# CVE-2026-66035: libssh2 Heap Buffer Overflow via ETM Cipher Negotiation

CVE-2026-66035 is fixed in 6 products on 1 platform, first by libssh2-1t64 1.11.0-4.1ubuntu0.24.04.4, released July 24, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-66035
- Priority: High
- CVSS: 7.5 (Important)
- EPSS: 0.0047
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jul 24, 2026
- Weakness: Heap-based Buffer Overflow
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-66035

## Description

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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | cargo-c amzn 2023 | 0.10.21-1.amzn2023.0.1 | [ALAS2023-2026-2064](https://patchblox.com/security/data/fix/linux/alas2023-2026-2064.md) | Aug 17, 2026 |
| Linux | cargo-c-debuginfo amzn 2023 | 0.10.21-1.amzn2023.0.1 | [ALAS2023-2026-2064](https://patchblox.com/security/data/fix/linux/alas2023-2026-2064.md) | Aug 17, 2026 |
| Linux | libssh2 Debian 13.5 (trixie) | 1.11.1-1+deb13u2 | [DEBIAN-CVE-2026-66035](https://patchblox.com/security/data/fix/linux/debian-cve-2026-66035.md) | Aug 1, 2026 |
| Linux | libssh2 Debian 14 | 1.11.1-5 | [DEBIAN-CVE-2026-66035](https://patchblox.com/security/data/fix/linux/debian-cve-2026-66035.md) | Aug 1, 2026 |
| Linux | libssh2-1t64 Ubuntu 24.04.5 LTS (noble) | 1.11.0-4.1ubuntu0.24.04.4 | [libssh2-1t64 1.11.0-4.1ubuntu0.24.04.4](https://patchblox.com/security/data/fix/linux/ubuntu-noble-libssh2-1t64-1.11.0-4.1ubuntu0.24.04.4.md) | Jul 24, 2026 |
| Linux | libssh2-1t64 Ubuntu 26.04.1 LTS (resolute) | 1.11.1-1ubuntu0.26.04.4 | [libssh2-1t64 1.11.1-1ubuntu0.26.04.4](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libssh2-1t64-1.11.1-1ubuntu0.26.04.4.md) | Jul 24, 2026 |
| Linux | libssh2_org opensuse-leap 16.0 | 1.11.1-160000.5.1 | [OPENSUSE-SU-2026:21549-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-21549-1.md) | Aug 10, 2026 |
| Linux | libssh2_org sles 15.6 | 1.11.0-150600.20.6.1 | [SUSE-SU-2026:3541-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-3541-1.md) | Aug 10, 2026 |
| Linux | libssh2_org sles 15.7 | 1.11.0-150600.20.6.1 | [SUSE-SU-2026:3541-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-3541-1.md) | Aug 10, 2026 |
| Linux | libssh2_org sles 16.0 | 1.11.1-160000.5.1 | [SUSE-SU-2026:23225-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-23225-1.md) | Aug 10, 2026 |
| Linux | rust-cargo-c-debugsource amzn 2023 | 0.10.21-1.amzn2023.0.1 | [ALAS2023-2026-2064](https://patchblox.com/security/data/fix/linux/alas2023-2026-2064.md) | Aug 17, 2026 |

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Data as of 2026-11-07T18:46:35Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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