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Grav CMS CVE-2026-75834

| EUVDEUVD-2026-60598 MEDIUM
Cross-site Scripting (XSS) (CWE-79)
2026-08-18 VulnCheck GHSA-993v-76jg-67xr
PHP XSS Grav
5.1
CVSS 4.0 · Vendor: VulnCheck
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Severity by source

Vendor (VulnCheck) PRIMARY
5.1 MEDIUM
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
vuln.today AI
5.4 MEDIUM

Requires low-privilege editor account (PR:L), victim must view the page (UI:R), and XSS executes in victim browser representing a scope change (S:C); no server-side confidentiality or availability impact.

3.1 AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N
4.0 AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

Primary rating from Vendor (VulnCheck).

CVSS VectorVendor: VulnCheck

CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
P
Scope
X

Lifecycle Timeline

2
Analysis Generated
Aug 18, 2026 - 12:03 vuln.today
Patch available
Aug 18, 2026 - 12:02 EUVD

DescriptionCVE.org

Grav before 2.0.14 contains a stored cross-site scripting vulnerability in the Security::detectXss() function (system/src/Grav/Common/Security.php). All XSS detection patterns use the PCRE /u (UTF-8) modifier, so a single invalid UTF-8 byte anywhere in page content causes preg_match() to return false for every pattern, silently bypassing the save-time XSS safety gate (Validation::checkSafety()). An authenticated attacker with page-edit permissions (without the security.xss_whitelist privilege) can store malicious JavaScript that executes in the browser of a visitor who views the affected page.

AnalysisAI

Stored XSS in Grav CMS before 2.0.14 allows authenticated page editors to bypass the save-time XSS detection gate by embedding a single invalid UTF-8 byte in page content, causing all PCRE /u-flagged patterns in Security::detectXss() to silently return false rather than flagging the payload. Any visitor who subsequently loads the affected page executes attacker-controlled JavaScript in their browser context. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

Recon
Obtain editor-level Grav credentials
Delivery
Craft page with invalid UTF-8 byte and XSS payload
Exploit
Submit page via web editor
Install
preg_match() returns false under PCRE /u mode
C2
Malicious script bypasses safety gate and persists on disk
Execute
Victim navigates to affected page
Impact
Stored JavaScript executes in victim browser
Sign in to reveal

Vulnerability AssessmentAI

Exploitation Exploitation requires an authenticated Grav user account with page-edit permissions who does not hold the security.xss_whitelist privilege - the bypass specifically targets the check applied to non-whitelisted editors. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The CVSS 4.0 score of 5.1 (Medium) accurately reflects the access prerequisites - PR:L (authenticated editor) and UI:P (passive victim interaction) - with downstream impact limited to the subsequent system (SC:L/SI:L), meaning the server itself is not directly compromised. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario An authenticated editor crafts a Grav page whose content begins with a single invalid UTF-8 byte (e.g., 0x80) followed by a standard script tag payload such as a cookie-exfiltration script. When saved, preg_match() returns false for every XSS pattern under /u mode, so checkSafety() passes the content without sanitization and the page is written to disk. …
Remediation Upgrade Grav to version 2.0.14 or later, which is the vendor-confirmed fix per the GitHub security advisory at https://github.com/getgrav/grav/security/advisories/GHSA-q2j8-x8hf-63ch; this is the only confirmed remediation. … Detailed patch versions, workarounds, and compensating controls in full report.
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CVE-2026-75834 vulnerability details – vuln.today
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