Skip to content
Noroxi
NVD / CVE Program· CNA redhat

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

Noroxi assessment

Priority: Monitor (action score 27/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 4.6% (FIRST EPSS); higher than 91.7% of all records.
  • Anyone who can reach it over the internet can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: libssh (AlmaLinux:8) 0.9.6-3.el8; libssh-config (AlmaLinux:8) 0.9.6-3.el8; libssh-devel (AlmaLinux:8) 0.9.6-3.el8.
  • Mentioned 4 times in Turkish national cyber security advisories (siberguvenlik.gov.tr).

Low priority for now. Upgrade to libssh (AlmaLinux:8) 0.9.6-3.el8; libssh-config (AlmaLinux:8) 0.9.6-3.el8; libssh-devel (AlmaLinux:8) 0.9.6-3.el8 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

MediumCVSS 6.5 · v3.1—No exploit Fix available
Published
Aug 31, 2021
Updated
Jun 17, 2026
EPSS
4.6% · 91th percentile

Report tools

JSON

Action score

27

Monitor

Low priority for now.

CVSS
26 / 40 · 6.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
1 / 30 · 4.6%

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2021-3634

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

One daily email digest (06:00 UTC); connect a webhook for KEV listings and weaponized exploits instantly.

Your free account also gets you

Running WordPress? Scan your site free with WP Lens

  • Stack alerts: add the products and versions you run, get the new CVEs that affect them
  • Uptime monitoring: 1 site, instant email when it goes down
  • Add your domain: subdomains and takeover risk scanned every week (1 domain)
  • 10 scans an hour on the security tools (3 a day without an account), an API key and MCP
  • All your scans in one panel: findings, what changed in the last scan, a report per domain or per client (PDF, share link)
All free tools

Sign up with email · Have an account? Sign in

Free account, no card. Unfollow any time.

Don't want an account? Get "Exploited this week" by email every MondayDon't want an account? RSS feed for libssh records

Running WordPress? Scan your site free with WP Lens

Is your version affected? Check it

Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

We use this product, ask for help

Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
libsshlibssh
redhatvirtualization
redhatenterprise linux
debiandebian linux
fedoraprojectfedora
oraclemysql workbench
netappcloud backup

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • debian debian linux10.0
  • debian debian linux11.0
  • fedoraproject fedora33
  • fedoraproject fedora34
  • fedoraproject fedora35
  • libssh libssh0.9.1 and later · before 0.9.6
  • netapp cloud backupall versions
  • oracle mysql workbenchup to 8.0.27 inclusive
  • redhat enterprise linux8.0
  • redhat virtualization4.0

Versions reported by the vendor

Affected version ranges reported by the assigning authority (redhat). Independent of NVD's CPE analysis and usually ahead of it.

  • n/a libssh

    • libssh 0.9.6affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:8libsshbefore 0.9.6-3.el80.9.6-3.el8
AlmaLinux:8libssh-configbefore 0.9.6-3.el80.9.6-3.el8
AlmaLinux:8libssh-develbefore 0.9.6-3.el80.9.6-3.el8
Alpine:v3.14libsshfrom 0.9.1 · before 0.9.6-r00.9.6-r0
Debian:11libsshbefore 0.9.5-1+deb11u10.9.5-1+deb11u1
Debian:12libsshbefore 0.9.6-10.9.6-1
openSUSE:Tumbleweedlibsshbefore 0.9.6-1.20.9.6-1.2
Red Hat:enterprise_linux:8::appstreamlibssh-debuginfobefore 0:0.9.6-3.el80:0.9.6-3.el8
Red Hat:enterprise_linux:8::appstreamlibssh-debugsourcebefore 0:0.9.6-3.el80:0.9.6-3.el8
Red Hat:enterprise_linux:8::appstreamlibssh-develbefore 0:0.9.6-3.el80:0.9.6-3.el8
Red Hat:enterprise_linux:8::baseoslibsshbefore 0:0.9.6-3.el80:0.9.6-3.el8
Red Hat:enterprise_linux:8::baseoslibssh-configbefore 0:0.9.6-3.el80:0.9.6-3.el8
Rocky Linux:8libsshbefore 0:0.9.6-3.el80:0.9.6-3.el8
SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSSlibsshbefore 0.9.8-150200.13.3.10.9.8-150200.13.3.1
SUSE:Linux Enterprise Software Development Kit 12 SP5libsshbefore 0.9.8-3.12.20.9.8-3.12.2

Same product

libssh: all records

Other highest-scoring records for the same primary product.

  • CVE-2018-10933A vulnerability was found in libssh's server-side state machine before versions 0.7.6 and 0.8.4.
    64This week
  • CVE-2019-14889A flaw was found with the libssh API function ssh_scp_new() in versions before 0.9.3 and before 0.8.8.
    36Monitor
  • CVE-2026-59851Libssh: libssh: authentication bypass via missing gssapi principal check
    35Monitor

Published before and after this one in the same product

  • ← EarlierCVE-2020-1730Apr 13, 2020 · A flaw was found in libssh versions before 0.8.9 and before 0.9.4 in the way it handled AES-CTR (or DES ciphers if enabled) ciphers.
    22Monitor
  • ← EarlierCVE-2020-16135Jul 29, 2020 · libssh 0.9.4 has a NULL pointer dereference in tftpserver.c if ssh_buffer_new returns NULL.
    24Monitor
  • Later →CVE-2023-1667May 26, 2023 · A NULL pointer dereference was found In libssh during re-keying with algorithm guessing.
    26Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
almalinux:libssh0.9.6-3.el8 · AlmaLinux:8Package registry (OSV)
almalinux:libssh-config0.9.6-3.el8 · AlmaLinux:8Package registry (OSV)
almalinux:libssh-devel0.9.6-3.el8 · AlmaLinux:8Package registry (OSV)
alpine:libssh0.9.6-r0 · Alpine:v3.14Package registry (OSV)
debian:libssh0.9.6-1 · Debian:12Package registry (OSV)
opensuse:libssh0.9.6-1.2 · openSUSE:TumbleweedPackage registry (OSV)
red hat:libssh0:0.9.6-3.el8 · Red Hat:enterprise_linux:8::baseosPackage registry (OSV)
red hat:libssh-config0:0.9.6-3.el8 · Red Hat:enterprise_linux:8::baseosPackage registry (OSV)
red hat:libssh-debuginfo0:0.9.6-3.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:libssh-debugsource0:0.9.6-3.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
red hat:libssh-devel0:0.9.6-3.el8 · Red Hat:enterprise_linux:8::appstreamPackage registry (OSV)
rocky linux:libssh0:0.9.6-3.el8 · Rocky Linux:8Package registry (OSV)
suse:libssh0.9.8-150200.13.3.1 · SUSE:Linux Enterprise High Performance Computing 15 SP2-LTSSPackage registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

National notice (Türkiye Cybersecurity Directorate / USOM)

Official security notices citing this record; remediation advice is on the agency's page.

All national notices →

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • A low-privileged account is enough.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
none
Integrity
none
Availability
high · the service can be disrupted
Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
None
Availability impact
High

Weakness class (CWE)

CWE-787 · Out-of-bounds Write

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (7 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records