CVE-2026-75827 in Grav
Summary
by MITRE • 08/18/2026
Grav before 2.0.15 contains an arbitrary file write vulnerability in the Blueprint dynamic-data bare-function validation that uses an incomplete denylist instead of a positive allowlist. Attackers with page-edit or blueprint-config access can invoke the error_log function through a data directive to append PHP payloads to web-accessible files, achieving remote code execution.
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Analysis
by VulDB Data Team • 08/18/2026
The vulnerability identified in Grav versions prior to 2.0.15 represents a critical security flaw rooted in improper input validation within the Blueprint dynamic-data bare-function mechanism. This component is designed to handle data directives that allow for flexible configuration and content manipulation, but it relies on an incomplete denylist approach rather than a robust positive allowlist strategy. In software security, relying on blocklists of known bad inputs is inherently risky because attackers can often discover novel bypass techniques or utilize edge cases not covered by the predefined list. This specific implementation flaw allows malicious actors to inject arbitrary function calls into the processing pipeline, specifically targeting PHP's error_log function in a manner that facilitates file system manipulation rather than simple log injection.
The technical exploitation of this vulnerability requires an attacker to possess at least page-edit or blueprint-config access privileges within the Grav application environment. Once these permissions are established, the attacker can craft specific data directives that bypass the insufficient validation checks. By invoking the error_log function through a manipulated directive, the attacker gains control over where log messages are written. Instead of writing to standard system logs which might be restricted by file permissions or rotated regularly, the vulnerability allows for appending PHP code directly into web-accessible files on the server. This technique effectively turns legitimate logging mechanisms into vectors for persistent backdoor installation, as the injected payload becomes part of a script that can be executed via HTTP requests.
The operational impact of this flaw is severe, leading to remote code execution (RCE) with the privileges of the web server process. Once PHP payloads are successfully appended and included or executed through subsequent interactions with the compromised files, an attacker gains full control over the underlying system. This level of access enables a wide range of malicious activities including data exfiltration, installation of additional malware such as cryptominers or ransomware, lateral movement within the network, and complete compromise of any sensitive data stored on or accessible from the server. The persistence achieved through file modification makes detection difficult without specialized monitoring tools that track changes to executable files in web directories.
From a classification perspective, this vulnerability aligns with CWE-94 Improper Control of Generation of Code (Code Injection) due to the injection of malicious PHP code into existing scripts. It also relates closely to CWE-732 Incorrect Permission Assignment for Critical Resource because the exploit leverages file write permissions that should be more strictly controlled or validated against a whitelist of allowed functions and targets. In terms of adversary behavior, this attack vector corresponds to ATT&CK technique T1059 Command and Scripting Interpreter, specifically through PHP scripts, as well as T1105 Ingress Tool Transfer if the attacker uses the RCE capability to download additional tools for further exploitation.
Mitigation strategies must prioritize immediate upgrading of Grav to version 2.0.15 or later where these validation checks have been corrected. For environments that cannot be immediately patched, administrators should enforce strict file permission settings on web-accessible directories to prevent write access by the web server user unless absolutely necessary for application functionality. Implementing a Web Application Firewall (WAF) with rules designed to detect attempts to invoke dangerous functions like error_log in unexpected contexts can provide an additional layer of defense. Furthermore, adopting a principle of least privilege ensures that users are granted only the minimum permissions required, reducing the attack surface available to potential intruders who might compromise lower-privileged accounts. Regular auditing of file integrity and monitoring for unauthorized modifications to PHP files in web roots is also recommended to detect any successful exploitation attempts early.