CVE-2017-6920 in Drupal
Summary
by MITRE
Drupal core 8 before versions 8.3.4 allows remote attackers to execute arbitrary code due to the PECL YAML parser not handling PHP objects safely during certain operations.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 10/20/2019
Drupal core version 8.3.4 and earlier contained a critical remote code execution vulnerability that stemmed from improper handling of PHP objects within the PECL YAML parser. This vulnerability exposed systems to arbitrary code execution attacks when Drupal processed YAML data containing serialized PHP objects. The flaw specifically occurred during the parsing operations where the YAML parser failed to sanitize or properly validate PHP objects, allowing malicious actors to inject and execute arbitrary code on affected systems. The vulnerability was classified as a remote code execution flaw with a CVSS score indicating high severity, making it particularly dangerous for web applications that relied on YAML parsing functionality. The root cause of this vulnerability aligns with CWE-502, which describes unsafe deserialization of untrusted data, and represents a classic example of how deserialization flaws can lead to remote code execution when objects are improperly handled during data parsing operations.
The technical exploitation of this vulnerability occurred when Drupal's YAML parsing functionality encountered maliciously crafted YAML input containing serialized PHP objects. Attackers could construct YAML data that, when processed by the PECL YAML parser, would trigger the deserialization of malicious PHP objects. This process allowed attackers to execute arbitrary code with the privileges of the web server, potentially leading to complete system compromise. The vulnerability was particularly concerning because it could be exploited through various attack vectors including user input fields, configuration files, or external data sources that Drupal might process through its YAML parsing mechanisms. The flaw demonstrated how third-party libraries integrated into web applications could introduce critical security risks when proper input validation and sanitization were not implemented.
The operational impact of CVE-2017-6920 was substantial across the Drupal ecosystem, affecting thousands of websites and applications that had not yet updated to the patched version. Organizations running vulnerable Drupal installations faced immediate risks including data breaches, unauthorized access, and potential complete system compromise. The vulnerability allowed attackers to establish persistent access to affected systems, making it particularly dangerous for environments handling sensitive data. Security teams had to urgently patch affected installations and monitor for signs of exploitation, as the vulnerability could be exploited automatically by malicious bots scanning for vulnerable targets. The incident highlighted the critical importance of keeping third-party libraries and components updated, as the vulnerability was not in Drupal core itself but in the PECL YAML parser that Drupal utilized for certain operations.
Mitigation strategies for this vulnerability required immediate patching of Drupal core to version 8.3.4 or later, which contained fixes addressing the unsafe YAML object handling. Organizations should have also implemented additional security measures including input validation for all YAML data sources, monitoring for suspicious file modifications, and network intrusion detection to identify exploitation attempts. The vulnerability demonstrated the importance of following security best practices such as the principle of least privilege, regular security updates, and comprehensive vulnerability assessment procedures. Security professionals recommended implementing web application firewalls to filter potentially malicious YAML input and conducting thorough security audits of all third-party components integrated into web applications. This vulnerability also emphasized the need for proper security training for developers regarding safe handling of serialized data and the importance of validating all external input before processing. The incident served as a reminder that even seemingly benign functionality like YAML parsing could become a critical attack surface when proper security controls were not in place, reinforcing the concepts outlined in the ATT&CK framework under the technique of deserialization of untrusted data.