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SSH Algorithm Security Scanner

A single-file Python tool for auditing and compliant the SSH algorithm configuration of one host or an entire network fleet. Identifies weak, deprecated, and NSA-linked algorithms; scores each host 0–100; and checks results against industry compliance frameworks (NIST, FIPS 140-2, BSI TR-02102, ANSSI, PRIVACY_FOCUSED) — making it suitable for periodic security reviews, pre/post-hardening verification, and compliance evidence collection.

Wraps the system ssh binary to probe each algorithm individually — no paramiko, no authentication, no credentials required. The tool never logs in and never executes commands; it only observes what the server is willing to negotiate.

Version: 3.6.4 | Author: Robert Tulke

Example

Table of Contents

What it can do

  • Probe every cipher, MAC, key exchange, and host key algorithm a server supports
  • Score each host 0–100 based on algorithm strength
  • Detect weak and deprecated algorithms (3DES, arcfour, HMAC-MD5, DSA, ...)
  • Flag algorithms with suspected NSA involvement (NIST P-curves and related)
  • Check results against five compliance frameworks: NIST, FIPS 140-2, BSI TR-02102, ANSSI, PRIVACY_FOCUSED
  • Scan a single host, a comma-separated list, a file (JSON/YAML/CSV/TXT), or stdin
  • Filter live output by algorithm category or host result
  • Rate-limit connections to protect fragile targets
  • Export results as JSON, CSV, or YAML
  • Run fully unattended with --summary-only (spinner on stderr, report at the end)

What it cannot do

  • Not a port scanner — assumes SSH is running on the given port; does not discover open ports
  • No authentication — uses PreferredAuthentications=none; never logs in, never executes commands
  • No configuration push — read-only analysis only
  • No CVE integration — does not map findings to CVE IDs
  • No daemon / continuous monitoring — single-run tool
  • Windows: works fine under WSL (Windows Subsystem for Linux); native Windows requires the OpenSSH client (winget install Microsoft.OpenSSH.Beta or via Optional Features) — the tool should run but is untested, and /etc/sshscan/ auto-discovery does not apply
  • No paramiko — depends on the system SSH binary; behavior reflects the installed SSH client version
  • No scan resume — interrupted scans cannot be continued

Requirements

Requirement Minimum
Python 3.8+
OpenSSH client ssh in PATH
PyYAML for YAML host files and YAML export 
pip install -r requirements.txt
# or individually:
pip install pyyaml

Supported OS

Scanner host (where you run sshscan.py)

Platform Status Notes
Ubuntu 20.04 / 22.04 / 24.04 Supported Primary development platform
Debian 11 (Bullseye) / 12 (Bookworm) Supported
Debian-based distros Supported Linux Mint, Raspberry Pi OS, Pop!_OS, Kali Linux, Parrot OS, MX Linux, Zorin OS and any other Debian/Ubuntu derivative — works as long as Python 3.8+ and openssh-client are installed
RHEL / Rocky Linux / AlmaLinux 8 Supported
RHEL / Rocky Linux / AlmaLinux 9 Supported
Fedora 38+ Supported
Arch Linux Supported
macOS 12 Monterey / 13 Ventura / 14 Sonoma / 15 Sequoia Supported Uses system OpenSSH; no extra dependencies
Windows — WSL (Ubuntu / Debian) Supported Run inside a WSL instance; full feature parity
Windows 10/11 — native OpenSSH Untested Requires OpenSSH client via winget install Microsoft.OpenSSH.Beta or Optional Features; /etc/sshscan/ auto-discovery does not apply
FreeBSD / OpenBSD / NetBSD Untested Should work with Python 3.8+ and OpenSSH client; not regularly tested
Alpine Linux Untested Requires openssh-client package; the BusyBox ssh stub is not sufficient

OpenSSH client version on the scanner host:

  • Minimum: OpenSSH 7.x — covers all algorithms except the post-quantum KEX entries
  • Full feature support: OpenSSH 9.9+ — required to probe mlkem768x25519-sha256 (ML-KEM)
  • OpenSSH 8.5–9.8 — can probe sntrup761x25519-sha512@openssh.com (NTRU Prime hybrid)

The scanner detects which algorithms the local SSH client supports and skips those it cannot probe, so operation is graceful on older OpenSSH versions.

Scan targets (what can be scanned)

Any SSH server is a valid target regardless of OS or hardware:

Target type Examples
Linux servers Any distribution running OpenSSH or Dropbear
macOS Built-in OpenSSH server
*BSD OpenBSD, FreeBSD, NetBSD
Windows Windows OpenSSH Server (sshd via Optional Features)
Network devices Cisco IOS/IOS-XE/NX-OS, Juniper JunOS, Palo Alto PAN-OS, F5 BIG-IP
Embedded / IoT Routers, NAS devices, industrial controllers with SSH
Cloud instances AWS EC2, GCP Compute, Azure VM — any SSH-accessible endpoint

Installation

macOS (Homebrew)

The recommended installation method on macOS. Homebrew manages Python, PyYAML, and future upgrades automatically.

brew tap rtulke/sshscan
brew install sshscan

Verify:

sshscan --version

Upgrade when a new version is released:

brew update
brew upgrade sshscan

Uninstall:

brew uninstall sshscan
brew untap rtulke/sshscan

The Homebrew tap is at https://github.com/rtulke/homebrew-sshscan.

Quick (system Python)

git clone https://github.com/rtulke/sshscan.git
cd sshscan
pip install pyyaml
chmod +x sshscan.py
./sshscan.py --version

Windows (WSL or native OpenSSH)

WSL — open a WSL terminal and follow the Linux instructions above.

Native Windows — install the OpenSSH client first, then:

winget install Microsoft.OpenSSH.Beta   # or via Settings > Optional Features
git clone https://github.com/rtulke/sshscan.git
cd sshscan
python -m venv .venv
.venv\Scripts\activate.bat
pip install -r requirements.txt
python sshscan.py --version

Note: /etc/sshscan/ config auto-discovery does not apply on Windows; use --config or place sshscan.conf in the working directory.

Virtual environment (recommended)

git clone https://github.com/rtulke/sshscan.git
cd sshscan
python3 -m venv .venv
source .venv/bin/activate
pip install -r requirements.txt
python3 sshscan.py --version

System-wide configuration

mkdir -p ~/.conf
cp sshscan.conf ~/.conf/sshscan.conf
# Edit to taste — loaded automatically without --config

Tab Completion (Linux)

The completion script lives in completion/sshscan.bash-completion and works with any distribution that has the bash-completion package installed.

Install bash-completion if not already present

# Debian / Ubuntu / Mint / Raspberry Pi OS
sudo apt install bash-completion

# RHEL / Rocky / AlmaLinux / Fedora
sudo dnf install bash-completion

# Arch Linux
sudo pacman -S bash-completion

Install the sshscan completion — system-wide

sudo cp completion/sshscan.bash-completion /etc/bash_completion.d/sshscan

Install for your user only (no sudo required)

mkdir -p ~/.local/share/bash-completion/completions
cp completion/sshscan.bash-completion ~/.local/share/bash-completion/completions/sshscan

Open a new shell and tab completion is active. To test immediately in the current session without installing:

source completion/sshscan.bash-completion

Configuration

Config file auto-discovery (no --config needed)

The scanner checks these locations in order and loads the first file found:

Priority Path Used when
1 ./sshscan.conf file exists in the current working directory
2 ~/.conf/sshscan.conf user-level config
3 /etc/sshscan/sshscan.conf system-wide config

--config FILE overrides auto-discovery entirely and always takes precedence. CLI arguments always override config file values.

[scanner] keys

Key Type Range Default CLI equivalent
threads int 1–500 20 --threads / -T
timeout int (s) 1–120 10 --timeout / -t
retry_attempts int 1–10 3 --retry-attempts
dns_cache_ttl int (s) 60–3600 300 —
banner_timeout int (s) 1–30 min(timeout, 5) --timeout-banner
rate_limit float 0.1–1000 unlimited --rate-limit
strict_host_key_checking string yes / no / accept-new accept-new --strict-host-key-checking
jump_host string [user@]host[:port] — --jump-host
proxy_command string ProxyCommand — --proxy-command

[compliance] keys

Key Type Default CLI equivalent
framework string none --compliance

Minimal config example

[scanner]
threads = 50
timeout = 10

[compliance]
framework = NIST

Full config example

[scanner]
threads = 30
timeout = 15
retry_attempts = 3
dns_cache_ttl = 600
banner_timeout = 3
rate_limit = 10.0
strict_host_key_checking = accept-new

[compliance]
framework = BSI_TR_02102

Privacy-focused preset

Use privacy_focus.conf (included) for a ready-made configuration that enforces the PRIVACY_FOCUSED compliance framework:

python3 sshscan.py --config privacy_focus.conf --file hosts.txt

Complete Parameter Reference

General

Parameter Short Description
--version -V Print version and author, then exit
--config FILE -c Load configuration from FILE (INI format)
--help -h Show help and exit

Host specification (mutually exclusive)

Parameter Short Description
--host HOSTS -H Single host or comma-separated list, e.g. host1,host2:2222
--file FILE -f File containing hosts (.json, .yaml, .csv, .txt)
--local -l Scan local SSH server at 127.0.0.1
(stdin) Pipe hosts when no other source is given

Scanning options

Parameter Short Default Description
--port PORT -p 22 Default SSH port for hosts without explicit port
--threads N -T 20 Concurrent scan threads
--timeout SEC -t 10 SSH connection timeout in seconds
--retry-attempts N 3 Retry attempts with exponential backoff
--rate-limit N unlimited Max new SSH connections per second
--timeout-banner SEC min(timeout,5) Timeout for initial SSH banner grab only
--strict-host-key-checking MODE accept-new SSH StrictHostKeyChecking: yes / no / accept-new
--jump-host [USER@]HOST[:PORT] — Route all connections through an SSH jump / bastion host
--proxy-command CMD — Route all connections via a ProxyCommand (SOCKS5/HTTP CONNECT)
--prefer-ipv6 off Prefer IPv6 (AAAA) when a host resolves to both A and AAAA records
--ipv6-only off Scan only via IPv6; skip hosts that have no AAAA record (implies --prefer-ipv6)

Algorithm testing

Parameter Short Description
--explicit ALGOS -e Test only the given comma-separated algorithms instead of the full set

Compliance

Parameter Description
--compliance FRAMEWORK Check results against a framework (see table below)
--list-frameworks List available compliance frameworks and exit
--list-algorithms List all scannable algorithms grouped by type, with weak/NSA annotations, and exit
--no-nsa-warnings Suppress NSA risk annotations in live output (analysis still runs; data included in exports)

Output

Parameter Short Description
--format FORMAT Export format: json, csv, yaml
--output FILE -o Write exported results to FILE (default: stdout)
--filter TOKENS Filter live output (see Filter section)
--list-filter List all filter tokens and exit
--summary Print aggregated summary after scan
--summary-only Suppress live output; show only summary (with spinner)
--verbose -v Verbose logging (INFO level)
--debug Full debug logging with function names and line numbers

Usage Examples

Single host

python3 sshscan.py --host example.com
python3 sshscan.py --host example.com:2222
python3 sshscan.py --host 192.168.1.1

# IPv6
python3 sshscan.py --host "[2001:db8::1]:22"
python3 sshscan.py --host "[::1]"
python3 sshscan.py --host 2001:db8::1
python3 sshscan.py --host "[2001:db8::1]:22" --prefer-ipv6

Multiple hosts

# Comma-separated inline
python3 sshscan.py --host "server1.com,server2.com:2222,192.168.1.100"

# From a text file
python3 sshscan.py --file hosts.txt

# From stdin (any mix of commas, spaces, newlines)
echo "server1.com server2.com:2222" | python3 sshscan.py
cat hosts.txt | python3 sshscan.py

# Multiple formats work
printf "server1.com\n192.168.1.1:2222\nserver3.com" | python3 sshscan.py

# IPv6 — mixed with IPv4 in a comma-separated list
python3 sshscan.py --host "192.168.1.1,[2001:db8::1]:22,[::1]"

# IPv6 — from a hosts file (bracket notation, one per line)
# hosts.txt:
#   [2001:db8::1]:22
#   [2001:db8::2]:2222
#   [::1]
python3 sshscan.py --file hosts.txt

# IPv6 — prefer AAAA when hostnames resolve to both A and AAAA
python3 sshscan.py --file hosts.txt --prefer-ipv6

# IPv6 — skip hosts that have no AAAA record
python3 sshscan.py --file hosts.txt --ipv6-only

Local SSH server

python3 sshscan.py --local
python3 sshscan.py --local --port 2222

Compliance checking

# Check against NIST framework
python3 sshscan.py --host example.com --compliance NIST

# Strict BSI check across a server fleet
python3 sshscan.py --file servers.txt --compliance BSI_TR_02102

# Privacy-focused: exclude all NIST/NSA-suspected algorithms
python3 sshscan.py --file hosts.txt --compliance PRIVACY_FOCUSED

# List all available frameworks
python3 sshscan.py --list-frameworks

Output filtering

# Show only NSA-flagged algorithms
python3 sshscan.py --host example.com --filter nsa

# Show only weak algorithms
python3 sshscan.py --file hosts.txt --filter weak

# Show all flagged algorithms (weak + NSA combined)
python3 sshscan.py --file hosts.txt --filter flagged

# Type tokens: show only KEX algorithms
python3 sshscan.py --host example.com --filter kex

# Type + category: show only weak KEX algorithms
python3 sshscan.py --file hosts.txt --filter kex,weak

# Type + category: show weak ciphers and MACs
python3 sshscan.py --file hosts.txt --filter cipher,mac,weak

# Output mode: show only security score and compliance line per host (no algo lines)
python3 sshscan.py --file hosts.txt --compliance NIST --filter security

# Output mode: show only SSH banners per host
python3 sshscan.py --file hosts.txt --filter banner

# Output mode + category: banners + NSA algorithm lines
python3 sshscan.py --file hosts.txt --filter banner,nsa

# Show only hosts that failed compliance
python3 sshscan.py --file hosts.txt --compliance NIST --filter failed

# Show only scan errors
python3 sshscan.py --file hosts.txt --filter error

# Combine: show NSA algorithms on failed hosts only
python3 sshscan.py --file hosts.txt --compliance NIST --filter nsa,failed

# List all filter tokens
python3 sshscan.py --list-filter

Export results

# JSON export to stdout
python3 sshscan.py --file hosts.txt --format json

# JSON export to file
python3 sshscan.py --file hosts.txt --format json --output results.json

# CSV for spreadsheet import
python3 sshscan.py --file hosts.txt --compliance NIST --format csv --output audit.csv

# YAML
python3 sshscan.py --file hosts.txt --format yaml --output results.yaml

Summary mode

# Suppress live output, show only report at the end
python3 sshscan.py --file hosts.txt --summary-only

# Live output + summary at the end
python3 sshscan.py --file hosts.txt --summary

# summary-only with compliance and JSON export
python3 sshscan.py --file hosts.txt --compliance NIST --summary-only --format json --output results.json

Performance tuning

# Fast scan of a large network
python3 sshscan.py --file large_network.txt --threads 100 --timeout 5

# Gentle scan for rate-sensitive targets (5 connections/sec)
python3 sshscan.py --file hosts.txt --rate-limit 5.0

# Slow network: long timeout, short banner timeout
python3 sshscan.py --file hosts.txt --timeout 30 --timeout-banner 5

# Conservative: 10 threads, 3 retries, long timeout
python3 sshscan.py --file hosts.txt --threads 10 --timeout 30 --retry-attempts 5

Specific algorithm testing

# Test only modern recommended algorithms
python3 sshscan.py --host example.com \
  --explicit "chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,curve25519-sha256,ssh-ed25519"

# Check if legacy weak algorithms are still supported
python3 sshscan.py --file hosts.txt \
  --explicit "3des-cbc,arcfour,hmac-md5,ssh-dss"

NSA analysis

# Show full NSA risk analysis per host (default)
python3 sshscan.py --host example.com

# Suppress NSA annotations in live output (data still in exports)
python3 sshscan.py --file hosts.txt --no-nsa-warnings --format json --output results.json

Using a config file

# Explicit config
python3 sshscan.py --config sshscan.conf --file servers.txt

# Override config values on the CLI
python3 sshscan.py --config sshscan.conf --threads 50 --timeout 20

# Use privacy preset
python3 sshscan.py --config privacy_focus.conf --file hosts.txt

# Auto-loaded from ~/.conf/sshscan.conf (no --config needed)
python3 sshscan.py --file servers.txt

Jump host / proxy

# All hosts through a bastion (SSH jump host)
python3 sshscan.py --file internal-hosts.txt --jump-host admin@bastion.corp:22

# Multiple jump hops (comma-separated, OpenSSH syntax)
python3 sshscan.py --file hosts.txt --jump-host "hop1.corp,admin@hop2.corp:2222"

# All hosts through a SOCKS5 proxy (e.g. an SSH -D tunnel)
python3 sshscan.py --file hosts.txt --proxy-command "nc -X 5 -x 127.0.0.1:1080 %h %p"

# All hosts through an HTTP CONNECT proxy
python3 sshscan.py --file hosts.txt --proxy-command "nc -X connect -x proxy.corp:3128 %h %p"

Per-host proxy via YAML host file:

# hosts.yaml
- host: internal1.corp
  via:
    type: jump
    host: bastion1.corp
    port: 22
    user: admin

- host: internal2.corp
  via:
    type: socks5
    host: 127.0.0.1
    port: 1080

- host: public.example.com   # no via — direct connection

Per-host proxy via CSV host file (columns: host,port,via_type,via_host[,via_port[,via_user]]):

internal1.corp,22,jump,bastion1.corp,22,admin
internal2.corp,22,socks5,127.0.0.1,1080
dmz-host.corp,2222,http,proxy.corp,3128
public.example.com,22

Debug and logging

# Verbose INFO logging
python3 sshscan.py --host example.com --verbose

# Full debug output (function names, line numbers)
python3 sshscan.py --host example.com --debug

Jump Hosts and Proxies

Route scan traffic through SSH jump hosts (bastions) or SOCKS5/HTTP CONNECT proxies. Useful for scanning internal networks that are not directly reachable from the scanning machine.

Global proxy (all hosts)

Apply the same proxy to every host in a scan run:

# SSH jump host
python3 sshscan.py --file internal.yaml --jump-host admin@bastion.corp

# SOCKS5 (e.g. an SSH -D dynamic tunnel)
python3 sshscan.py --file hosts.txt --proxy-command "nc -X 5 -x 127.0.0.1:1080 %h %p"

# HTTP CONNECT proxy
python3 sshscan.py --file hosts.txt --proxy-command "nc -X connect -x proxy.corp:3128 %h %p"

Or set a global configuration in sshscan.conf:

[scanner]
jump_host = admin@bastion.corp:22
# proxy_command = nc -X 5 -x socks5proxy.corp:1080 %h %p

Per-host proxy (mixed environments)

Different hosts can have different proxies by adding a via field to JSON/YAML host files, or extra columns to CSV files.

YAML — via dict with keys: type (jump/socks5/http), host, port, user (optional):

- host: internal-db.corp
  via: {type: jump, host: bastion1.corp, port: 22, user: dbteam}

- host: dmz-web.corp
  via: {type: http, host: proxy.corp, port: 3128}

- host: external.example.com
  # no via — direct connection

JSON — same structure:

[
  {"host": "internal-db.corp", "port": 22, "via": {"type": "jump", "host": "bastion1.corp", "user": "dbteam"}},
  {"host": "dmz-web.corp",     "port": 22, "via": {"type": "http",  "host": "proxy.corp", "port": 3128}},
  {"host": "external.example.com"}
]

CSV — extra columns after host,port: via_type,via_host[,via_port[,via_user]]:

internal-db.corp,22,jump,bastion1.corp,22,dbteam
dmz-web.corp,22,http,proxy.corp,3128
external.example.com,22

Priority order

Per-host via → --jump-host (global) → --proxy-command (global) → direct connection.

Notes

  • Jump hosts require SSH key auth (no password prompt) — BatchMode=yes is always set
  • When a proxy is active, the SSH banner is fetched via the SSH binary rather than a raw socket
  • Use --rate-limit to avoid overwhelming the bastion with parallel connections

Output Filtering

--filter accepts comma-separated tokens from any of the groups below. All groups are composable. Type and category tokens combine with AND within their group. Run --list-filter for a full reference with examples.

Category tokens

Filter by security classification:

Token Shows Marker
supported Algorithms the server supports, no warning [x]
unsupported Algorithms the server does not support [-]
flagged All flagged algorithms (weak + NSA combined) [!]
weak Weak/deprecated algorithms only (subset of flagged) [!]
nsa NSA-suspected algorithms only (subset of flagged) [!]

Type tokens

Filter by protocol layer. Composable with category tokens (--filter kex,weak = weak KEX only):

Token Shows
cipher Cipher / encryption algorithm lines
mac MAC algorithm lines
kex Key exchange algorithm lines
hostkey Host key algorithm lines

Output mode tokens

Suppress per-algorithm lines entirely; show only the specified host-level line. Pairing with type or category tokens re-enables those matching algorithm lines.

Token Shows
security Security score and compliance line per host
banner SSH banner line per host

Host tokens

Show only hosts matching the condition. All output for a host is buffered until the scan completes, then flushed or discarded based on the result.

Token Shows
passed Hosts that passed the compliance check (requires --compliance)
failed Hosts that failed the compliance check (requires --compliance)
error Hosts where the scan failed (connection error, timeout, DNS failure)

Compliance Frameworks

Framework Description Minimum Score
NIST NIST Cybersecurity Framework — balanced baseline 70
FIPS_140_2 FIPS 140-2 Level 1 — requires NIST curves, forbids Curve25519 90
BSI_TR_02102 German BSI TR-02102-4 — very strict, requires ETM MACs 85
ANSSI French ANSSI guidelines — highest strictness 90
PRIVACY_FOCUSED Anti-surveillance — forbids all NIST curves, requires Curve25519/Ed25519 95

Each framework defines required and forbidden algorithms per category (cipher, MAC, KEX, host key) and a minimum security score. A host is compliant only when all required algorithms are present, no forbidden algorithms are present, and the score meets the minimum.

NSA Backdoor Detection

The scanner flags algorithms with suspected NSA design influence based on public research and the Snowden disclosures.

High-risk (NIST P-curves)

Category Algorithms
Key exchange ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521
Host keys ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521

Recommended alternatives

Purpose Recommended
Key exchange curve25519-sha256, mlkem768x25519-sha256 (post-quantum)
Host keys ssh-ed25519
Encryption chacha20-poly1305@openssh.com, aes256-gcm@openssh.com
MAC hmac-sha2-256-etm@openssh.com, hmac-sha2-512-etm@openssh.com

NSA analysis always runs. Use --no-nsa-warnings to suppress the annotations in live output; the data is still included in all exports.

Security Scoring

Each host is scored 0–100 based on the algorithms it supports.

Score Rating
90–100 Excellent — modern algorithms only
70–89 Good — mostly modern, few legacy
50–69 Fair — mixed
30–49 Poor — many weak algorithms
0–29 Critical — predominantly weak or NSA-flagged

Weak algorithms reduce the score proportionally. NSA-flagged algorithms apply a 1.5× penalty. The score must meet the framework's minimum_score threshold for compliance to pass.

Weak algorithms detected

Category Algorithms
Cipher DES, 3DES-CBC, Blowfish-CBC, CAST128-CBC, Arcfour, AES-CBC modes
MAC HMAC-MD5, HMAC-SHA1, HMAC-SHA1-96, UMAC-64 (and ETM variants)
KEX DH-Group1-SHA1, DH-Group14-SHA1, DH-GEX-SHA1
Host keys DSA, RSA

Input File Formats

Text file (.txt)

server1.example.com
server2.example.com:2222
192.168.1.100
# Lines starting with # are ignored
[::1]:22

JSON (.json)

[
  "server1.example.com",
  "server2.example.com:2222",
  {"host": "server3.example.com", "port": 2222},
  "192.168.1.100"
]

YAML (.yaml / .yml)

- server1.example.com
- server2.example.com:2222
- host: server3.example.com
  port: 2222

CSV (.csv)

server1.example.com,22
server2.example.com,2222
192.168.1.100,22

Duplicate hosts (same IP + port) are silently skipped regardless of file format. IPv6 addresses can be specified in three ways:

  • Bracket notation with port: [::1]:22, [2001:db8::1]:2222
  • Bracket notation without port (uses --port default): [::1], [2001:db8::1]
  • Bare address without port: 2001:db8::1

Exit Codes

Code Meaning
0 All hosts scanned successfully, no compliance failures
1 Fatal error or usage error (bad arguments, file not found, etc.)
2 One or more hosts failed compliance check
3 One or more hosts had scan errors (connection failure, timeout, DNS)

Codes 2 and 3 can occur together; code 2 takes precedence.

Troubleshooting

All hosts time out Increase timeout or reduce threads: --timeout 30 --threads 10

"No matching cipher found" for everything The local ssh binary may not support the algorithm. Check: ssh -Q cipher

Scan is too slow Increase threads: --threads 50. Ensure DNS resolves quickly (or use IP addresses directly).

Scan is hammering a target Use --rate-limit 2.0 to cap at 2 new connections per second.

IPv6 hosts not connecting Use bracket notation: --host "[2001:db8::1]:22" or a bare address --host "2001:db8::1" (uses default port). If the host resolves to both A and AAAA records and IPv4 is being used, add --prefer-ipv6 to force IPv6. To skip all hosts that have no AAAA record, use --ipv6-only.

All algorithms show as not supported on a specific host The host may be blocking the connection entirely (firewall, wrong port). Check the SSH banner: if it's empty, the host is not reachable on that port.

Hardening Guide

For a comprehensive step-by-step guide covering firewall rules, 2FA, certificate authorities, jump hosts, cloud environments, and incident response, see the SSH Hardening Guide. Ready-to-use config profiles are also available in hardening-examples/.

Once sshscan reports weak or NSA-flagged algorithms, the next step is removing them from the server configuration and optionally restricting what the SSH client will accept. The configs below map directly to sshscan's compliance frameworks.

Before applying any server config: ensure you have out-of-band console access (KVM, cloud console, serial port). A broken sshd config that prevents the daemon from starting will lock you out if SSH is your only access path. Always run sudo sshd -t before reloading.

OpenSSH Server (/etc/ssh/sshd_config)

Balanced — no weak algorithms, NIST curves allowed

Removes all weak and deprecated algorithms. NIST P-curves (ecdh-sha2-nistp*, ecdsa-sha2-nistp*) are kept for broader client compatibility. Matches the NIST, BSI_TR_02102, and ANSSI compliance frameworks.

# /etc/ssh/sshd_config

# Modern AEAD ciphers only — no CBC, no arcfour, no 3DES
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr

# ETM MACs preferred; SHA-2 only — no MD5, no SHA-1, no 64-bit UMAC
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com

# Modern key exchange — DH group 14+ (SHA-2), Curve25519, NIST ECDH
KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group16-sha512,diffie-hellman-group14-sha256,diffie-hellman-group-exchange-sha256,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256

# Host key types to advertise — Ed25519 and RSA-SHA2; no DSA, no legacy RSA/SHA-1
HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-256,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com

Strict — no NIST curves (anti-surveillance / PRIVACY_FOCUSED)

Also removes NIST P-curves. Only Curve25519 and ChaCha20/AES-GCM remain. Requires OpenSSH 7.x+ on the client side. Matches the PRIVACY_FOCUSED compliance framework.

# /etc/ssh/sshd_config

# ChaCha20 and AES-GCM only — no NIST-derived constructs
Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com

# ETM MACs only
MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com

# Curve25519 only — no NIST P-curves
# sntrup761x25519-sha512@openssh.com adds post-quantum protection (requires OpenSSH 8.5+)
KexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org

# Ed25519 only — no RSA, no ECDSA
HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com

Apply and verify

# 1. Test the config — catches syntax errors and unsupported algorithm names
sudo sshd -t

# 2. Reload sshd without dropping existing sessions
sudo systemctl reload sshd
# or on older init systems:
sudo service ssh reload

# 3. Confirm the daemon is still running
sudo systemctl status sshd

Generate a missing Ed25519 host key

The strict config requires an Ed25519 host key. Generate one if it does not exist yet:

sudo ssh-keygen -t ed25519 -f /etc/ssh/ssh_host_ed25519_key -N ""

Then make sure sshd_config includes the key:

HostKey /etc/ssh/ssh_host_ed25519_key

OpenSSH Client (~/.ssh/config)

Client restrictions apply to all outgoing connections from your machine. Replace Host * with a specific hostname to restrict only one target.

Balanced — no weak algorithms, NIST curves allowed

# ~/.ssh/config

Host *
    Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
    MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512,hmac-sha2-256,umac-128@openssh.com
    KexAlgorithms curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group18-sha512,diffie-hellman-group16-sha512,diffie-hellman-group14-sha256,diffie-hellman-group-exchange-sha256,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256
    HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,rsa-sha2-512,rsa-sha2-256,rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com

Strict — no NIST curves

# ~/.ssh/config

Host *
    Ciphers chacha20-poly1305@openssh.com,aes256-gcm@openssh.com
    MACs hmac-sha2-512-etm@openssh.com,hmac-sha2-256-etm@openssh.com,umac-128-etm@openssh.com
    KexAlgorithms sntrup761x25519-sha512@openssh.com,curve25519-sha256,curve25519-sha256@libssh.org
    HostKeyAlgorithms ssh-ed25519,ssh-ed25519-cert-v01@openssh.com

A strict client config will refuse to connect to servers that only offer NIST curves or legacy host keys. Use a per-host override to allow exceptions:

# Allow legacy algorithms only for this specific host
Host legacy.example.com
    KexAlgorithms +ecdh-sha2-nistp256
    HostKeyAlgorithms +ecdsa-sha2-nistp256,rsa-sha2-256

Verify with sshscan

After applying server changes, re-scan to confirm the result:

# Check the local server — should show no [!] lines
python3 sshscan.py --local --filter flagged

# Full compliance check against specific framework
python3 sshscan.py --local --compliance PRIVACY_FOCUSED --summary

# Remote host after hardening
python3 sshscan.py --host server.example.com --filter weak,nsa
python3 sshscan.py --host server.example.com --compliance BSI_TR_02102

About

A single-file Python tool for auditing SSH algorithm configuration across one or many hosts. Wraps the system ssh binary to probe each algorithm individually — no paramiko, no authentication.

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ssh security-audit algorithm cipher scanner ssh-server ssh-key security-tools cipher-algorithms audit-tools ssh-scanner

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MIT license
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