CVE-2021-41269: Cron-utils Template Injection RCE Vulnerability

CVE-2021-41269 Overview

CVE-2021-41269 is a critical template injection vulnerability in cron-utils, a Java library used to define, parse, validate, and migrate cron expressions. The vulnerability allows attackers to inject arbitrary Java Expression Language (EL) expressions through untrusted cron input, resulting in unauthenticated Remote Code Execution (RCE). Applications using the @Cron annotation to validate user-supplied cron expressions are particularly susceptible to exploitation.

Critical Impact

Unauthenticated attackers can achieve Remote Code Execution by injecting malicious Java EL expressions through cron validation, potentially leading to complete system compromise.

Affected Products

• cron-utils versions up to 9.1.2
• Applications using the @Cron annotation for validation
• Java applications parsing untrusted cron expressions

Discovery Timeline

• 2021-11-15 - CVE CVE-2021-41269 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-41269

Vulnerability Analysis

This vulnerability stems from improper handling of error messages in cron-utils' validation logic. When parsing a malformed or specially crafted cron expression, the library would include the raw user-supplied input directly in constraint violation messages and exception strings. Because Java Bean Validation frameworks evaluate constraint violation messages as EL expressions, an attacker could embed malicious EL code within a cron expression that would be executed when the validation error message was processed.

The attack requires no authentication and can be exploited remotely over the network. Successful exploitation grants attackers the ability to execute arbitrary code within the context of the Java application, potentially leading to complete system compromise, data exfiltration, or lateral movement within the target environment.

Root Cause

The root cause lies in the unsafe construction of error messages that directly interpolate user-controlled input. In the CronValidator.java class, the e.getMessage() value (which contains the user's malicious cron expression) was passed directly to buildConstraintViolationWithTemplate(). Similarly, in CronParser.java, the raw expression was included in exception messages using String.format(). This pattern allowed Java EL expressions embedded in the input to be evaluated during error message processing.

Attack Vector

The vulnerability is exploited through network-accessible interfaces where applications accept user-supplied cron expressions for validation. An attacker crafts a malicious cron string containing embedded Java EL expressions. When the application attempts to validate this input using the @Cron annotation, the invalid expression triggers an error. The error handling code then constructs a violation message containing the attacker's payload, which the Bean Validation framework interprets and executes as an EL expression.

The following patches demonstrate the security fix applied to remediate this vulnerability:

Patch in CronValidator.java:

             return true;
         } catch (IllegalArgumentException e) {
             context.disableDefaultConstraintViolation();
-            context.buildConstraintViolationWithTemplate(e.getMessage()).addConstraintViolation();
+            context.buildConstraintViolationWithTemplate("Error parsing the Cron expression").addConstraintViolation();
             return false;
         }
     }

Source: GitHub Commit d6707503e

Patch in CronParser.java:

                 }
                 return new SingleCron(cronDefinition, results).validate();
             } catch (final IllegalArgumentException e) {
-                throw new IllegalArgumentException(String.format("Failed to parse '%s'. %s", expression, e.getMessage()), e);
+                throw new IllegalArgumentException(String.format("Failed to parse cron expression. %s", e.getMessage()), e);
             }
         }
     }

Source: GitHub Commit cfd2880f

The fix removes user-controlled input from error messages, replacing dynamic content with static strings that cannot be interpreted as EL expressions.

Detection Methods for CVE-2021-41269

Indicators of Compromise

• Unusual cron expression validation errors containing EL syntax such as ${ or #{ patterns
• Application logs showing exception messages with Java EL expression fragments
• Unexpected process spawning or network connections originating from Java application processes
• Anomalous system commands or file operations executed under the Java application's user context

Detection Strategies

• Implement application-level logging to capture and analyze all cron expression validation attempts
• Monitor for suspicious patterns in web application firewall (WAF) logs, specifically requests containing ${...} or #{...} syntax in cron-related parameters
• Deploy runtime application self-protection (RASP) solutions to detect and block EL injection attempts
• Use software composition analysis (SCA) tools to identify vulnerable cron-utils versions in your codebase

Monitoring Recommendations

• Configure alerts for Java process behaviors indicative of RCE, such as spawning shell processes or establishing outbound connections
• Enable detailed logging in Bean Validation frameworks to capture validation failure details
• Implement centralized log aggregation to correlate suspicious activity across application instances
• Regularly audit dependency versions using tools like OWASP Dependency-Check or Snyk

How to Mitigate CVE-2021-41269

Immediate Actions Required

• Upgrade cron-utils to version 9.1.6 or later immediately across all affected applications
• Audit application code for any usage of the @Cron annotation that validates external or user-supplied input
• Review application logs for evidence of exploitation attempts prior to patching
• Consider implementing input validation at the application boundary to reject cron expressions containing EL syntax

Patch Information

The vulnerability has been addressed in cron-utils version 9.1.6. The fix replaces user-controlled content in error messages with static strings, preventing EL expression injection. Security patches are available in the following commits:

• GitHub Commit cfd2880f - Fix in CronParser.java
• GitHub Commit d6707503e - Fix in CronValidator.java

For additional details, refer to the GitHub Security Advisory GHSA-p9m8-27x8-rg87.

Workarounds

• There are no known workarounds for this vulnerability; upgrading to version 9.1.6 or later is the only effective remediation
• As a temporary measure, avoid using the @Cron annotation to validate any untrusted or user-supplied cron expressions
• Implement strict input validation to sanitize cron expressions before processing, blocking patterns that contain ${ or #{ sequences

# Maven dependency update example
# Update pom.xml to use patched version:
# <dependency>
#     <groupId>com.cronutils</groupId>
#     <artifactId>cron-utils</artifactId>
#     <version>9.1.6</version>
# </dependency>

# Verify current cron-utils version in your project
mvn dependency:tree | grep cron-utils