CVE-2021-41269
📋 TL;DR
CVE-2021-41269 is a critical template injection vulnerability in cron-utils Java library that allows attackers to inject arbitrary Java Expression Language (EL) expressions through untrusted cron expressions. This leads to unauthenticated remote code execution (RCE) on affected systems. Only projects using the @Cron annotation to validate untrusted cron expressions are vulnerable.
💻 Affected Systems
- cron-utils Java library
📦 What is this software?
Cron Utils by Cron Utils Project
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise with attacker gaining full control over the affected server, allowing data theft, lateral movement, and persistent backdoor installation.
Likely Case
Remote code execution leading to application compromise, data exfiltration, and potential ransomware deployment.
If Mitigated
No impact if proper input validation and patching are implemented, or if @Cron annotation is not used with untrusted inputs.
🎯 Exploit Status
Exploitation requires the attacker to provide a malicious cron expression containing Java EL injection payloads to an endpoint using @Cron annotation.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 9.1.6
Vendor Advisory: https://github.com/jmrozanec/cron-utils/security/advisories/GHSA-p9m8-27x8-rg87
Restart Required: Yes
Instructions:
1. Update pom.xml or build.gradle to use cron-utils version 9.1.6 or higher. 2. Run dependency update command (mvn clean install or gradle build). 3. Restart the Java application. 4. Verify the updated version is in use.
🔧 Temporary Workarounds
Input validation and sanitization
allImplement strict input validation to reject cron expressions containing EL expressions or suspicious patterns before they reach @Cron annotation processing.
🧯 If You Can't Patch
- Disable or remove @Cron annotation usage with untrusted inputs
- Implement web application firewall (WAF) rules to block requests containing Java EL expression patterns
🔍 How to Verify
Check if Vulnerable:
Check if your Java application uses cron-utils version ≤9.1.2 and uses @Cron annotation with user-supplied cron expressions.Check Version:
mvn dependency:tree | grep cron-utils OR gradle dependencies | grep cron-utilsVerify Fix Applied:
Verify cron-utils version is ≥9.1.6 in dependency files and running application. Test that malicious cron expressions containing EL injection are properly rejected.📡 Detection & Monitoring
Log Indicators:
- Java EL expression parsing errors
- Unexpected cron expression patterns in logs
- Unusual process execution from Java application
Network Indicators:
- HTTP requests containing Java EL expression patterns to endpoints processing cron expressions
SIEM Query:
source="application_logs" AND ("EL expression" OR "cron-utils" OR "@Cron") AND (error OR exception)🔗 References
- https://github.com/jmrozanec/cron-utils/commit/cfd2880f80e62ea74b92fa83474c2aabdb9899da
- https://github.com/jmrozanec/cron-utils/commit/d6707503ec2f20947f79e38f861dba93b39df9da
- https://github.com/jmrozanec/cron-utils/issues/461
- https://github.com/jmrozanec/cron-utils/security/advisories/GHSA-p9m8-27x8-rg87
- https://github.com/jmrozanec/cron-utils/commit/cfd2880f80e62ea74b92fa83474c2aabdb9899da
- https://github.com/jmrozanec/cron-utils/commit/d6707503ec2f20947f79e38f861dba93b39df9da
- https://github.com/jmrozanec/cron-utils/issues/461
- https://github.com/jmrozanec/cron-utils/security/advisories/GHSA-p9m8-27x8-rg87
