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

> A malicious SSH server can trigger a heap buffer overflow in libssh2 clients during handshake, potentially leading to remote code execution.

- **Severity:** High 7.7 (`CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 24 Jul 2026
- **Updated:** 28 Jul 2026
- **Also known as:** ALPINE-CVE-2026-66035, AZL-93399, BELL-CVE-2026-66035, DEBIAN-CVE-2026-66035, ECHO-a13b-3527-8590, GHSA-v6rf-8q4r-r495, JLSEC-2026-1088, RHSA-2026:46955, ROOT-OS-DEBIAN-11-CVE-2026-66035, UBUNTU-CVE-2026-66035

## What is CVE-2026-66035?

CVE-2026-66035 is a high-severity memory corruption vulnerability in libssh2, published on 24 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official 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.

Weakness: CWE-122 (Heap-based Buffer Overflow)

## Which versions are affected by CVE-2026-66035?

It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `libssh2` | Red Hat:hummingbird:1 | All versions | 0:1.11.1-10.3.hum1 |
| `libssh2-devel` | Red Hat:hummingbird:1 | All versions | 0:1.11.1-10.3.hum1 |
| `libssh2-docs` | Red Hat:hummingbird:1 | All versions | 0:1.11.1-10.3.hum1 |

Products: Libssh2 Libssh2.

## How do I fix CVE-2026-66035?

Apply the vendor's patch.

## Is CVE-2026-66035 being exploited?

There is no known exploitation of CVE-2026-66035 in the wild and no public exploit code.

## Other vulnerabilities in libssh2

- [CVE-2026-55200](https://insights.sbomify.com/CVE-2026-55200/): libssh2 - Out-of-Bounds Write via Unchecked packet_length in transport.c (critical 9.2)
- [CVE-2026-66032](https://insights.sbomify.com/CVE-2026-66032/): libssh2 Double-Free Heap Corruption via sftp_open() (high 8.7)
- [CVE-2026-66033](https://insights.sbomify.com/CVE-2026-66033/): libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation (high 8.7)
- [CVE-2026-58050](https://insights.sbomify.com/CVE-2026-58050/): libssh2 - Integer Overflow in publickey Subsystem Attribute Allocation (high 8.3)
- [CVE-2026-58051](https://insights.sbomify.com/CVE-2026-58051/): libssh2 - Free of Uninitialized Pointer in publickey List Cleanup (high 8.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-66035 and fix it if it is affected.

What it is: CVE-2026-66035 is a high-severity memory corruption vulnerability in libssh2, published on 24 Jul 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-66035 in the wild and no public exploit code.

Affected product: libssh2. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-66035/ (Markdown: https://insights.sbomify.com/CVE-2026-66035/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66035
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66035
- OSV: https://osv.dev/vulnerability/CVE-2026-66035

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66035/
Summaries are generated and may contain mistakes; the official description above is the record.