Intel

CVE-2026-66033

ubuntu libssh2: Integer Underflow (Wrap or Wraparound)

highEPSS 0.6%

Affected

  • ubuntu/libssh2 1.11.0-4.1ubuntu0.24.04.4..*
  • ubuntu/libssh2 1.11.1-1ubuntu0.26.04.4..*

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.

Source: suse_csafUbuntu Security NoticesMicrosoft Security Update GuideNVD (NIST)cvelistv5CVE.org (MITRE)BSI WID PortalBSI CSAF

Affected operating systems

  • linux

    ubuntu / libssh2noble

  • linux

    ubuntu / libssh2resolute

Metrics

XIEM Effective CVSS
8.7
Source: nvd-v4
EPSS percentile
46.2 %
High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
KEV status
—
Show all metrics
Severity
high
Priority score
83.63
Exploit maturity
no public PoC known
CVSS v3
7.5
CVSS v4
8.7
EPSS score
0.6 %
Low — model estimates < 1% exploitation likelihood.
Published
2026-07-24 17:17 UTC
CWE
CWE-191, CWE-125

Weakness classes (CWE)

  • CWE-191Base

    Integer Underflow (Wrap or Wraparound)

    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.mitre.org →

  • CWE-125Base

    Out-of-bounds Read

    The product reads data past the end, or before the beginning, of the intended buffer.

    cwe.mitre.org →

References & sources

Reanalysis & status changes

Chronological NVD audit events for this CVE — reanalyses, CVSS updates, CPE diffs.

  1. Initial Analysis2026-07-30 15:44 UTC· nvd@nist.gov
    • CPE Configuration: OR *cpe:2.3:a:libssh2:libssh2:*:*:*:*:*:*:*:* versions up to (including) 1.11.1
    • Reference Type: VulnCheck: https://github.com/libssh2/libssh2/commit/a2ed82d40964bbc0d64cd717aa0a5a892117d2e6 Types: Patch
    • Reference Type: VulnCheck: https://github.com/libssh2/libssh2/pull/2401 Types: Issue Tracking, Patch
    • Reference Type: VulnCheck: https://www.vulncheck.com/advisories/libssh2-integer-underflow-dos-via-aes-gcm-cipher-negotiation Types: Patch, Third Party Advisory
  2. CVE Modified2026-07-27 17:16 UTC· 134c704f-9b21-4f2e-91b3-4a467353bcc0
    • SSVC: {"id":"CVE-2026-66033","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technical…

Linked advisories