The vulnerability CVE-2025-15661 exists in libssh2's sftp_symlink() function, where a crafted SSH_FXP_NAME response from a malicious SSH server or man-in-the-middle attacker can cause an out-of-bounds heap read. This can disclose sensitive heap memory contents or crash the application, causing denial of service. The issue is classified as CWE-125 (Out-of-bounds Read). The Red Hat advisory for Hardened Images RPMs including libssh2-1.11.1-9.hum1 packages describes this flaw and recommends mitigation by restricting SSH/SFTP traffic to trusted endpoints and enforcing strict SSH host key verification. No explicit patch or fix version is provided in the advisory, and the vendor recommends mitigation measures to reduce risk.

An attacker controlling an SSH server or positioned as a man-in-the-middle can exploit this vulnerability to read out-of-bounds heap memory, potentially disclosing sensitive information such as cryptographic keys or memory addresses. Additionally, the attacker can cause the client application to crash, resulting in denial of service. The confidentiality impact is low, integrity impact is none, and availability impact is high as per the Red Hat CVSS assessment. This vulnerability could also aid attackers in bypassing protection mechanisms like ASLR by leaking memory layout information.

No official patch or fix is explicitly stated in the Red Hat advisory. Mitigation involves implementing strict network egress filtering to allow outbound SSH/SFTP traffic only to known, trusted endpoints. Additionally, enforce strict SSH host key checking in libssh2 client applications to reject unknown servers and prevent man-in-the-middle attacks. Users should monitor Red Hat advisories for any forthcoming patches and apply updates promptly once available.