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CVE-2026-66033— libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation

CVSS 7.5 · High EPSS 0.39% · P31

Possible ATT&CK Techniques 1AI

T1190 · Exploit Public-Facing Application

Affected Version Matrix 2

VendorProductVersion RangeStatus
libssh2libssh2≤ 1.11.1affected
a2ed82d40964bbc0d64cd717aa0a5a892117d2e6unaffected
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I. Basic Information for CVE-2026-66033

Vulnerability Information
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Vulnerability Title
libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation
Source: CVE Program / CVE List V5
Vulnerability 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: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
整数下溢(超界折返)
Source: CVE Program / CVE List V5

Affected Products

VendorProductAffected VersionsCPESubscribe
libssh2libssh2 0 ~ 1.11.1 -

II. Public POCs for CVE-2026-66033

#POC DescriptionSource LinkShenlong Link
AI-Generated POCPremium
Qwen3.6-35B-A3B · 8213 chars
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III. Intelligence Information for CVE-2026-66033

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Patches & Fixes for CVE-2026-66033 (2)

Vendor Advisories for CVE-2026-66033 (1)

Same Patch Batch · libssh2 · 2026-07-24 · 4 CVEs total

CVE-2026-660328.8 HIGHlibssh2 Double-Free Heap Corruption via sftp_open()
CVE-2026-660347.5 HIGHlibssh2 Heap Out-of-Bounds Read via publickey subsystem
CVE-2026-660357.5 HIGHlibssh2 Heap Buffer Overflow via ETM Cipher Negotiation

IV. Related Vulnerabilities

Same product: libssh2
Same vendor: libssh2
Same weakness: CWE-191

V. Comments for CVE-2026-66033

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