CVE-2026-91051 in EWWW Image Optimizer Plugin
Summary
by MITRE • 09/30/2026
The EWWW Image Optimizer WordPress plugin before 8.8.0 does not prevent authenticated users with author-level permissions from storing a serialized value in a post meta field that is deserialized when the post is rendered, allowing them to perform PHP Object Injection, which can lead to remote code execution when a suitable gadget chain is present via another installed EWWW Image Optimizer WordPress plugin before 8.8.0 or .
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Analysis
by VulDB Data Team • 09/30/2026
The vulnerability identified in versions of the EWWW Image Optimizer plugin prior to version 8.8.0 represents a critical server-side request forgery and deserialization flaw that compromises the integrity of the WordPress application environment. This security issue stems from an insufficient validation mechanism within the plugin's handling of post metadata fields. Specifically, the software fails to sanitize or restrict input data submitted by authenticated users who possess author-level permissions. In standard WordPress architectures, author-level accounts are intended for content creation and management but should not have access to modify low-level system configurations or execute arbitrary code. However, due to this oversight, these users can inject malicious payloads into post meta fields that the plugin subsequently processes without adequate scrutiny.
The core technical flaw involves the storage of serialized PHP objects within a database field associated with posts. When an author submits content containing specially crafted data, they are able to store a serialized value in a specific metadata key managed by EWWW Image Optimizer. This action alone is not immediately exploitable for code execution but sets the stage for a more severe attack vector known as PHP Object Injection. The vulnerability manifests when the plugin attempts to render or process the post content later. During this rendering phase, the application retrieves the stored meta value and passes it directly into a PHP unserialize function without verifying that the data originates from a trusted source or conforms to expected formats. This lack of input validation allows an attacker-controlled serialized object to be instantiated within the application's memory space.
The operational impact of this vulnerability is severe because successful exploitation can lead to Remote Code Execution on the target server. Once the malicious serialized object is deserialized, it triggers the instantiation of a PHP class defined by the attacker or included in the environment. If the WordPress installation includes other plugins or themes that contain vulnerable gadget chains—sequences of method calls that result in dangerous side effects such as file inclusion or command execution—the injected object can leverage these existing classes to execute arbitrary system commands. This effectively bypasses authentication and authorization controls, granting the attacker full control over the web server's underlying operating system depending on the privileges under which the PHP process runs. Such an outcome allows for data exfiltration, defacement, installation of backdoors, or use of the compromised host as a pivot point for further network attacks.
This vulnerability aligns with Common Weakness Enumeration (CWE) categories including CWE-502 Deserialization of Untrusted Data and CWE-94 Improper Control of Generation of Code PHP Object Injection. From an ATT&CK perspective, this exploitation technique falls under the Tactic Execution and Persistence domains, specifically mapping to techniques involving server-side request forgery or injection attacks that leverage legitimate application features for malicious purposes. The presence of gadget chains from other installed plugins is a critical factor in determining exploitability, highlighting the risk associated with maintaining multiple third-party extensions on a single WordPress instance where one component's weakness can be chained with another's vulnerabilities to achieve full system compromise.
Mitigation strategies must focus primarily on immediate patching and rigorous input validation practices. Administrators running affected versions of EWWW Image Optimizer should upgrade to version 8.8.0 or later, which implements proper sanitization checks before storing data in post meta fields and ensures that only expected data types are processed during deserialization operations. For organizations unable to update immediately due to compatibility concerns, temporary mitigations include restricting author-level user permissions where possible, implementing Web Application Firewall rules to detect patterns associated with serialized PHP object injection payloads, and auditing installed plugins for known vulnerabilities that could serve as gadget chains. Long-term remediation requires adopting secure coding standards such as avoiding the use of native PHP unserialize functions on untrusted input or utilizing safe alternatives like JSON encoding/decoding where applicable, thereby eliminating the attack surface entirely regardless of external plugin dependencies.