CVE-2026-66033Network attack vector

Client crash via AES-GCM length underflow

Published Jul 24, 2026 · Updated Jul 24, 2026

Integer underflow in libssh2 1.11.1 and earlier allows remote attackers controlling an SSH server to crash connecting clients. The ssh2_cipher_crypt() function subtracts AAD and authentication-tag lengths from blocksize without runtime bounds checks, wrapping cryptlen before a near-SIZE_MAX memcpy call. Exploitation occurs before authentication when a client negotiates AES-GCM with a malicious server, and the bounded consequence is an immediate client-process crash.

CVSS severity8.7
High
EPSS probability0.58%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Integer underflow in libssh2 1.11.1 and earlier allows remote attackers controlling an SSH server to crash connecting clients. The ssh2_cipher_crypt() function subtracts AAD and authentication-tag lengths from blocksize without runtime bounds checks, wrapping cryptlen before a near-SIZE_MAX memcpy call. Exploitation occurs before authentication when a client negotiates AES-GCM with a malicious server, and the bounded consequence is an immediate client-process crash.

The record

CVE
CVE-2026-66033
Published
Jul 24, 2026
Updated
Jul 24, 2026
Vendor
libssh2
Product
libssh2
Classifications
CWE-125, CWE-191, T1499
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jul 24, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 24, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <=1.11.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

libssh2 · libssh2version=0 <=1.11.1

Published CVSS scores

7.5VulnCheckCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
8.7VulnCheckCVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-125Out-of-bounds Read
CWE-191Integer Underflow (Wrap or Wraparound)
T1499Endpoint Denial of Service

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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