CVE-2025-4877: libssh Buffer Overflow Vulnerability

CVE-2025-4877 Overview

A vulnerability exists in the libssh package where an integer overflow can occur when a libssh consumer passes an unexpectedly large input buffer to the ssh_get_fingerprint_hash() function. When this happens, the bin_to_base64() function experiences an integer overflow that leads to memory under-allocation. The subsequent out-of-bounds write operation can result in heap corruption, potentially allowing attackers to manipulate program memory.

Critical Impact

This integer overflow vulnerability can lead to heap corruption through out-of-bounds writes, affecting the integrity and stability of applications using 32-bit builds of libssh.

Affected Products

• libssh (32-bit builds only)
• Applications and services utilizing libssh for SSH connectivity on 32-bit architectures
• Linux distributions shipping 32-bit libssh packages

Discovery Timeline

• 2025-08-20 - CVE-2025-4877 published to NVD
• 2025-08-20 - Last updated in NVD database

Technical Details for CVE-2025-4877

Vulnerability Analysis

This vulnerability (CWE-787: Out-of-bounds Write) stems from improper handling of large input buffers in the ssh_get_fingerprint_hash() function. When a consumer application passes an oversized input buffer, the internal bin_to_base64() function performs size calculations that can overflow on 32-bit systems due to the limited integer range. The overflow causes the function to allocate significantly less memory than required for the output buffer, while the subsequent write operations proceed with the original (larger) calculated size, writing beyond the allocated heap memory boundaries.

The vulnerability is particularly relevant for embedded systems, IoT devices, and legacy applications that still rely on 32-bit builds. While 64-bit systems have a much larger integer range making this overflow impractical to trigger, 32-bit builds remain vulnerable when processing maliciously crafted or unexpectedly large fingerprint data.

Root Cause

The root cause is an integer overflow in the size calculation within the bin_to_base64() function. When converting binary data to base64 encoding, the function calculates the required output buffer size. On 32-bit systems, passing an input buffer large enough causes this calculation to wrap around due to integer overflow, resulting in a much smaller allocation than actually needed. The CWE-787 classification indicates this is an out-of-bounds write vulnerability where data is written past the end of the intended buffer.

Attack Vector

The attack requires local access with low privileges to exploit. An attacker must be able to influence the input to the ssh_get_fingerprint_hash() function, either through a malicious application using libssh or by manipulating data processed by an existing application. The attack complexity is high due to several factors: the attacker must trigger the precise conditions for integer overflow, the target system must be running a 32-bit build of libssh, and heap layout must be favorable for successful exploitation.

The vulnerability mechanism works as follows: the ssh_get_fingerprint_hash() function receives an input buffer for fingerprint processing. Internally, the bin_to_base64() function calculates the output size needed for base64 encoding. On 32-bit systems with large input, this calculation overflows, causing under-allocation. When the conversion proceeds, data is written beyond the allocated buffer into adjacent heap memory.

For detailed technical information, refer to the Libssh CVE-2025-4877 Advisory.

Detection Methods for CVE-2025-4877

Indicators of Compromise

• Unexpected crashes or segmentation faults in applications using libssh on 32-bit systems
• Heap corruption errors or memory-related exceptions in SSH connection handling code
• Abnormal behavior in fingerprint verification or key exchange processes
• Core dumps showing memory corruption patterns near base64 encoding routines

Detection Strategies

• Monitor for application crashes in libssh-dependent services with heap corruption signatures
• Implement runtime memory sanitizers (AddressSanitizer) in development environments to detect out-of-bounds writes
• Review application logs for SSH fingerprint processing errors or unexpected buffer handling exceptions
• Deploy SentinelOne's behavioral analysis to detect exploitation attempts through anomalous memory access patterns

Monitoring Recommendations

• Enable detailed logging for SSH authentication and key fingerprint operations
• Configure heap memory corruption detection in system monitoring tools
• Monitor 32-bit applications using libssh for stability issues and unusual resource consumption
• Track version information of libssh installations across the infrastructure to identify vulnerable deployments

How to Mitigate CVE-2025-4877

Immediate Actions Required

• Identify all 32-bit builds of libssh in your environment and prioritize them for updates
• Review applications consuming libssh to assess potential exposure to malicious input buffers
• Consider migrating vulnerable 32-bit deployments to 64-bit builds where feasible
• Apply the vendor patch from the stable-0.11 branch as soon as available for your distribution

Patch Information

The libssh project has addressed this vulnerability in their stable branch. The fix is available in the libssh commit 6fd9cc8ce3958092a1aae11f1f2e911b2747732d. Red Hat has also published tracking information for this vulnerability through their security advisory and Bugzilla ID 2376193. Check with your Linux distribution vendor for updated packages containing the fix.

Workarounds

• Migrate affected services to 64-bit builds of libssh where the integer overflow is not practically exploitable
• Implement input validation in applications using libssh to reject unexpectedly large buffers before passing to fingerprint functions
• Isolate 32-bit libssh applications in sandboxed environments to limit potential exploitation impact
• Consider temporary service restrictions on affected systems until patches can be applied

# Check libssh architecture on Linux systems
file /usr/lib/libssh.so* 2>/dev/null || file /usr/lib64/libssh.so*

# Verify installed libssh version (package manager specific)
rpm -q libssh 2>/dev/null || dpkg -l libssh* 2>/dev/null