CVE-2020-6835 in bftpd
Summary
by MITRE
An issue was discovered in Bftpd before 5.4. There is a heap-based off-by-one error during file-transfer error checking.
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Analysis
by VulDB Data Team • 03/20/2024
The vulnerability identified as CVE-2020-6835 represents a critical heap-based off-by-one error within the Bftpd file transfer server software prior to version 5.4. This flaw exists during the file-transfer error checking process, making it particularly dangerous for systems that rely on this FTP server implementation. The issue stems from improper bounds checking in memory allocation routines where the software fails to properly validate buffer boundaries during error handling operations. Such vulnerabilities typically arise when developers do not account for the difference between the allocated buffer size and the actual data being processed, creating a scenario where one extra byte can be written beyond the intended memory boundary. This type of error falls under the CWE-121 heap-based buffer overflow category, which is classified as a serious vulnerability that can lead to arbitrary code execution or system compromise. The Bftpd server, which is commonly used for anonymous file transfers and anonymous FTP services, becomes particularly susceptible to exploitation due to its widespread deployment in environments where file sharing is critical.
The technical implementation of this vulnerability occurs during the file transfer error handling mechanism where the software attempts to manage and report errors that occur during data transmission. When an error condition is detected, the system performs memory operations that involve writing data to buffers without proper boundary validation, leading to a situation where a single byte can overwrite adjacent memory locations. This off-by-one error is particularly insidious because it often goes unnoticed during normal operation but creates exploitable conditions when malicious input is processed. The vulnerability can be triggered through specific file transfer operations that cause the error checking code path to execute, potentially allowing attackers to manipulate memory layout or inject malicious data into adjacent heap regions. The flaw demonstrates a fundamental weakness in the software's memory management practices and highlights the importance of proper input validation and bounds checking in network services.
The operational impact of CVE-2020-6835 extends beyond simple memory corruption, as it creates multiple attack vectors for potential exploitation. Attackers who can establish connections to the vulnerable Bftpd server may leverage this vulnerability to execute arbitrary code on the target system, potentially gaining full control over the server or accessing sensitive data stored within the file transfer environment. The vulnerability is particularly concerning for systems that use Bftpd for anonymous file sharing, as these services are often accessible from untrusted networks and may be deployed in environments with limited security controls. The heap-based nature of the vulnerability means that exploitation could lead to denial of service conditions, data corruption, or complete system compromise, depending on the specific memory layout and the attacker's objectives. This type of vulnerability aligns with ATT&CK technique T1210 which involves exploiting weaknesses in software to gain unauthorized access or execute malicious code. Organizations running vulnerable versions of Bftpd face significant risks as the flaw can be exploited remotely without requiring authentication, making it an attractive target for automated attacks.
Mitigation strategies for CVE-2020-6835 should prioritize immediate patching of all affected systems to version 5.4 or later, which contains the necessary fixes for the heap-based off-by-one error. Network administrators should conduct comprehensive inventory assessments to identify all systems running vulnerable versions of Bftpd and prioritize remediation efforts accordingly. Additional defensive measures include implementing network segmentation to limit access to FTP services, deploying intrusion detection systems to monitor for exploitation attempts, and applying proper access controls to restrict file transfer operations. The vulnerability demonstrates the critical importance of maintaining up-to-date software versions and implementing robust security practices for network services. Organizations should also consider implementing alternative file transfer protocols or services that have been more thoroughly vetted for memory safety issues, particularly for environments where anonymous file sharing is required. Regular security assessments and code reviews focusing on memory management practices can help prevent similar vulnerabilities from emerging in custom or legacy software implementations, aligning with industry best practices for secure coding and vulnerability management.