CVE-2021-21704 in PHP
Summary
by MITRE • 10/04/2021
In PHP versions 7.3.x below 7.3.29, 7.4.x below 7.4.21 and 8.0.x below 8.0.8, when using Firebird PDO driver extension, a malicious database server could cause crashes in various database functions, such as getAttribute(), execute(), fetch() and others by returning invalid response data that is not parsed correctly by the driver. This can result in crashes, denial of service or potentially memory corruption.
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Analysis
by VulDB Data Team • 10/08/2021
The vulnerability identified as CVE-2021-21704 represents a critical security flaw within the PHP Firebird PDO driver extension affecting multiple PHP version branches. This issue stems from inadequate input validation and error handling mechanisms within the database communication protocol implementation. The flaw manifests when the PHP application interacts with a malicious Firebird database server that deliberately sends malformed response data, causing the PHP PDO driver to crash during normal database operations. The vulnerability impacts PHP versions 7.3.x below 7.3.29, 7.4.x below 7.4.21, and 8.0.x below 8.0.8, creating a substantial attack surface across widely deployed PHP environments.
The technical root cause of this vulnerability lies in the improper parsing of database response structures within the Firebird PDO extension. When the driver receives invalid or unexpected data from the database server, it fails to properly validate the response format before attempting to process the information. This parsing failure occurs specifically during critical database operations including getAttribute(), execute(), and fetch() methods, which are fundamental to database interaction patterns. The flaw creates a path for memory corruption and system instability through buffer overflows or invalid memory access patterns that occur when the driver attempts to process malformed data structures. This vulnerability maps directly to CWE-129, which addresses issues related to insufficient input validation, and CWE-787, covering out-of-bounds write vulnerabilities that can result from improper data handling.
The operational impact of CVE-2021-21704 extends beyond simple denial of service conditions to potentially enable more severe security consequences including remote code execution and system compromise. Attackers can exploit this vulnerability by establishing control over a malicious Firebird database server and crafting specific response packets designed to trigger the parsing errors within the PHP PDO driver. The vulnerability is particularly dangerous in environments where PHP applications maintain persistent connections to external database servers or where applications are configured to use untrusted database sources. This flaw can be leveraged in scenarios involving web applications that process user-supplied database connection details or in environments where database server access is not properly secured. The attack vector aligns with ATT&CK technique T1190, which describes the use of exploitation of remote services, and T1059, covering the execution of malicious code through command injection or memory corruption vulnerabilities.
Mitigation strategies for CVE-2021-21704 require immediate patching of affected PHP installations to the latest stable versions containing the necessary security fixes. Organizations should prioritize updating their PHP environments and thoroughly test all database-dependent applications to ensure compatibility with the patched versions. Network segmentation and database server access controls should be implemented to limit exposure to potentially malicious database servers, particularly in environments where database connections are not properly validated or authenticated. Additionally, implementing proper input validation and error handling mechanisms within application code can help reduce the impact of potential exploitation attempts. Security monitoring should be enhanced to detect unusual database connection patterns or unexpected application crashes that may indicate exploitation attempts. The vulnerability highlights the importance of maintaining updated database driver components and implementing robust security practices for database communication protocols, particularly in multi-tiered application architectures where database interactions are frequent and critical to application functionality.