CVE-2026-55200

An out-of-bounds write vulnerability exists in the libssh2 client. A remote attacker can exploit this by sending a specially crafted SSH packet with an abnormally large length value. This corrupts the application's memory and can potentially allow the attacker to execute arbitrary code on the affected system.

An Important out-of-bounds write vulnerability was discovered in libssh2. As it only impacts client installations of the library, exploitation would require a victim to initiate an SSH connection to an attacker-controlled server. This means an attacker must first redirect client connections via DNS poisoning, a man-in-the-middle, or compromise of a trusted host. While the vulnerability does not require authentication and requires no special configuration, the client redirection prerequisites significantly limit the practical attack surface compared to a server-side flaw.

The integer overflow provides uncontrolled access to the heap, which reliably crashes the client process but is unlikely to achieve remote code execution in practice. Weaponizing the overflow for code execution would require a separate information disclosure vulnerability to defeat ASLR, along with a specific heap layout to place exploitable structures adjacent to the undersized allocation. On RHEL, ASLR is enabled by default and glibc's heap metadata integrity checks further raise the bar, making denial of service the realistic impact for most deployments.

RHEL 8.1 and later do not include libssh2 as a supported product component. Hence, they are not affected by this flaw.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

The primary mitigation is strict network access control. Ensure libssh2 clients only connect to trusted SSH servers, and use firewalls to block untrusted incoming connections if libssh2 is deployed as a server-side application.