CVE-2026-66034 in libssh2
Summary
by MITRE • 07/24/2026
libssh2 through 1.11.1, fixed in commit a13bb6c, contains a missing bounds check vulnerability that allows a malicious SSH server to trigger an arbitrary-length heap out-of-bounds read and a free of an uninitialized pointer via the publickey subsystem. In libssh2_publickey_list_fetch(), the version 1 response parser reads a server-controlled comment_len value and advances the parse pointer without verifying sufficient bytes remain in the buffer, causing the out-of-bounds read to leak heap pointers from adjacent allocations defeating ASLR, followed by heap allocator state corruption when the error cleanup path frees an uninitialized pointer from a non-zeroed realloc() region.
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Analysis
by VulDB Data Team • 07/25/2026
The vulnerability in libssh2 versions through 1.11.1 represents a critical security flaw that demonstrates the dangerous consequences of inadequate input validation in cryptographic libraries. This issue specifically affects the publickey subsystem functionality where the library processes server responses during public key operations. The vulnerability stems from a missing bounds check in the version 1 response parser implementation within the libssh2_publickey_list_fetch() function, creating a scenario where malicious SSH servers can exploit the protocol parsing logic to execute arbitrary code through memory corruption techniques.
The technical flaw manifests as an out-of-bounds read condition that occurs when processing the comment_len field from server responses. The parser reads this server-controlled value without first validating that sufficient data remains in the buffer for the subsequent operations, allowing attackers to manipulate the parsing pointer beyond valid memory boundaries. This particular vulnerability falls under CWE-129, which addresses improper validation of array indices, and more specifically relates to CWE-787, which covers out-of-bounds write vulnerabilities. The heap-based nature of this issue creates opportunities for attackers to leak memory contents including heap pointers from adjacent allocations, effectively bypassing modern address space layout randomization protections that are designed to prevent exploitation.
The operational impact of this vulnerability extends beyond simple information disclosure, as the memory corruption occurs during error cleanup paths when the library attempts to free an uninitialized pointer. This particular aspect of the flaw creates a scenario where the heap allocator state becomes corrupted due to freeing memory that was allocated through realloc() but not properly zeroed, leading to potential denial of service conditions or more serious exploitation opportunities. The attack vector requires a malicious SSH server to be present in the network, making this vulnerability particularly concerning for applications that connect to untrusted SSH servers without proper certificate validation or host key verification mechanisms.
The security implications of this vulnerability align with ATT&CK technique T1566, which covers credential access through network sniffing and protocol manipulation. The heap corruption during error handling creates potential for exploitation through techniques such as heap spraying or use-after-free conditions that could be leveraged to achieve privilege escalation or arbitrary code execution in affected applications. Organizations using libssh2 libraries in their SSH client implementations must consider this vulnerability as a critical threat that requires immediate attention, particularly in environments where untrusted SSH servers may be encountered during normal operations.
Mitigation strategies should prioritize updating to libssh2 version containing the fix from commit a13bb6c, which properly implements bounds checking for the comment_len field and validates buffer boundaries before memory operations. Additionally, administrators should implement strict host key verification policies that prevent connections to untrusted SSH servers, as this vulnerability requires a malicious server to be present in the network for exploitation. Network segmentation and monitoring solutions should also be employed to detect anomalous SSH traffic patterns that might indicate exploitation attempts, while regular security audits of applications using libssh2 should verify proper implementation of error handling and memory management practices to prevent similar vulnerabilities from being introduced in custom code implementations.