CVE-2021-3634A 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…
- 6.5MediumCVSS base score
- 4.7%Exploitation in 30 daysEPSS · top 8.1% of all CVEs
- NoKnown exploitednot in CISA's KEV catalog
- NonePublic exploitsnone known yet
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.
Timeline
ID reserved
By redhat
- 2 months later
· 2 months later
Published
- 3 years later
· 3 years later
Record updated
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| NVDNVD · Modified | 3.1 | Medium6.5 | CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H |
- EPSS (exploitation in 30 days)
- 4.7% · top 8.1% of all CVEs (15 Nov 2026)
The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 6.5.
Affected products
libssh
- affected libssh 0.9.6
References
References8
- https://bugzilla.redhat.com/show_bug.cgi?id=1978810 (opens in a new tab)x_transferred
- DSA-4965 (opens in a new tab)vendor-advisoryx_transferred
- FEDORA-2021-ec797b6a96 (opens in a new tab)vendor-advisoryx_transferred
- FEDORA-2021-288925ac19 (opens in a new tab)vendor-advisoryx_transferred
- FEDORA-2021-f2a020a065 (opens in a new tab)vendor-advisoryx_transferred
- https://www.oracle.com/security-alerts/cpujan2022.html (opens in a new tab)x_transferred
- https://security.netapp.com/advisory/ntap-20211004-0003/ (opens in a new tab)x_transferred
- GLSA-202312-05 (opens in a new tab)vendor-advisoryx_transferred
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- EUVD EUVD-2021-26935 (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisories (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://cveledger.app/cve/CVE-2021-3634)Record last updated .