CVE-2023-6918 in libssh
Summary
by MITRE • 12/19/2023
A flaw was found in the libssh implements abstract layer for message digest (MD) operations implemented by different supported crypto backends. The return values from these were not properly checked, which could cause low-memory situations failures, NULL dereferences, crashes, or usage of the uninitialized memory as an input for the KDF. In this case, non-matching keys will result in decryption/integrity failures, terminating the connection.
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Analysis
by VulDB Data Team • 03/17/2025
The vulnerability identified as CVE-2023-6918 resides within the libssh library's implementation of message digest operations through its abstract layer that interfaces with various crypto backends. This flaw manifests in the improper handling of return values from underlying cryptographic operations, creating a cascade of potential security and stability issues that affect the entire cryptographic processing pipeline. The issue stems from insufficient validation of function return codes during digest computation, which allows for the propagation of error states into critical cryptographic operations.
The technical implementation of this vulnerability involves the abstract message digest layer that serves as an interface between libssh's high-level cryptographic APIs and the underlying crypto backend implementations. When these backend functions fail to return expected success codes, the abstraction layer fails to properly validate these return values before proceeding with subsequent operations. This validation failure creates multiple attack surfaces where memory management errors can occur, including low-memory condition failures that may lead to system instability, NULL pointer dereferences that cause application crashes, and the utilization of uninitialized memory segments as inputs for key derivation functions. The specific context of this vulnerability becomes critical when considering that KDF operations require properly initialized cryptographic inputs to generate valid keys for decryption and integrity verification processes.
The operational impact of CVE-2023-6918 extends beyond simple application crashes to encompass potential cryptographic security weaknesses that could be exploited by adversaries. When non-matching keys result from the flawed KDF processing, the system experiences decryption and integrity verification failures that terminate connections, effectively creating a denial of service condition for legitimate users. This vulnerability particularly affects systems relying on libssh for secure communication protocols such as ssh, sftp, and scp implementations. The potential for uninitialized memory usage as KDF inputs creates a scenario where attackers could potentially manipulate the cryptographic processing flow to achieve unauthorized access or cause service disruption. From a cybersecurity perspective, this vulnerability aligns with CWE-248, which addresses improper exception handling in software implementations, and represents a significant concern for network infrastructure security.
Mitigation strategies for CVE-2023-6918 should prioritize immediate patching of affected libssh installations, as this addresses the root cause of the improper return value validation. Organizations should implement comprehensive monitoring for connection termination patterns that could indicate exploitation attempts, particularly focusing on authentication failures and decryption errors that align with the vulnerability's behavior. The implementation of proper error handling procedures within cryptographic libraries, including validation of return codes from all backend operations, serves as a fundamental defensive measure against similar issues. Additionally, system administrators should consider implementing network segmentation and access controls to limit exposure of systems running vulnerable versions of libssh, while also establishing incident response procedures specifically designed to handle cryptographic error conditions that could indicate exploitation of this vulnerability.