CVE-2026-66033: libssh2 Out-of-bounds Read — Fix

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.

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CVSS v3.1 Score

7.5

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS — Exploit Prediction

0.0037

Probability of exploitation

0.30%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-125 Out-of-bounds Read

CWE-191 CWE-191

Affected Products

Vendor	Product	Versions
libssh2	libssh2	< 1.11.1

References

Advisories & Patches

https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 https://github.com/libssh2/libssh2/pull/2401 https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation

Frequently Asked Questions

What is CVE-2026-66033? +

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. It has a CVSS v3.1 base score of 7.5 (HIGH).

How severe is CVE-2026-66033? +

CVE-2026-66033 has a CVSS v3.1 score of 7.5 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0037, placing it in the 0th percentile for exploitation probability.

What products are affected by CVE-2026-66033? +

CVE-2026-66033 affects products from libssh2, specifically: libssh2. Check the affected products table above for specific version ranges.

How do I check if I'm vulnerable to CVE-2026-66033? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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