package main import "strings" // --------------------------------------------------------------------------- // The ssh_config keyword table // --------------------------------------------------------------------------- // // This is the documented subset SSHDesk recognises. A keyword outside this // table is reported as unknown with a nearest-match suggestion; it is still // parsed and still resolved, because ssh itself would only warn. // knownKeywords maps the lowercased keyword to its canonical spelling. var knownKeywords = map[string]string{} // canonicalKeywords is the list used to build knownKeywords and to power the // nearest-match suggestion for unknown keywords. var canonicalKeywords = []string{ "AddKeysToAgent", "AddressFamily", "BatchMode", "BindAddress", "BindInterface", "CanonicalDomains", "CanonicalizeFallbackLocal", "CanonicalizeHostname", "CanonicalizeMaxDots", "CanonicalizePermittedCNAMEs", "CASignatureAlgorithms", "CertificateFile", "ChannelTimeout", "CheckHostIP", "Ciphers", "ClearAllForwardings", "Compression", "ConnectionAttempts", "ConnectTimeout", "ControlMaster", "ControlPath", "ControlPersist", "DynamicForward", "EnableEscapeCommandline", "EnableSSHKeysign", "EscapeChar", "ExitOnForwardFailure", "FingerprintHash", "ForkAfterAuthentication", "ForwardAgent", "ForwardX11", "ForwardX11Timeout", "ForwardX11Trusted", "GatewayPorts", "GlobalKnownHostsFile", "GSSAPIAuthentication", "GSSAPIDelegateCredentials", "HashKnownHosts", "Host", "HostbasedAcceptedAlgorithms", "HostbasedAuthentication", "HostKeyAlgorithms", "HostKeyAlias", "HostName", "IdentitiesOnly", "IdentityAgent", "IdentityFile", "IgnoreUnknown", "Include", "IPQoS", "KbdInteractiveAuthentication", "KbdInteractiveDevices", "KexAlgorithms", "KnownHostsCommand", "LocalCommand", "LocalForward", "LogLevel", "LogVerbose", "MACs", "Match", "NoHostAuthenticationForLocalhost", "NumberOfPasswordPrompts", "ObscureKeystrokeTiming", "PasswordAuthentication", "PermitLocalCommand", "PermitRemoteOpen", "PKCS11Provider", "Port", "PreferredAuthentications", "ProxyCommand", "ProxyJump", "ProxyUseFdpass", "PubkeyAcceptedAlgorithms", "PubkeyAuthentication", "RekeyLimit", "RemoteCommand", "RemoteForward", "RequestTTY", "RequiredRSASize", "RevokedHostKeys", "SecurityKeyProvider", "SendEnv", "ServerAliveCountMax", "ServerAliveInterval", "SessionType", "SetEnv", "StdinNull", "StreamLocalBindMask", "StreamLocalBindUnlink", "StrictHostKeyChecking", "SyslogFacility", "TCPKeepAlive", "Tag", "Tunnel", "TunnelDevice", "UpdateHostKeys", "User", "UserKnownHostsFile", "VerifyHostKeyDNS", "VisualHostKey", "XAuthLocation", } // deprecatedKeywords are accepted by some ssh versions but should not appear // in a modern config. The value is the advice printed by `check`. var deprecatedKeywords = map[string]string{ "cipher": "removed in OpenSSH 7.6; use Ciphers", "compressionlevel": "removed in OpenSSH 7.4; it only ever applied to SSHv1", "protocol": "removed in OpenSSH 7.6; SSHv1 no longer exists", "rhostsrsaauthentication": "removed in OpenSSH 7.4 (SSHv1 only)", "rsaauthentication": "removed in OpenSSH 7.4 (SSHv1 only)", "useprivilegedport": "removed in OpenSSH 7.5", "useroaming": "removed in OpenSSH 7.1p2 (CVE-2016-0777)", "gssapitrustdns": "not present in portable OpenSSH by default", "challengeresponseauthentication": "renamed to KbdInteractiveAuthentication " + "in OpenSSH 8.7", } // listKeywords accumulate: every matching block contributes a value instead of // the first one winning outright. This is real OpenSSH behaviour and it is the // only exception to first-obtained-value-wins. var listKeywords = map[string]bool{ "identityfile": true, "certificatefile": true, "localforward": true, "remoteforward": true, "dynamicforward": true, "sendenv": true, "setenv": true, "canonicalizepermittedcnames": true, } func init() { for _, k := range canonicalKeywords { knownKeywords[strings.ToLower(k)] = k } // OpenSSH accepts "Hostname" and "HostName" identically; the lowercase // map handles that, but the alias below keeps the canonical spelling. knownKeywords["hostname"] = "HostName" knownKeywords["macs"] = "MACs" } func isKnownKeyword(lower string) bool { _, ok := knownKeywords[lower] if ok { return true } _, ok = deprecatedKeywords[lower] return ok } func canonicalKeyword(lower string) string { if c, ok := knownKeywords[lower]; ok { return c } return lower } // --------------------------------------------------------------------------- // Nearest-match suggestion // --------------------------------------------------------------------------- // editDistance is the standard Levenshtein distance over bytes. func editDistance(a, b string) int { if a == b { return 0 } if len(a) == 0 { return len(b) } if len(b) == 0 { return len(a) } prev := make([]int, len(b)+1) cur := make([]int, len(b)+1) for j := 0; j <= len(b); j++ { prev[j] = j } for i := 1; i <= len(a); i++ { cur[0] = i for j := 1; j <= len(b); j++ { cost := 1 if a[i-1] == b[j-1] { cost = 0 } del := prev[j] + 1 ins := cur[j-1] + 1 sub := prev[j-1] + cost m := del if ins < m { m = ins } if sub < m { m = sub } cur[j] = m } prev, cur = cur, prev } return prev[len(b)] } // suggestKeyword returns the closest known keyword to the given unknown one, // or "" when nothing is close enough to be worth printing. func suggestKeyword(unknown string) string { u := strings.ToLower(unknown) best, bestDist := "", 1<<30 for _, k := range canonicalKeywords { d := editDistance(u, strings.ToLower(k)) if d < bestDist || (d == bestDist && k < best) { best, bestDist = k, d } } // Allow roughly one edit per three characters, capped at 4. limit := len(u)/3 + 1 if limit > 4 { limit = 4 } if bestDist > limit { return "" } return best } // --------------------------------------------------------------------------- // Settings that quietly weaken security // --------------------------------------------------------------------------- type weakRule struct { Key string // lowercased keyword Value string // lowercased value that triggers it, "" for any value Severity string Message string } var weakRules = []weakRule{ {"stricthostkeychecking", "no", "error", "host key verification is disabled: ssh will silently accept any key, " + "which is exactly what a man-in-the-middle needs"}, {"stricthostkeychecking", "off", "error", "host key verification is disabled ('off' is a synonym for 'no')"}, {"stricthostkeychecking", "accept-new", "info", "new host keys are accepted without asking; changed keys are still " + "refused. Weaker than 'yes' but far better than 'no'"}, {"userknownhostsfile", "/dev/null", "error", "host keys are thrown away, so every connection is a first connection " + "and key changes can never be detected"}, {"forwardagent", "yes", "warning", "agent forwarding lets anyone with root on the remote host use your " + "keys for as long as you are connected"}, {"forwardx11", "yes", "warning", "X11 forwarding exposes your local X server to the remote host"}, {"forwardx11trusted", "yes", "warning", "trusted X11 forwarding disables the X11 security extension entirely"}, {"gssapidelegatecredentials", "yes", "warning", "delegating GSSAPI credentials hands your Kerberos ticket to the server"}, {"checkhostip", "no", "info", "the host key is no longer checked against the server's IP address"}, {"nohostauthenticationforlocalhost", "yes", "warning", "host keys are not verified for localhost, which matters when local " + "ports are forwarded to somewhere else"}, {"permitlocalcommand", "yes", "info", "the remote side of an escape sequence can run local commands"}, {"batchmode", "yes", "info", "password and passphrase prompts are disabled; connections fail " + "instead of prompting"}, {"identitiesonly", "no", "info", "every key in your agent is offered to the server, which leaks the " + "list of hosts you have keys for"}, {"hashknownhosts", "no", "info", "known_hosts is stored in clear text, so a reader of the file learns " + "every host you connect to"}, } // weakAlgorithms flags obsolete primitives inside an algorithm list. var weakAlgorithmKeywords = map[string]bool{ "ciphers": true, "macs": true, "kexalgorithms": true, "hostkeyalgorithms": true, "pubkeyacceptedalgorithms": true, "hostbasedacceptedalgorithms": true, "casignaturealgorithms": true, } var weakAlgorithms = map[string]string{ "3des-cbc": "56-bit effective 3DES", "blowfish-cbc": "64-bit block cipher, birthday-bound attacks", "cast128-cbc": "64-bit block cipher", "arcfour": "RC4 is broken", "arcfour128": "RC4 is broken", "arcfour256": "RC4 is broken", "hmac-md5": "MD5 is broken", "hmac-md5-96": "MD5 is broken", "hmac-sha1": "SHA-1 is deprecated", "hmac-sha1-96": "SHA-1 is deprecated", "umac-64@openssh.com": "64-bit tag is too short", "diffie-hellman-group1-sha1": "1024-bit group, SHA-1", "diffie-hellman-group14-sha1": "SHA-1", "diffie-hellman-group-exchange-sha1": "SHA-1", "ssh-dss": "DSA is limited to 1024 bits", "ssh-dss-cert-v01@openssh.com": "DSA is limited to 1024 bits", "ssh-rsa": "RSA with SHA-1 signatures", "ssh-rsa-cert-v01@openssh.com": "RSA with SHA-1 signatures", "rijndael-cbc@lysator.liu.se": "non-standard alias for AES-CBC", "hmac-ripemd160": "RIPEMD-160 is deprecated", }