion through its own classes, the risk remains critical due to the interconnected nature of modern web applications. If another plugin or theme installed on the same server contains a vulnerable class with accessible magic methods such as __destruct(), __wakeup(), or __toString(), an attacker can leverage this deserialization flaw to trigger those chains. Successful exploitation could lead to severe consequences including arbitrary file deletion, retrieval of sensitive database credentials or user data, and full remote code execution depending on the specific POP chain available in the environment.
Mitigation strategies must address both immediate remediation and long-term security posture improvements. The most effective solution is to upgrade ConvertPlus to a version where this vulnerability has been patched by the vendor. In cases where upgrading is not immediately feasible, administrators should restrict access to WordPress admin pages using web application firewalls or server-level configurations that block unauthorized AJAX requests lacking valid nonces. Additionally, implementing strict allowlisting for deserialization targets can prevent exploitation even if other vulnerable plugins are present. Developers and site owners should also audit their plugin ecosystem regularly to identify any components with known POP chains, ensuring that the principle of least privilege is enforced across all user roles and that sensitive functions are protected by robust capability checks rather than relying solely on nonce validation or input sanitization alone.