# CVE-2021-3634: Memory corruption in libssh

> A memory corruption flaw in libssh allows attackers to execute arbitrary code or cause denial of service via key re-exchange.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 31 Aug 2021
- **Updated:** 3 Aug 2024
- **Also known as:** ALSA-2022:2031, CLSA-2023-1685023099, CLSA-2023-1685023501, DEBIAN-CVE-2021-3634, DSA-4965-1, GHSA-4wwm-f449-qwpq, MGASA-2021-0441, OESA-2021-1343, RHSA-2022:2031, RLSA-2022:2031, UBUNTU-CVE-2021-3634, USN-5053-1

## What is CVE-2021-3634?

CVE-2021-3634 is a medium-severity memory corruption vulnerability in Debian Debian Linux, published on 31 Aug 2021. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

A flaw has been found in libssh in versions prior to 0.9.6. The SSH protocol keeps track of two shared secrets during the lifetime of the session. One of them is called secret_hash and the other session_id. Initially, both of them are the same, but after key re-exchange, previous session_id is kept and used as an input to new secret_hash. Historically, both of these buffers had shared length variable, which worked as long as these buffers were same. But the key re-exchange operation can also change the key exchange method, which can be based on hash of different size, eventually creating "secret_hash" of different size than the session_id has. This becomes an issue when the session_id memory is zeroed or when it is used again during second key re-exchange.

Weakness: CWE-787 (Out-of-bounds Write)

## Which versions are affected by CVE-2021-3634?

It affects 7 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `libssh` | AlmaLinux:8 | All versions | 0.9.6-3.el8 |
| `libssh-config` | AlmaLinux:8 | All versions | 0.9.6-3.el8 |
| `libssh-devel` | AlmaLinux:8 | All versions | 0.9.6-3.el8 |
| `libssh-debuginfo` | Red Hat:enterprise_linux:8::appstream | All versions | 0:0.9.6-3.el8 |
| `libssh-debugsource` | Red Hat:enterprise_linux:8::appstream | All versions | 0:0.9.6-3.el8 |
| `libssh-devel` | Red Hat:enterprise_linux:8::appstream | All versions | 0:0.9.6-3.el8 |
| `libssh` | Red Hat:enterprise_linux:8::baseos | All versions | 0:0.9.6-3.el8 |
| `libssh-config` | Red Hat:enterprise_linux:8::baseos | All versions | 0:0.9.6-3.el8 |
| `libssh-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:0.9.6-3.el8 |
| `libssh-debugsource` | Red Hat:enterprise_linux:8::baseos | All versions | 0:0.9.6-3.el8 |
| `libssh` | Rocky Linux:8 | All versions | 0:0.9.6-3.el8 |

Products: Debian Debian Linux, Fedoraproject Fedora, Libssh Libssh, Netapp Cloud Backup, Oracle Mysql Workbench, Redhat Enterprise Linux, Redhat Virtualization.

## How do I fix CVE-2021-3634?

Upgrade to a fixed version.

## Is CVE-2021-3634 being exploited?

There is no known exploitation of CVE-2021-3634 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-3634 and fix it if it is affected.

What it is: CVE-2021-3634 is a medium-severity memory corruption vulnerability in Debian Debian Linux, published on 31 Aug 2021. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-3634 in the wild and no public exploit code.

Affected product: Debian Debian Linux. 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-2021-3634/ (Markdown: https://insights.sbomify.com/CVE-2021-3634/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3634
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3634
- OSV: https://osv.dev/vulnerability/CVE-2021-3634

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-3634/
Summaries are generated and may contain mistakes; the official description above is the record.