CVE-2026-66033 libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation
A malicious SSH server can crash connecting clients by exploiting an integer underflow in libssh2 during AES-GCM cipher negotiation.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-66033?
CVE-2026-66033 is a high-severity vulnerability in libssh2, published on 24 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
libssh2 through 1.11.1, fixed in commit a2ed82d, contains a pre-authentication integer underflow vulnerability in the ssh2_cipher_crypt() function in src/openssl.c that allows a malicious SSH server to crash any connecting client by negotiating AES-GCM ciphers during handshake. Attackers can exploit the underflow in the expression computing blocksize minus aadlen minus authentication tag length to trigger an out-of-bounds read and a memcpy call with a near-SIZE_MAX length argument, causing immediate process crash before any authentication occurs.
Which versions are affected by CVE-2026-66033?
It affects 1 product.
Products include Libssh2 Libssh2.
How do I fix CVE-2026-66033?
Apply the vendor's patch.
Is CVE-2026-66033 being exploited?
There is no known exploitation of CVE-2026-66033 in the wild and no public exploit code.
Other vulnerabilities in libssh2
- CVE-2026-55200 libssh2 - Out-of-Bounds Write via Unchecked packet_length in transport.c
- CVE-2026-66032 libssh2 Double-Free Heap Corruption via sftp_open()
- CVE-2026-58050 libssh2 - Integer Overflow in publickey Subsystem Attribute Allocation
- CVE-2026-58051 libssh2 - Free of Uninitialized Pointer in publickey List Cleanup
- CVE-2025-15661 libssh2 - Heap Buffer Over-read via sftp_symlink() in sftp.c
Sources
Also known as ALPINE-CVE-2026-66033, AZL-93402, AZL-97428, BELL-CVE-2026-66033, DEBIAN-CVE-2026-66033, ECHO-bd2e-2758-1b23 and 4 more.