CVE-2026-39573 Overview
CVE-2026-39573 is an unauthenticated PHP Object Injection vulnerability affecting the Mildhill WordPress theme in versions 1.5 and earlier. The flaw maps to [CWE-502] Deserialization of Untrusted Data and allows remote attackers to inject crafted serialized PHP objects without authentication. When the application deserializes attacker-controlled data, gadget chains present in WordPress core, plugins, or other themes can trigger remote code execution, arbitrary file operations, or data exfiltration.
Critical Impact
Unauthenticated attackers can submit serialized payloads to the Mildhill theme that trigger PHP object instantiation, potentially leading to full WordPress site compromise when usable gadget chains are present.
Affected Products
- Mildhill WordPress theme versions 1.5 and earlier
- WordPress installations using the vulnerable theme
- Sites with additional plugins or themes providing exploitable PHP gadget chains
Discovery Timeline
- 2026-06-17 - CVE-2026-39573 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2026-39573
Vulnerability Analysis
The vulnerability stems from unsafe handling of serialized data within the Mildhill theme. The theme passes attacker-supplied input to PHP's unserialize() function without validating its origin or contents. PHP object deserialization automatically invokes magic methods such as __wakeup(), __destruct(), and __toString() on reconstructed objects. Attackers exploit this behavior by crafting object graphs that chain these magic methods through classes already loaded in the WordPress runtime.
The attack does not require authentication, increasing the exposed attack surface. Successful exploitation depends on the presence of a usable gadget chain inside WordPress core, installed plugins, or other active themes. EPSS data assigns the issue a 0.395% probability with a 31.1 percentile, reflecting that exploitation requires environment-specific gadget availability.
Root Cause
The root cause is the use of unserialize() on untrusted input, classified as [CWE-502]. The theme does not enforce signature verification, type restrictions, or allowed_classes constraints when reconstructing PHP objects. Any input path reaching the vulnerable sink permits arbitrary class instantiation.
Attack Vector
The attack vector is network-based and requires no privileges or user interaction. An attacker submits a crafted serialized payload through a request parameter, cookie, or form field handled by the Mildhill theme. The vulnerable endpoint deserializes the payload, instantiating attacker-controlled object structures. When magic methods execute on those objects, attackers chain operations that may write files, execute system commands, or manipulate the WordPress database.
Technical details are available in the Patchstack Vulnerability Report.
Detection Methods for CVE-2026-39573
Indicators of Compromise
- HTTP request bodies, query strings, or cookies containing serialized PHP markers such as O:, a:, s:, or __PHP_Incomplete_Class
- Unexpected PHP files written to wp-content/uploads/ or theme directories
- New or modified WordPress administrator accounts created without audit trail
- Outbound connections from the web server to unfamiliar hosts following deserialization activity
Detection Strategies
- Monitor web server access logs for requests targeting Mildhill theme endpoints carrying serialized object patterns
- Inspect PHP error logs for warnings referencing unserialize(), __wakeup, or unknown class instantiation
- Apply web application firewall rules that flag serialized object syntax in untrusted parameters
- Compare installed theme file hashes against vendor-released versions to detect tampering
Monitoring Recommendations
- Track creation of new PHP files under wp-content/ and alert on writes by the web server user
- Monitor wp_users and wp_options tables for unauthorized modifications
- Alert on web server processes spawning shell interpreters such as sh, bash, or cmd.exe
- Forward WordPress and web server logs to a centralized analytics platform for correlation
How to Mitigate CVE-2026-39573
Immediate Actions Required
- Identify all WordPress sites using the Mildhill theme and inventory their versions
- Disable or replace the Mildhill theme on sites running version 1.5 or earlier until a patched release is verified
- Deploy WAF signatures that block serialized PHP payloads in unauthenticated request paths
- Review web server and WordPress logs for prior exploitation attempts predating mitigation
Patch Information
At publication, no vendor-released patched version is documented in the available references. Administrators should consult the Patchstack Vulnerability Report for updates and apply any vendor fix as soon as it is published.
Workarounds
- Replace the Mildhill theme with a maintained alternative until a vendor patch is available
- Restrict access to theme-handled endpoints using authentication or IP allow-listing where feasible
- Configure a virtual patching rule at the WAF to reject requests containing PHP serialized object syntax
- Audit installed plugins and themes to remove unused components that could provide gadget chains
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
