CVE-2024-21742: Apache James Mime4j Header Injection Flaw

CVE-2024-21742 Overview

CVE-2024-21742 is a header injection vulnerability affecting the Apache James MIME4J library. The vulnerability stems from improper input validation when using MIME4J DOM for composing messages. This flaw can be exploited by an attacker to inject unintended headers into MIME messages, potentially enabling various attack scenarios including email spoofing, security bypass, and manipulation of email routing behavior.

Critical Impact

Attackers can exploit improper input validation to inject arbitrary headers into MIME messages, potentially bypassing email security controls and enabling phishing or spoofing attacks.

Affected Products

• Apache James MIME4J (all versions prior to patched release)

Discovery Timeline

• 2024-02-27 - CVE-2024-21742 published to NVD
• 2025-05-06 - Last updated in NVD database

Technical Details for CVE-2024-21742

Vulnerability Analysis

This vulnerability is classified as CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component), commonly known as injection. The flaw exists within the MIME4J DOM API when composing email messages. The library fails to properly validate and sanitize user-supplied input that is incorporated into MIME message headers, allowing attackers to inject arbitrary header content by including special characters such as carriage return (CR) and line feed (LF) sequences.

When an application uses the vulnerable MIME4J DOM to construct email messages with attacker-controlled data, malicious input containing header delimiters can break out of the intended header context and inject additional headers. This type of attack is particularly dangerous in email systems because MIME headers control critical aspects of message processing, routing, and display.

Root Cause

The root cause of CVE-2024-21742 lies in insufficient input validation within the MIME4J DOM message composition functionality. The library does not adequately sanitize or reject input containing newline characters (CRLF sequences) when setting header values. In MIME and HTTP protocols, CRLF sequences serve as delimiters between headers, and failing to neutralize these characters in user-supplied input allows injection of additional headers.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by providing malicious input to an application that uses the MIME4J library to compose email messages. By crafting input containing CRLF sequences followed by additional header content, the attacker can inject arbitrary headers into the resulting MIME message.

For example, if an application allows users to specify a display name that gets incorporated into email headers, an attacker could provide a malformed display name containing embedded header injections. The injected headers could include additional recipients (BCC), modified sender information, or headers that affect email security controls. For technical details on the vulnerability mechanism, see the Apache Security Notification.

Detection Methods for CVE-2024-21742

Indicators of Compromise

• Anomalous email headers with unexpected CRLF sequences or multiple instances of typically single-occurrence headers
• Email logs showing messages with suspicious header patterns originating from applications using MIME4J
• Reports of spoofed or unexpected emails appearing to originate from legitimate systems using the affected library

Detection Strategies

• Implement application-level logging to capture the full headers of composed MIME messages for forensic analysis
• Monitor email gateway logs for messages with malformed or duplicate headers that may indicate injection attempts
• Review application dependencies to identify usage of vulnerable Apache James MIME4J versions
• Deploy network-level inspection to detect CRLF injection patterns in email traffic

Monitoring Recommendations

• Configure email security gateways to alert on messages with anomalous header structures
• Establish baseline header patterns for applications using MIME4J and alert on deviations
• Monitor for unexpected changes in email routing or delivery patterns that could indicate header manipulation

How to Mitigate CVE-2024-21742

Immediate Actions Required

• Identify all applications using Apache James MIME4J library for message composition
• Update to the latest patched version of Apache James MIME4J as specified in the vendor advisory
• Implement input validation at the application layer to sanitize user input before passing to MIME4J
• Review and audit code paths where user-controlled data is incorporated into email headers

Patch Information

Apache has released a security update to address this vulnerability. Organizations should consult the Apache Security Notification for specific patch version information and upgrade instructions. The patch implements proper input validation to prevent header injection through CRLF sequences.

Workarounds

• Implement strict input validation at the application layer to reject or sanitize any input containing CR or LF characters before using with MIME4J
• Use allowlist-based validation for header values, permitting only expected character sets
• Consider using alternative email composition libraries with built-in injection protections while awaiting patched deployment

# Example: Checking MIME4J version in Maven projects
mvn dependency:tree | grep mime4j

# Update pom.xml to use patched version (consult Apache advisory for specific version)
# Rebuild and redeploy applications after updating dependencies