# CVE-2026-66033: libssh2 Integer Underflow DoS via AES-GCM Cipher Negotiation

> A malicious SSH server can crash connecting clients by exploiting an integer underflow in libssh2 during AES-GCM cipher negotiation.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/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:** 24 Jul 2026
- **Updated:** 28 Jul 2026
- **Also known as:** ALPINE-CVE-2026-66033, AZL-93402, AZL-97428, BELL-CVE-2026-66033, DEBIAN-CVE-2026-66033, ECHO-bd2e-2758-1b23, GHSA-2g8c-wcg8-r65c, JLSEC-2026-1086, ROOT-OS-DEBIAN-11-CVE-2026-66033, UBUNTU-CVE-2026-66033

## What is CVE-2026-66033?

CVE-2026-66033 is a high-severity vulnerability in libssh2, published on 24 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

libssh2 through 1.11.1, fixed in commit a2ed82d, contains a pre-authentication integer underflow vulnerability in the ssh2_cipher_crypt() function in src/openssl.c that allows a malicious SSH server to crash any connecting client by negotiating AES-GCM ciphers during handshake. Attackers can exploit the underflow in the expression computing blocksize minus aadlen minus authentication tag length to trigger an out-of-bounds read and a memcpy call with a near-SIZE_MAX length argument, causing immediate process crash before any authentication occurs.

Weakness: CWE-191 (Integer Underflow (Wrap or Wraparound)), CWE-125 (Out-of-bounds Read)

## Which versions are affected by CVE-2026-66033?

It affects 1 product.


Products: Libssh2 Libssh2.

## How do I fix CVE-2026-66033?

Apply the vendor's patch.

## Is CVE-2026-66033 being exploited?

There is no known exploitation of CVE-2026-66033 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-2026-58050](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-2026-66033 and fix it if it is affected.

What it is: CVE-2026-66033 is a high-severity vulnerability in libssh2, published on 24 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-66033 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-2026-66033/ (Markdown: https://insights.sbomify.com/CVE-2026-66033/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66033
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66033
- OSV: https://osv.dev/vulnerability/CVE-2026-66033

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66033/
Summaries are generated and may contain mistakes; the official description above is the record.