CVE-2026-66033
Received Received - Intake

Integer Underflow in libssh2 Pre-Authentication

Publication date: 2026-07-24

Last updated on: 2026-07-24

Assigner: VulnCheck

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.

CVSS Scores

EPSS Scores

Probability:
Percentile:

Meta Information

Published
2026-07-24
Last Modified
2026-07-24
Generated
2026-07-24
AI Q&A
2026-07-24
EPSS Evaluated
N/A
NVD
CVE-2026-66033
EUVD
EUVD-2026-48720

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
libssh2 libssh2 to 1.11.1 (inc)

Helpful Resources

Exploitability

CWE
CWE Icon
KEV
KEV Icon
CWE ID Description
CWE-191 The product subtracts one value from another, such that the result is less than the minimum allowable integer value, which produces a value that is not equal to the correct result.
CWE-125 The product reads data past the end, or before the beginning, of the intended buffer.

Attack-Flow Graph

AI Quick Actions

Instant insights powered by AI
Executive Summary

libssh2 through version 1.11.1 has a pre-authentication integer underflow flaw in the ssh2_cipher_crypt() function. This allows a malicious SSH server to crash any connecting client by negotiating AES-GCM ciphers during the handshake. The underflow occurs when computing blocksize minus aadlen minus authentication tag length, leading to an out-of-bounds read and a memcpy call with an extremely large length, causing the client to crash before authentication.

Detection Guidance

This vulnerability can be detected by checking the version of libssh2 in use. If your system uses libssh2 through 1.11.1, it is vulnerable. Commands like 'dpkg -l | grep libssh2' (Debian/Ubuntu) or 'rpm -qa | grep libssh2' (RHEL/CentOS) can help identify the installed version.

Impact Analysis

This vulnerability allows attackers to crash any vulnerable SSH client by exploiting a malicious server. The crash happens before authentication, meaning users cannot connect to the server. It disrupts secure communication and may lead to denial-of-service conditions for affected systems.

Compliance Impact

This vulnerability could impact compliance by disrupting secure data transmission, potentially violating requirements for secure communication channels in standards like GDPR and HIPAA. Downtime or service disruption due to crashes may also lead to non-compliance with availability requirements.

Mitigation Strategies

Immediately update libssh2 to a version that includes the fix (commit a2ed82d or later). If updating is not possible, disable AES-GCM ciphers in SSH server configurations to prevent exploitation during handshake.

Chat Assistant

Ask questions about this CVE
Hi! I’m here to help you understand CVE-2026-66033. Ask me anything about the vulnerability, its impact, or mitigation strategies.
0/70

EPSS Chart