CVE-2026-39554: Fidalgo PHP Object Injection Vulnerability

CVE-2026-39554 Overview

CVE-2026-39554 is an unauthenticated PHP Object Injection vulnerability affecting the Fidalgo WordPress theme in versions up to and including 1.2.2. The flaw is categorized under [CWE-502] Deserialization of Untrusted Data. Attackers can submit crafted serialized payloads to the theme without authentication, triggering object instantiation inside the PHP runtime. When suitable PHP classes (gadgets) are present in the WordPress environment, this can lead to remote code execution, arbitrary file operations, or authentication bypass.

Critical Impact
Unauthenticated attackers can inject PHP objects into vulnerable Fidalgo theme deployments, with full confidentiality, integrity, and availability impact when chained with usable gadget classes.

Affected Products

Fidalgo WordPress theme versions <= 1.2.2
WordPress installations using the Fidalgo theme as the active or parent theme
Any site exposing Fidalgo theme entry points to the public internet

Discovery Timeline

2026-06-17 - CVE-2026-39554 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2026-39554

Vulnerability Analysis

The vulnerability stems from unsafe deserialization of untrusted input inside the Fidalgo theme. The theme passes attacker-controlled data into PHP's unserialize() function without validating its origin or structure. PHP then reconstructs arbitrary objects defined by the serialized payload.

Object instantiation invokes magic methods such as __wakeup, __destruct, or __toString on the resulting objects. Attackers chain these magic methods through gadgets present in WordPress core, plugins, or other themes. The result can be remote code execution, arbitrary file deletion, or sensitive data disclosure depending on the available gadget chain.

The attack does not require authentication. The CVSS vector indicates higher attack complexity, reflecting the dependency on a viable gadget chain inside the target environment.

Root Cause

The root cause is direct deserialization of attacker-supplied input through unserialize() without input validation, allow-listing, or use of safer formats such as JSON. The theme treats serialized data from request parameters, cookies, or stored values as trusted state.

Attack Vector

The attack vector is network-based. An unauthenticated attacker sends an HTTP request containing a serialized PHP payload to a vulnerable endpoint exposed by the Fidalgo theme. The payload references a class whose magic methods produce a side effect when the object is constructed or destroyed.

Technical exploitation details and proof-of-concept information are described in the Patchstack Vulnerability Report. No public exploit code is currently listed in ExploitDB, and the vulnerability is not present on the CISA KEV list. The EPSS score is 0.395%.

Detection Methods for CVE-2026-39554

Indicators of Compromise

HTTP request bodies, query strings, or cookies containing serialized PHP markers such as O:, a:, or s: followed by class names and lengths
Unexpected child processes spawned by the PHP-FPM or web server user after requests to Fidalgo theme endpoints
New or modified PHP files under wp-content/themes/fidalgo/ or wp-content/uploads/
Outbound network connections from the web server to unfamiliar hosts immediately following theme requests

Detection Strategies

Inspect web server access logs for requests targeting Fidalgo theme paths that include URL-encoded serialized payloads
Enable PHP error_log monitoring for warnings emitted by unserialize() on unexpected classes
Deploy WAF rules that flag PHP serialization signatures in request parameters not normally expected to carry binary data
Compare installed theme file hashes against a known-good baseline from the vendor distribution

Monitoring Recommendations

Forward WordPress, PHP, and web server logs to a centralized analytics platform for correlation across requests, file changes, and process executions
Alert on creation of new administrator accounts or modifications to wp_options immediately following anomalous theme requests
Track outbound DNS and HTTP traffic from web server workloads for command-and-control indicators

How to Mitigate CVE-2026-39554

Immediate Actions Required

Identify all WordPress installations using the Fidalgo theme at version 1.2.2 or earlier and prioritize them for remediation
Restrict public access to Fidalgo theme endpoints using web server access controls or a WAF until a fix is applied
Audit wp-content/themes/, wp-content/plugins/, and the WordPress database for signs of unauthorized modification
Rotate WordPress administrator passwords, secret keys in wp-config.php, and any API tokens stored on the host

Patch Information

At the time of NVD publication, the advisory references the Patchstack Vulnerability Report as the authoritative source. Administrators should consult that advisory for the fixed release version and apply the updated Fidalgo theme as soon as it is available. If a vendor patch is not yet published, replace the theme or take the site offline until remediation is possible.

Workarounds

Switch the active theme to a maintained alternative until a patched Fidalgo release is available
Add WAF rules that block request bodies and query strings containing PHP serialization syntax targeting theme endpoints
Restrict access to /wp-content/themes/fidalgo/ paths at the reverse proxy or CDN layer for non-authenticated traffic
Disable any Fidalgo theme features that accept serialized input from cookies or form parameters

bash

# Configuration example: block serialized PHP payloads at the web server (nginx)
location ~* /wp-content/themes/fidalgo/ {
    if ($request_uri ~* "(O%3A[0-9]+%3A|a%3A[0-9]+%3A%7B)") {
        return 403;
    }
    if ($request_body ~* "(O:[0-9]+:\"|a:[0-9]+:\{)") {
        return 403;
    }
}