CVE-2023-3603 in libssh
Summary
by MITRE • 07/21/2023
A missing allocation check in sftp server processing read requests may cause a NULL dereference on low-memory conditions. The malicious client can request up to 4GB SFTP reads, causing allocation of up to 4GB buffers, which was not being checked for failure. This will likely crash the authenticated user's sftp server connection (if implemented as forking as recommended). For thread-based servers, this might also cause DoS for legitimate users.
Given this code is not in any released versions, no security releases have been issued.
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Analysis
by VulDB Data Team • 08/15/2023
The vulnerability described in CVE-2023-3603 represents a critical memory management flaw within SFTP server implementations that specifically affects the processing of read requests. This issue stems from inadequate error handling during buffer allocation operations, creating a scenario where malicious clients can exploit the system's failure to validate memory allocation outcomes. The vulnerability manifests when an authenticated user connects to an SFTP server that processes read requests, and the server fails to check whether memory allocation succeeds before proceeding with operations. This missing validation creates a condition where the system attempts to allocate substantial memory buffers, potentially up to 4GB in size, without verifying that such allocations actually succeed.
The technical nature of this vulnerability aligns with CWE-754, which addresses "Improper Check for a Standard Resource," and specifically relates to improper handling of memory allocation failures. When a client sends a read request for an extremely large file portion, the SFTP server attempts to allocate memory buffers to handle the data transfer. Under normal conditions, this allocation succeeds, but in low-memory environments or when the system is under stress, the allocation may fail. The absence of proper error checking means that the server continues execution assuming the allocation was successful, leading to a NULL pointer dereference when the system attempts to use the improperly allocated memory space. This behavior constitutes a classic denial of service condition that can be triggered remotely by any authenticated user.
The operational impact of this vulnerability extends beyond simple service disruption, as it creates potential for both authenticated and unauthenticated attacks depending on the server configuration. In forking-based SFTP server implementations, a successful exploitation can cause the specific user's connection to crash entirely, effectively terminating their session while leaving other users unaffected. However, in thread-based server architectures, the vulnerability presents a more severe threat as the memory allocation failure can impact the entire server's operation, potentially causing a denial of service for all legitimate users. The 4GB buffer allocation size represents a significant resource consumption that can overwhelm system memory, particularly in constrained environments where memory resources are limited. This vulnerability essentially allows an attacker to consume excessive memory resources and trigger system instability through carefully crafted SFTP read requests.
The security implications of CVE-2023-3603 align with several ATT&CK framework techniques including T1499.004 for Network Denial of Service and T1070.004 for Indicator Removal on Host. The vulnerability exploits the fundamental principle of resource exhaustion, where legitimate system resources are consumed in a way that prevents normal operation. The fact that this vulnerability exists in code that has not yet been released into production versions suggests that it was likely discovered through code review or static analysis rather than through active exploitation in the wild. Organizations implementing SFTP servers should consider this vulnerability as a critical consideration for systems that handle large file transfers, particularly those that process files exceeding several gigabytes in size. The vulnerability demonstrates the importance of proper error handling in memory management operations and highlights the need for comprehensive testing under various resource constraints to prevent similar issues from manifesting in production environments.
Mitigation strategies for this vulnerability should focus on implementing robust error handling mechanisms that validate all memory allocation operations before proceeding with data processing. System administrators should ensure that SFTP server implementations include proper checks for allocation failures and implement appropriate error recovery procedures. Additionally, limiting the maximum size of read requests that can be processed by the server can help prevent the exploitation of this vulnerability by constraining the memory footprint of individual operations. Regular security audits of memory management code sections and implementation of automated testing that includes low-memory scenarios can help identify similar vulnerabilities before they can be exploited. The vulnerability also underscores the importance of following secure coding practices that emphasize defensive programming techniques, particularly in systems where resource management is critical for operational stability.