CVE-2021-27850: Critical Vulnerability in Apache Tapestry

A critical unauthenticated remote code execution vulnerability was found in all recent versions of Apache Tapestry. The affected versions include 5.4.5, 5.5.0, 5.6.2, and 5.7.0. This vulnerability allows attackers to bypass a previous fix (CVE-2019-0195) that was intended to prevent unauthorized access to certain class files.

The vulnerability stems from an insufficient blacklist filter that fails to properly validate URLs, allowing for a crafted asset file URL to be exploited. Attackers can append a slash to the URL, bypassing the filter and downloading sensitive files such as the HMAC secret key contained in `AppModule.class`. This key can then be used to sign serialized Java objects, potentially leading to remote code execution.

Organizations should prioritize patching immediately. The recommended upgrades are to versions 5.6.2 or later for 5.4.0 to 5.6.1, and to 5.7.1 or later for 5.7.0 to mitigate this vulnerability.

The vulnerability has a CVSS score of 9.8, indicating its critical severity, and is classified as a network attack with low complexity and no required privileges. It poses a significant risk to organizations utilizing Apache Tapestry, making it essential for security teams to take immediate action.

In addition to patching, organizations should monitor for any signs of exploitation and review their security policies regarding file access and URL validation.

For further information on best practices for vulnerability management, organizations may refer to the vulnerability management program design.

Vulnerability Details

The official description of this vulnerability states that it is a critical unauthenticated remote code execution vulnerability found in all recent versions of Apache Tapestry. The vulnerability allows attackers to bypass a fix implemented for CVE-2019-0195, thus compromising the security of the system.

The CVSS score for this vulnerability is 9.8, indicating a critical severity level. This high score reflects the potential impact on confidentiality, integrity, and availability as a result of successful exploitation.

The affected products include Apache Tapestry, specifically versions 5.4.5, 5.5.0, 5.6.2, and 5.7.0. The vulnerability was published on April 15, 2021, and has been classified under multiple CWE identifiers, including CWE-200 and CWE-502.

Technical Analysis

The root cause of this vulnerability lies in the improper implementation of a blacklist filter intended to prevent access to sensitive files. While the fix for CVE-2019-0195 involved blocking URLs ending with certain file extensions, attackers can exploit the system by appending a slash to the URL, effectively bypassing this filter.

The attack vector for this vulnerability is network-based, requiring no authentication or user interaction, making it particularly dangerous. Attack complexity is low, allowing attackers to exploit this vulnerability with minimal effort.

The impact of this vulnerability includes high confidentiality, integrity, and availability risks. Attackers may gain unauthorized access to sensitive content, modify data, or disrupt service availability.

Risk & Impact Analysis

The risk to organizations includes potential unauthorized access to sensitive information and the ability for attackers to execute arbitrary code remotely. The implications of such exploitation can be severe, leading to data breaches, loss of customer trust, and significant financial repercussions.

Organizations should assess their exposure and take immediate steps to patch affected versions of Apache Tapestry. Given the critical nature of this vulnerability, it should be treated with the highest urgency.

The overall blast radius of this vulnerability is extensive due to the wide range of affected versions. It is crucial for organizations to understand the urgency based on the CVSS score and the potential impact described.

Affected Versions

The versions of Apache Tapestry affected by this vulnerability are:

• 5.4.5 • 5.5.0 • 5.6.2 • 5.7.0 Organizations using any of these versions should upgrade immediately to mitigate the risk.

Mitigation & Remediation

To remediate this vulnerability, organizations should take the following actions:

• Upgrade to Apache Tapestry 5.6.2 or later for versions 5.4.0 to 5.6.1. • Upgrade to Apache Tapestry 5.7.1 or later for version 5.7.0. • Review URL access configurations to ensure proper validation and filtering. • Implement monitoring for unusual access patterns that may indicate attempts to exploit this vulnerability.

For further insights on effective security strategies, organizations may consider engaging in penetration testing to assess their security posture.

Detection Guidance

Organizations should monitor for the following indicators to detect potential exploitation of this vulnerability:

• Unusual HTTP requests to asset files, particularly those ending with `.class`. • Access attempts with appended slashes or unusual URL patterns. • Logs indicating the retrieval of class files or other sensitive resources.

AppSecure Threat Intelligence Insight

The long-term significance of this vulnerability underscores the need for robust URL validation mechanisms within web applications. As attackers continuously adapt, security teams must learn from incidents like this to enhance their defenses.

This incident highlights a broader trend of vulnerabilities arising from insufficient input validation and the importance of comprehensive testing during development and after deployment.

To strengthen defenses against similar vulnerabilities, organizations should implement continuous security assessments and ensure that security practices evolve with emerging threats. For further reading on security assessment strategies, organizations may refer to the penetration testing methodology to maintain a proactive security posture.

Finally, organizations should consider reviewing their vulnerability management program to ensure they are equipped to respond effectively to future threats.