CVE-2026-61453 in Grav
Summary
by MITRE • 07/15/2026
Grav v2.0.0 contains a cross-site scripting vulnerability (fixed in 2.0.1). The XSS blueprint validator (Security::detectXss()) runs on raw page content before Twig processing. When Twig content processing is enabled (twig_content.process_enabled: true), an attacker with page-write API permission can use Twig's string concatenation operator (~) to dynamically construct event handler names, dangerous tag names, or dangerous protocols at render time (e.g. {% set x = "on" ~ "error" %}). The validator sees only the harmless Twig expression and allows the content, but after Twig rendering the output (rendered via {{ page.content|raw }}) contains an active payload such as <img src=1 onerror=alert(1)>, executing arbitrary JavaScript in visitors' browsers.
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Analysis
by VulDB Data Team • 07/15/2026
The vulnerability in Grav v2.0.0 represents a sophisticated cross-site scripting flaw that exploits a fundamental mismatch between input validation and output rendering processes. This issue manifests through the Security::detectXss() function which operates on raw page content prior to Twig template processing, creating a window where malicious payloads can bypass initial security checks. The vulnerability specifically targets the blueprint validator's inability to properly analyze dynamically constructed content that emerges only after Twig processing completes. When Twig content processing is enabled through the configuration twig_content.process_enabled: true, the system creates an environment where attackers can leverage Twig's string concatenation operator to construct dangerous HTML attributes and event handlers at runtime.
The technical exploitation mechanism leverages Twig's dynamic capabilities to generate XSS payloads that appear benign during validation but become active upon rendering. An attacker with page-write API permissions can craft malicious content such as {% set x = "on" ~ "error" %} where the concatenation operator combines string literals to form dangerous JavaScript event handlers like onerror. The Security::detectXss() validator only examines the original Twig expression without understanding that it will be processed into executable HTML attributes, allowing the malicious content to pass validation. This represents a classic case of improper input sanitization where the validator operates at an inappropriate layer of abstraction, failing to account for the transformational nature of template engines.
The operational impact of this vulnerability extends beyond simple XSS execution to potentially compromise entire user sessions and enable more sophisticated attacks including session hijacking, credential theft, and data exfiltration. Visitors who encounter the rendered malicious content experience arbitrary JavaScript execution in their browsers, which can be used to steal cookies, redirect users to malicious sites, or perform actions on behalf of authenticated users. The vulnerability is particularly dangerous because it requires minimal privileges for exploitation - only page-write API permissions are needed, which many content management systems grant to editors and contributors. This makes the attack surface significantly larger than typical XSS vulnerabilities that require more elevated access levels.
The security implications align with CWE-79 Cross-site Scripting flaws and map directly to ATT&CK technique T1531 Credential Access through Web Session Hijacking. Mitigation strategies should focus on implementing proper input validation at the final rendered output level rather than relying solely on pre-processing validation, ensuring that all content is analyzed after transformation by template engines. The fix implemented in Grav v2.0.1 addresses this by strengthening the XSS detection mechanism to account for dynamic content construction that occurs during Twig processing, requiring the security validator to analyze content after it has been fully processed rather than before. Organizations should also implement Content Security Policy headers as additional defense-in-depth measures and consider restricting Twig processing capabilities where possible to limit the attack surface available to authenticated users with write permissions.