# CVE-2026-55200: libssh2 - Out-of-Bounds Write via Unchecked packet_length in transport.c

> Remote attackers can achieve code execution by sending crafted SSH packets that trigger an out-of-bounds write in the libssh2 library.

- **Severity:** Critical 9.2 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/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:** Not needed
- **Fix available:** Yes
- **Published:** 17 Jun 2026
- **Updated:** 22 Oct 2026
- **Also known as:** ALPINE-CVE-2026-55200, AZL-90927, BELL-CVE-2026-55200, DEBIAN-CVE-2026-55200, ECHO-81b6-9726-71c6, GHSA-r8mh-x5qv-7gg2, JLSEC-2026-661, MINI-h8c6-pq27-86w4, ROOT-OS-DEBIAN-11-CVE-2026-55200, UBUNTU-CVE-2026-55200

## What is CVE-2026-55200?

CVE-2026-55200 is a critical memory corruption vulnerability in libssh2, published on 17 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

libssh2 1.11.1, fixed in commit 97acf3d, contains an out-of-bounds write vulnerability in ssh2_transport_read() that fails to enforce upper bounds on packet_length field. Remote attackers can send crafted SSH packets with excessively large packet_length values to corrupt heap memory and achieve remote code execution.

Weakness: CWE-680 (Integer Overflow to Buffer Overflow)

## Which versions are affected by CVE-2026-55200?

It affects 1 product.


Products: Libssh2 Libssh2.

## How do I fix CVE-2026-55200?

Apply the vendor's patch.

## Is CVE-2026-55200 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55200 exists.

Public exploit code:

- CVE-2026-55200 (github_poc): https://github.com/0xredash/CVE-2026-55200
- CVE-2026-55200 - Critical libssh2 Remote Code Execution Vulnerability (github_poc): https://github.com/xd20111/CVE-2026-55200
- https://github.com/kaleth4/CVE-2026-55200 (github_poc): https://github.com/kaleth4/CVE-2026-55200

## Other vulnerabilities in libssh2

- [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)
- [CVE-2025-15661](https://insights.sbomify.com/CVE-2025-15661/): libssh2 - Heap Buffer Over-read via sftp_symlink() in sftp.c (high 8.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55200 and fix it if it is affected.

What it is: CVE-2026-55200 is a critical memory corruption vulnerability in libssh2, published on 17 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55200 exists.

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-2026-55200/ (Markdown: https://insights.sbomify.com/CVE-2026-55200/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55200
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55200
- OSV: https://osv.dev/vulnerability/CVE-2026-55200

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55200/
Summaries are generated and may contain mistakes; the official description above is the record.