CVE-2026-66033: libssh2 through 1.11.1, fixed in commit a2ed82d, contains a pre-authentication integer underflow vulnerability

Q: How do I fix CVE-2026-66033?

libssh2: upgrade to newer than 1.11.1. Exact commands for each OS are on this page.

Q: Is CVE-2026-66033 being exploited?

CVE-2026-66033 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-66033 affect?

CVE-2026-66033 affects libssh2. Severity is high (CVSS 7.5). 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 negotia

Q: How urgent is CVE-2026-66033?

Priority 43/100. High severity; Remotely exploitable, no login, no user action

This month

HIGH 7.5REMOTEUPGRADE

CVE-2026-66033 is a high-severity flaw (CVSS 7.5) affecting libssh2. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 w

Why this priority

High severity
Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	7.5 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.6% (top 52%)
CISA exploited list	No
Weakness	CWE-125, CWE-191
Published	2026-07-24
Last modified	2026-07-30

Publisher (CNA): VulnCheck · Vendor: libssh2

Last updated 2026-07-30 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-07-24 17:17 UTCPublished CVE record published
2026-07-30 15:44 UTCUpdated Record last modified

libssh2 libssh2 UPGRADE

Affected: <= 1.11.1

Fixed in: newer than 1.11.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy libssh2 || rpm -q libssh2 || libssh2 --version

2. UPGRADE to newer than 1.11.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade libssh2

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade libssh2

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-66033?

libssh2: upgrade to newer than 1.11.1. Exact commands for each OS are on this page.

Is CVE-2026-66033 being exploited?

CVE-2026-66033 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-66033 affect?

CVE-2026-66033 affects libssh2. Severity is high (CVSS 7.5). 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 negotia

How urgent is CVE-2026-66033?

Priority 43/100. High severity; Remotely exploitable, no login, no user action

Software pages

OpenSSL vulnerabilities

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References

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