WordPress GutenBee Plugin Arbitrary File Upload Flaw

The GutenBee – Gutenberg Blocks plugin for WordPress is vulnerable to Arbitrary File Upload in all versions up to, and including, 2.20.1 via the gutenbee_file_and_ext_json function. This is due to a flawed strpos() substring check that only verifies whether the filename contains the string '.json' rather than confirming the filename ends with a .json extension, allowing double-extension filenames like shell.json.php to bypass validation. This makes it possible for authenticated attackers, with author-level access and above, to upload files that may be executable, which makes remote code execution possible.

Casky was already ahead

This CVE exploits attack patterns that Casky's 419matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

The GutenBee – Gutenberg Blocks plugin for WordPress contains a critical arbitrary file upload vulnerability in versions up to 2.20.1. The flaw stems from improper input validation in the gutenbee_file_and_ext_json function, which uses a flawed strpos() check to verify file extensions. Rather than confirming that a filename ends with .json, the function merely checks if the string '.json' appears anywhere in the filename. This allows attackers to bypass validation using double-extension filenames like shell.json.php, enabling arbitrary code execution. With a CVSS score of 8.8, this vulnerability poses a significant risk to WordPress installations using this plugin. Authenticated attackers can upload malicious PHP files disguised as JSON, leading to remote code execution and complete site compromise.

Casky's 419 matching skills leverage Claude AI's extended reasoning to detect the attack patterns associated with this CVE across three MITRE ATT&CK techniques: Initial Access (TA0001), Persistence (TA0043), and Execution (TA0002). Practitioners using Casky would observe findings related to suspicious file upload requests with double-extension payloads, anomalous PHP execution from upload directories, and credential-based access patterns preceding malicious uploads. The platform's security skills would flag validation bypass attempts that check for substring presence rather than proper file extension validation, as well as post-upload behaviors indicative of webshell activity. By correlating these attack signals across the threat chain—from initial file upload through persistence and command execution—Casky enables practitioners to identify both active exploitation attempts and post-compromise indicators.

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Casky Risk Score

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EPSS Probability

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Shodan Exposure

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GreyNoise Signal

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419 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-9227.

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malware analysis · medium

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MITRE ATT&CK Techniques

TA0001 TA0043 TA0002

Investigate the attack patterns behind CVE-2026-9227

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