libssh2 Heap Buffer Overflow via ETM Cipher Negotiation
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- Passive
- Attack complexity
- High
- Attack requirements
- None
- Impact type
- Code execution
- Confidentiality · integrity · availability
- High · High · 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-122 and the CVSS impact.
Summary
libssh2 through 1.11.1, fixed in commit 42e33d8, contains a pre-authentication heap buffer overflow vulnerability that allows a malicious SSH server to corrupt heap metadata in any connecting client by sending a packet with a packet_length smaller than the cipher's block size during Encrypt-then-MAC cipher negotiation. In the fullpacket() function in src/transport.c, the ETM path allocates a buffer of packet_length bytes but copies blocksize minus one bytes via memcpy, causing an overflow that on 32-bit glibc writes attacker-controlled bytes into an adjacent chunk's SIZE field, enabling tcache bin confusion, overlapping live objects, and function pointer overwrite during the session handshake before authentication.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- No
- Technical impact
- Total
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-2026:46955 Released 2026-07-27 · Red Hat rating ImportantRed Hat Hardened Images RPMs Security Update
- Red Hat Hardened Images libssh2-1.11.1-10.3.hum1@aarch64, libssh2-1.11.1-10.3.hum1@src, libssh2-1.11.1-10.3.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-2026:46955 |
Not affected: 1 product
- Red Hat Enterprise Linux 7
Timeline
- 2026-10-02 Threatist record created from CVE-2026-66035
- 2026-10-03 Alias added: GHSA-v6rf-8q4r-r495
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-2026-66035 · CVE Program
- CVE-2026-66035 · NVD
- FIRST EPSS
- GHSA-v6rf-8q4r-r495 · GitHub Advisory Database · CC BY 4.0
- CVE-2026-66035 · 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