CVE-2026-39576: SingleMalt Auth Bypass Vulnerability

CVE-2026-39576 Overview

CVE-2026-39576 is an unauthenticated PHP Object Injection vulnerability affecting the SingleMalt WordPress theme in versions 1.5 and earlier. The flaw is classified 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 a suitable property-oriented programming (POP) chain is available in WordPress core, an active plugin, or another theme, exploitation can lead to remote code execution, arbitrary file operations, or data tampering. The vulnerability has a network attack vector and does not require user interaction.

Critical Impact
Unauthenticated attackers can inject PHP objects into the application context, potentially leading to remote code execution, file deletion, or full site compromise when a usable POP chain is present.

Affected Products

SingleMalt WordPress theme versions <= 1.5
WordPress installations running the vulnerable theme
Any site exposing the theme's entry points to the public internet

Discovery Timeline

2026-06-17 - CVE-2026-39576 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2026-39576

Vulnerability Analysis

The SingleMalt theme passes attacker-controlled input to a PHP deserialization function without validating the data. PHP's unserialize() reconstructs objects from serialized strings and invokes magic methods such as __wakeup(), __destruct(), or __toString() during the process. When attacker data reaches this sink, the attacker controls which classes are instantiated and what property values those objects carry.

Exploitation depends on the presence of gadget classes loaded in the WordPress runtime. WordPress core and popular plugins contain known POP chains that translate object injection into file writes, file deletion, or command execution. The unauthenticated network reachability removes the primary access control barrier, leaving the attack complexity tied only to chain availability.

Root Cause

The root cause is unsafe deserialization of user-supplied input inside the theme. The code calls unserialize() on data sourced from HTTP parameters, cookies, or option storage without applying allow-listing, integrity checks, or safer formats such as JSON. This pattern is the canonical [CWE-502] weakness.

Attack Vector

An unauthenticated attacker sends an HTTP request containing a serialized PHP payload to a theme endpoint that triggers deserialization. The crafted payload references gadget classes available in the WordPress process. Upon deserialization, magic methods execute and drive the POP chain toward its sink, such as file write or command execution. No credentials, MFA, or user interaction is required.

The vulnerability is described in the Patchstack Vulnerability Report. Refer to the advisory for endpoint-level technical details.

Detection Methods for CVE-2026-39576

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.
New or modified PHP files in wp-content/uploads/, wp-content/themes/singlemalt/, or other writable directories.
Outbound connections from the web server to unfamiliar hosts shortly after suspicious POST requests.
Unexpected WordPress administrator accounts or modified wp_options entries containing serialized payloads.

Detection Strategies

Inspect web server and WAF logs for serialized PHP patterns in requests targeting SingleMalt theme paths.
Hash the theme directory and compare against a known-good baseline to detect tampering.
Enable PHP error logging and alert on unserialize() warnings or unexpected class instantiation messages.
Run integrity checks on wp-config.php and core WordPress files for unauthorized modifications.

Monitoring Recommendations

Forward WordPress and web server logs to a central log platform and alert on the serialization indicators above.
Monitor process creation on the web host for PHP spawning shells such as /bin/sh, bash, or cmd.exe.
Track file write events in theme and upload directories using endpoint or host-based monitoring.

How to Mitigate CVE-2026-39576

Immediate Actions Required

Identify all WordPress sites running the SingleMalt theme at version 1.5 or earlier.
Deactivate and remove the SingleMalt theme until a patched release is confirmed available from the vendor.
Rotate WordPress administrator passwords, secret keys in wp-config.php, and any API tokens stored on the host.
Review the WordPress site for unknown administrator accounts, scheduled tasks, and modified plugin or theme files.

Patch Information

No fixed version is listed in the available CVE data at the time of publication. Consult the Patchstack Vulnerability Report for vendor patch status and upgrade guidance.

Workarounds

Restrict access to the WordPress site behind a web application firewall configured to block serialized PHP payloads.
Block requests containing PHP serialization patterns such as O:\d+:" or a:\d+:{ at the WAF or reverse proxy layer.
Disable or replace the SingleMalt theme with a maintained alternative until remediation is confirmed.

bash

# Example ModSecurity rule to block serialized PHP payloads in requests
SecRule ARGS|REQUEST_COOKIES|REQUEST_BODY "@rx (?:^|[=&;])(O|a|s):\d+:(?:\"|{)" \
  "id:1026039576,phase:2,deny,status:403,log,msg:'Potential PHP Object Injection - CVE-2026-39576'"