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No exploitation reportedFix available

CVE-2021-3634

No exploitation reported by the sources we read.

  • Published2021-08-31
  • Fixed in0.9.6
  • WeaknessCWE-787

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

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

dates in UTC
  1. 2021-08-3117:15CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-11-03 15:37 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2021-3634?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2021-3634?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD
Libssh
BranchAffectedFixed in
0.9≥ 0.9.1, < 0.9.60.9.6
Fedoraproject fedora
BranchAffectedFixed in
33.x= 33no fix published
34.x= 34no fix published
35.x= 35no fix published
Debian debian linux
BranchAffectedFixed in
10.0= 10.0no fix published
11.0= 11.0no fix published

4 more products in the sources.

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • libsshUbuntu:22.04:LTSall versions · fixed in 0.9.6-1
  • libsshUbuntu:20.04:LTSall versions · fixed in 0.9.3-2ubuntu2.2
  • libsshTuxCare:CentOS:8.5all versions · fixed in 0.9.4-3.el8.tuxcare.els1and 2 other packages
  • libsshTuxCare:CentOS:8.4all versions · fixed in 0.9.4-3.el8.tuxcare.els1and 2 other packages
  • libsshSUSE:Linux Enterprise Software Development Kit 12 SP5all versions · fixed in 0.9.8-3.12.2
  • libsshSUSE:Linux Enterprise Server for SAP Applications 15 SP3all versions · fixed in 0.9.8-150200.13.3.1
  • libsshSUSE:Linux Enterprise Server for SAP Applications 15 SP2all versions · fixed in 0.9.8-150200.13.3.1
  • libsshSUSE:Linux Enterprise Server for SAP Applications 12 SP5all versions · fixed in 0.9.8-3.12.2

32 more package ranges on OSV.dev

Full record on NVD

Fix

  1. Update Libssh to 0.9.6

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub
Metasploit modules
none listed
Metasploit Framework

Sources

  • NVDCVSS 3.16.5read 2026-10-23
  • NVDCVSS 2.04read 2026-10-23
  • CNAweakness (CWE)CWE-787known since 2026-10-23
  • NVDweakness (CWE)CWE-787read 2026-10-23
  • FIRST EPSSEPSS score4.7 %2026-11-03
  • OSVaffected packages—

Identifiers

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Published
2021-08-31
Last Modified
2026-06-17
NVD status
Modified

Frequently asked questions

What is CVE-2021-3634?

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.

Is CVE-2021-3634 actively exploited?

Active exploitation of CVE-2021-3634 has not been confirmed. Its EPSS score was 4.7% on 2026-11-03, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-3634?

CVE-2021-3634 has a CVSS v3 base score of 6.5 (MEDIUM severity), with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.

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Sources for this record