CVE-2025-66303: Getgrav Grav DoS Vulnerability

CVE-2025-66303 Overview

CVE-2025-66303 is a Denial of Service (DoS) vulnerability affecting Grav, a popular file-based Web platform. The vulnerability stems from improper input validation in the handling of scheduled_at parameters used for cron expressions. When an attacker with administrative privileges submits a malicious input (such as a single quote character) through the scheduled_at parameter, the application fails to sanitize this input properly, resulting in a corrupted cron expression that renders the admin panel non-functional.

This vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption) and carries a CVSS score of 4.9 (Medium severity) with vector string CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H. While high privileges are required to exploit this vulnerability, the impact on availability is significant as recovery requires manual server access to modify the backup.yaml file.

Critical Impact

Successful exploitation renders the Grav admin panel completely non-functional, disrupting administrative operations. Recovery requires direct server access to manually correct corrupted cron expressions in the backup.yaml configuration file.

Affected Products

• Grav CMS versions prior to 1.8.0-beta.27
• Grav CMS 1.8.0-beta1 through 1.8.0-beta26
• All stable Grav CMS versions before the fix

Discovery Timeline

• 2025-12-01 - CVE-2025-66303 published to NVD
• 2025-12-03 - Last updated in NVD database

Technical Details for CVE-2025-66303

Vulnerability Analysis

The vulnerability exists in Grav's scheduler component, specifically within the IntervalTrait.php file that handles cron expression parsing. The at() method accepts a cron expression string and passes it directly to CronExpression::factory() without proper validation or error handling. When an invalid or malicious expression is provided, the resulting exception is not caught, leading to application instability.

The CVSS vector CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H indicates:

• Network-accessible attack vector - exploitable remotely
• Low attack complexity - easy to execute
• High privileges required - admin access needed
• No user interaction - works without victim action
• High availability impact - complete denial of service

The EPSS (Exploit Prediction Scoring System) data shows a probability of 0.045% with a percentile of 13.91%, indicating relatively low likelihood of active exploitation in the wild.

Root Cause

The root cause is insufficient input validation in the scheduler's cron expression handling. The vulnerable code directly instantiates a CronExpression object from user-supplied input without:

1. Validating the input format before processing
2. Implementing try-catch error handling around the factory method
3. Sanitizing special characters that could corrupt the expression

When invalid input is stored in backup.yaml, subsequent attempts to load the admin panel fail due to the corrupted cron configuration.

Attack Vector

An authenticated attacker with administrative privileges can exploit this vulnerability by:

1. Accessing the Grav admin panel's backup scheduling functionality
2. Injecting a malicious value (e.g., a single quote ') into the scheduled_at parameter
3. Submitting the form to save the corrupted cron expression to backup.yaml
4. The admin panel becomes inaccessible due to parsing failures

The following patch demonstrates the security fix implemented in version 1.8.0-beta.27:

{
    /**
     * Set the Job execution time.
     *
     * @param  string  $expression
     * @return self
     */
    public function at($expression)
    {
        $this->at = $expression;
        try {
            $this->executionTime = CronExpression::factory($expression);
        } catch (InvalidArgumentException $e) {
            // Invalid cron expression - set to null to prevent DoS
            $this->executionTime = null;
        }

        return $this;
    }

Source: https://github.com/getgrav/grav/commit/9d11094e4133f059688fad1e00dbe96fb6e3ead7

The fix wraps the CronExpression::factory() call in a try-catch block, gracefully handling invalid expressions by setting executionTime to null rather than allowing the exception to propagate and crash the application.

Detection Methods for CVE-2025-66303

Indicators of Compromise

• Unexplained admin panel unavailability or HTTP 500 errors
• Malformed or corrupted entries in user/config/backups.yaml containing invalid cron syntax
• Application error logs showing InvalidArgumentException related to cron expression parsing
• Unexpected special characters (quotes, semicolons) in scheduled backup configurations

Detection Strategies

Organizations can detect potential exploitation attempts through:

Log Analysis:

• Monitor web server logs for POST requests to backup configuration endpoints with unusual characters
• Review PHP error logs for CronExpression parsing failures
• Track administrative actions that modify scheduler settings

File Integrity Monitoring:

• Implement monitoring on user/config/backups.yaml for unexpected modifications
• Alert on configuration files containing malformed cron expressions
• Use checksum validation to detect unauthorized changes

Application Monitoring:

• Set up health checks for admin panel availability
• Monitor for sudden increases in HTTP 500 responses from admin routes
• Track failed authentication attempts followed by configuration changes

Monitoring Recommendations

Deploy comprehensive monitoring for Grav CMS installations:

1. Real-time alerting on admin panel availability drops
2. Configuration auditing for all scheduler-related parameter changes
3. Input validation logging to capture rejected or suspicious inputs
4. Administrative session tracking to correlate privilege abuse with configuration modifications

SentinelOne Singularity platform can provide behavioral detection for web application attacks, monitoring for anomalous patterns in admin panel access and file system modifications that may indicate exploitation attempts.

How to Mitigate CVE-2025-66303

Immediate Actions Required

• Update Grav CMS to version 1.8.0-beta.27 or later immediately
• Review user/config/backups.yaml for any corrupted cron expressions
• Audit administrative access logs for suspicious backup configuration changes
• Implement Web Application Firewall (WAF) rules to filter malicious scheduler inputs
• Restrict admin panel access to trusted IP addresses or VPN-only access

Patch Information

The vulnerability is fixed in Grav CMS version 1.8.0-beta.27. The security patch is available at:

• Commit:9d11094e4133f059688fad1e00dbe96fb6e3ead7
• Repository:https://github.com/getgrav/grav
• Advisory:https://github.com/getgrav/grav/security/advisories/GHSA-x62q-p736-3997

The patch implements proper exception handling around the cron expression factory method, preventing malformed input from causing application-wide failures. Update using Grav's built-in update mechanism or by manually applying the latest release.

Workarounds

If immediate patching is not possible, implement these temporary mitigations:

# Manual recovery if admin panel is already affected:
# 1. SSH into the server hosting Grav
# 2. Navigate to Grav's user config directory
cd /path/to/grav/user/config/

# 3. Backup and edit the backups.yaml file
cp backups.yaml backups.yaml.bak

# 4. Remove or correct corrupted cron expressions
# Look for lines with 'scheduled_at' containing invalid characters
nano backups.yaml

# 5. Set a valid cron expression or disable scheduling
# Example valid cron: "0 3 * * *" (daily at 3 AM)
# Or remove the scheduled_at line entirely

# 6. Clear Grav's cache
cd /path/to/grav
bin/grav cache

Additional protective measures:

• Restrict administrative user accounts to only trusted personnel
• Enable two-factor authentication for all admin accounts
• Consider placing the admin panel behind additional authentication layers
• Regularly backup configuration files to enable quick recovery