libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Attack requirements
- None
- Impact type
- Denial of service
- Confidentiality · integrity · availability
- None · None · High
- Fix status
- Partial Red Hat: 1 fixed, 1 without a fix
Attack profile from CVSS 4.0 · CNA. Fix status from CNA and Red Hat. Impact type from CWE-125 and the CVSS impact.
Summary
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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- Yes
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| The libssh2 Project | libssh2 | up to 1.11.1 | — | Patch available | CNA |
- libssh2 VendorThe libssh2 Project Affected up to 1.11.1 Fixed in — Patch available CNA
CPE configurations NVD · 1
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:libssh2:libssh2:*:*:*:*:*:*:*:* | up to 1.11.1 | Vulnerable | The libssh2 Project |
- cpe:2.3:a:libssh2:libssh2:*:*:*:*:*:*:*:* Versions up to 1.11.1 Vulnerable ·The libssh2 Project
Remediation
Red Hat 1 advisory
- RHSA-2026:46927 Released 2026-07-27 · Red Hat rating ImportantRed Hat Hardened Images RPMs Security Update
- Red Hat Hardened Images libssh2-1.11.1-10.2.hum1@aarch64, libssh2-1.11.1-10.2.hum1@src, libssh2-1.11.1-10.2.hum1@x86_64 +4
Vendor assessments
Red Hat
Red Hat rating: Important. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Red Hat Enterprise Linux 6 | Affected | No fix planned: Out of support scope |
| Red Hat Hardened Images | Fixed | RHSA-2026:46927 |
Not affected: 1 product
- Red Hat Enterprise Linux 7
Timeline
- 2026-10-02 Threatist record created from CVE-2026-66033
- 2026-10-03 Alias added: GHSA-2g8c-wcg8-r65c
- 2026-10-09 EPSS moved to the below the 50th percentile
References
Patch 2
More The libssh2 Project Libssh2 vulnerabilities
- TI-2026-39880 libssh2 - Out-of-Bounds Write via Unchecked packet_length in transport.c CVSS 4.0CRITICAL 9.2
- TI-2026-06681 libssh2 userauth.c userauth_password integer overflow CVSS 4.0MEDIUM 6.9
- TI-2023-26623 The SSH transport protocol with certain OpenSSH extensions, found in OpenSSH before 9.6… CVSS 3.1MEDIUM 5.9
- TI-2026-41806 libssh2 - Free of Uninitialized Pointer in publickey List Cleanup CVSS 4.0HIGH 8.3
- TI-2019-02869 Out-of-bounds read in libssh2 CVSS 3.1CRITICAL 9.1
- TI-2019-02865 Integer overflow in libssh2 CVSS 3.1HIGH 8.8
Sources
- CVE-2026-66033 · CVE Program
- CVE-2026-66033 · NVD
- FIRST EPSS
- GHSA-2g8c-wcg8-r65c · GitHub Advisory Database · CC BY 4.0
- CVE-2026-66033 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences