Critical libssh2 Flaws Expose SSH Clients to RCE, DoS, and Memory Disclosure

Multiple vulnerabilities in libssh2 have exposed SSH clients and products that embed the library to attacks from malicious or impersonated SSH servers. The most severe issue, CVE-2026-55200, is a pre-authentication out-of-bounds heap write in ssh2_transport_read() that affects all versions through 1.11.1 and could allow remote code execution, while CVE-2026-55199 can force clients into a CPU-intensive loop during key exchange and cause denial of service. The library is widely used in curl, backup software, IoT firmware, and network appliances, raising broad supply-chain risk, especially where libssh2 is statically linked or bundled into downstream products.

A separate flaw, CVE-2025-15661, affects sftp_symlink() in src/sftp.c and can be triggered during SFTP READLINK or REALPATH operations when a malicious server or man-in-the-middle sends a crafted SSH_FXP_NAME response with an oversized link_len value. That bug causes a heap buffer over-read that may disclose memory contents or crash applications. Fixes were reported in commits 97acf3d, 1762685, and 2dae302, but at the time of reporting no official libssh2 release containing the newer patches had been published, leaving downstream vendors responsible for shipping their own updates.