# CVE-2026-66034: libssh2 Heap Out-of-Bounds Read via publickey subsystem

> A malicious SSH server can trigger a heap out-of-bounds read and free of an uninitialized pointer in libssh2, leaking heap pointers and corrupting memory.

- **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-2026-66034, AZL-93393, AZL-97410, BELL-CVE-2026-66034, DEBIAN-CVE-2026-66034, ECHO-3f90-0490-a5db, GHSA-6h6q-9mwg-r6x7, JLSEC-2026-1087, ROOT-OS-DEBIAN-11-CVE-2026-66034, UBUNTU-CVE-2026-66034

## What is CVE-2026-66034?

CVE-2026-66034 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 a13bb6c, contains a missing bounds check vulnerability that allows a malicious SSH server to trigger an arbitrary-length heap out-of-bounds read and a free of an uninitialized pointer via the publickey subsystem. In libssh2_publickey_list_fetch(), the version 1 response parser reads a server-controlled comment_len value and advances the parse pointer without verifying sufficient bytes remain in the buffer, causing the out-of-bounds read to leak heap pointers from adjacent allocations defeating ASLR, followed by heap allocator state corruption when the error cleanup path frees an uninitialized pointer from a non-zeroed realloc() region.

Weakness: CWE-908 (Use of Uninitialized Resource), CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-66034?

It affects 1 product.


Products: Libssh2 Libssh2.

## How do I fix CVE-2026-66034?

Apply the vendor's patch.

## Is CVE-2026-66034 being exploited?

There is no known exploitation of CVE-2026-66034 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-2026-66034 and fix it if it is affected.

What it is: CVE-2026-66034 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-2026-66034 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-2026-66034/ (Markdown: https://insights.sbomify.com/CVE-2026-66034/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66034
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66034
- OSV: https://osv.dev/vulnerability/CVE-2026-66034

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66034/
Summaries are generated and may contain mistakes; the official description above is the record.