Grav Flex Objects Server-Side Template Injection via Frontmatter

The bundled Grav Flex Objects plugin (getgrav/grav-plugin-flex-objects) before 1.4.0 contains a stored server-side template injection vulnerability. When rendering dynamic collection or object titles, the plugin passes user-controlled frontmatter values (page.header.flex.collection.title or page.header.flex.object.title) to Twig's template_from_string(), causing them to be evaluated as Twig code rather than treated as text. This path bypasses Grav's Security::cleanDangerousTwig() sanitization. An attacker who can control the title frontmatter of a publicly reachable Flex Objects page can achieve arbitrary Twig execution and escalate to remote command execution via access to internal Grav services such as the scheduler.

Casky was already ahead

This CVE exploits attack patterns that Casky's 322matched 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

CVE-2026-58655 is a stored server-side template injection (SSTI) vulnerability in the Grav Flex Objects plugin that allows attackers to execute arbitrary code through malicious frontmatter in page headers. The vulnerability exists because user-controlled values in page.header.flex.collection.title and page.header.flex.object.title are passed directly to Twig's template_from_string() function, bypassing Grav's Security::cleanDangerousTwig() sanitization layer. With a CVSS score of 8.8, this high-severity flaw affects any Grav installation using the bundled Flex Objects plugin before version 1.4.0. Attackers can inject Twig template syntax into page metadata, which gets evaluated as executable code during rendering, leading to complete server compromise.

Casky's 322 matched skills leverage Claude AI's extended reasoning to detect the attack patterns behind this vulnerability by analyzing both TA0001 (Initial Access) and TA0043 (Reconnaissance) techniques. Practitioners using Casky would observe detection signals including: suspicious Twig function calls in page rendering contexts, particularly template_from_string() invocations with user-supplied input; abnormal frontmatter containing template syntax like {{ }} or {%...%} tags; CWE-94 (Code Injection) patterns where untrusted data flows directly into template engines; and exploitation attempts leveraging page metadata modification to achieve remote code execution. The skills enable security teams to identify both reconnaissance activity (scanning for vulnerable Flex Objects versions) and initial access attempts (uploading or modifying pages with malicious frontmatter) before exploitation occurs.

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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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Composite risk scoring from EPSS, CISA KEV, Shodan, and GreyNoise — 21 security APIs correlated into a single Casky Risk Score. Coming in Casky Pro. Join early access →

322 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-58655.

Abuse of Public-Facing API: Mobile API Abuse

mobile security · low

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abusing-dpapi-for-credential-access

red teaming · high

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abusing-shadow-credentials-for-privesc

red teaming · high

Run

MITRE ATT&CK Techniques

TA0001 TA0043

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