CVE-2026-66033
libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation
- State
- Published
- Severity
- 8.7 High
- Published
- Jul 24, 2026
- Updated
- Jul 28, 2026
- Reserved
- Jul 23, 2026
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.
Scores
| CVSS | Score | Vector | Source |
|---|---|---|---|
| 4.0 | 8.7 High | 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 | VulnCheck |
| 3.1 | 7.5 High | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H | VulnCheck |
Weaknesses
Affected Products
| Product | Versions |
|---|---|
libssh2libssh2 |
|
References
- Pull Requestissue-trackingpatch
- Patch Commitpatch
- https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiationthird-party-advisory
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