CVE-2026-66033
High
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.
SeverityHigh
CVSS7.5
Exploit probability48. percentile0.6%
ProductLibssh2
PublishedJul 24, 2026
First seenJul 24, 2026
Weakness typeCWE-191CWE-191CWE-125Out-of-bounds read
How to fix
Affected products
| Product | Affected versions | Fixed in |
|---|---|---|
| libssh2 libssh2 | v ≤ 1.11.1 | — |
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