Name: CVE-2026-66033: Integer Underflow (Wrap or Wraparound)
Creator: Rapid7
Published: 2026-07-24T00:00:00.000Z
License: https://www.rapid7.com/legal/website-terms-of-use/
Keywords: CVE, CVE-2026-66033, CWE-125, CWE-191, CVSS 7.5, EPSS 0.00583

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.

CVSS Details

CVSS 4.0 Base Score: 8.7 (HIGH)
CVSS 4.0 Vector: (CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X)
CVSS 3.1 Base Score: 7.5
CVSS 3.1 Vector: (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)

Covered by Rapid7

Product	Vendor Advisory	Solution File	Added	Published
Alpine Linux	—	Upgrade libssh2	Oct 28, 2026	Jul 24, 2026
Amazon_linux_2023	—	Upgrade cargo-cUpgrade cargo-c-debuginfoUpgrade rust-cargo-c-debugsource	Aug 18, 2026	Jul 24, 2026
Debian	—	Upgrade libssh2	Sep 21, 2026	Jul 24, 2026
Redhat_linux	—	No solution exists	Jul 29, 2026	Jul 24, 2026
Ubuntu	—	Upgrade libssh2-1t64	Sep 14, 2026	Sep 3, 2026
Vmware Photon_os	—	Use 'tdnf update' to upgrade all packages to the latest version.	Sep 21, 2026	Jul 24, 2026

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CVSS Details

Covered by Rapid7

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