Category Archives: Security

How to configure 802.1X client and server in FreeBSD

This article describes the steps required to configure 802.1X client and server using EAP-TLS both client and server side in FreeBSD.

Background

I recently bought a new switch of ebay capable of 802.1X PNAC (Port based Network Access Control). I wanted to have this set up for a long time, but it wasn’t unil now I had a switch thats actually supported it. This article describes how I got it up and running.

Certificates

Since I already have EAP-TLS set up for my wifi (authentication using X.509 client certificates) I will also use EAP-TLS for wired access. So I configured a private CA in order to issue both server certificates for the radius server and also client certificates for all clients that will use my network.  I have a more general post about how set up the CA here.

Radius server (Authentication server)

You will need to configure a Radius server to handle the authentication requests. I already have EAP-TLS configured using hostpad and its internal radius server for my wifi. But that server is very limited, so I decided to give FreeRADIUS a go. This also means that my wifi clients will be authenticated using the FreeRADIUS server from now on.

I understand that FreeRADIUS is very flexible and “easy” to customize, but I really think that the configuration is very hard do grasp. It would be virtually impossible to configure it without some guide to follow. The two big problems are that the configuration is split up into MANY files and that all the documentation is inside the config files, which makes them really hard to read. Luckily I found this guide online that did exactly what I wanted. So please have a look at that guide under “Configuration” to see how I configured FreeRADIUS. Its basically just a few minor changes in four files.

The switch (Authenticator)

This article will not cover the switch configuration needed for this setup. The configuration you will have to do is very depended on what brand of switch you have and what software it is running.  I have a Juniper EX2200-C and there is good online documentation on how to set up 802.1X.

The Supplicant (802.1X client)

In 802.1X the client is called the supplicant. To authenticate against the radius server you will basically need a small supplicant software installed on the client that will handle the authentication. This is done using EAPOL-packages that are sent out on the network and then handled by the switch (The Authenticator). The switch then talks to the raidus server (The Authentication server) to verify the client.

In Linux and FreeBSD the most commonly used supplicant software is called wpa_supplicant. Most of you who know of wpa_supplicant have used it for wifi authentication in differents forms. It can handle alot of different security types like WPA2 Enterprise, WPA2 or even WEP. But it can also work with wired network authentication. The configuration is actually very straight forward and similar to the wifi configs.

network={
        key_mgmt=IEEE8021X
        eap=TLS
        identity="identity"
        ca_cert="/etc/ssl/chain.pem"
        client_cert="/etc/ssl/client.cert.pem"
        private_key="/etc/ssl/client.key"
        private_key_passwd="passw0rd"
}

This is all you will need to have wpa_supplicant authenticate using client certificates over ethernet.

To have the wpa_supplicant automatically started when you FreeBSD machine boots you can just add the WPA keyword to your interface declaration in /etc/rc.conf like this:

ifconfig_ue0="DHCP WPA"

Smart Card/HSM backed OpenSSL CA

This article describes how to set up a Smart Card/HSM backed OpenSSL CA using a Smart Card HSM or any PKCS11 enabled device.

Background

Since some years back I use WPA2 Enterprise with EAP-TLS (Certificate authentication) for my wifi at home. Historically I have used certificates from a public CA for this purpose. This is not best practice since you don’t have control over the certificates that are issued.

Also, I recently bought a new switch capable of 802.1X authentication on all ports. For this purpose I want all my machines (even those without wifi) to have certificates. So I decided to go through the hassle of setting up my own private CA.

Setting up CA

For the basic setup of the CA I followed Jamies excellent guide on setting up a CA. So in this post you can assume that all the basic stuff like folders structure and basic commands are the same. I will only show you the differences needed to have the Root CA key stored on a PKCS11 device like a HSM, Smart Card HSM or a Yubikey. I will even try to follow his topic names so you can follow along.

Configure PKCS11 Engine

I will not discuss the operating system part of getting PKCS11 devices to work in this article. But basically you just need to install some packages, you can read about it here.

First of all we need to configure OpenSSL to talk to your PKCS11 device. This can be done from configuration or interactively on the command line.

From conf:

# At beginning of conf (before everything else)
openssl_conf            = openssl_def

# At end of conf (after everything else)
[openssl_def]
engines = engine_section

[engine_section]
pkcs11 = pkcs11_section

[pkcs11_section]
engine_id = pkcs11
dynamic_path = /usr/local/lib/engines/pkcs11.so
MODULE_PATH = /usr/local/lib/opensc-pkcs11.so
init = 0

From cli:

OpenSSL> engine -t dynamic -pre SO_PATH:/usr/local/lib/engines/pkcs11.so -pre ID:pkcs11 -pre LIST_ADD:1 -pre LOAD -pre MODULE_PATH:/usr/local/lib/opensc-
pkcs11.so

Create the root pair

First of all we need to have a RSA key pair on the PKCS11 device:

# pkcs11-tool --module /usr/local/lib/opensc-pkcs11.so -l --keypairgen --key-type rsa:2048 --label "SSL Root CA"
Using slot 0 with a present token (0x0)
Logging in to "HSM 2 (UserPIN)".
Please enter User PIN:
Key pair generated:
Private Key Object; RSA
  label:      SSL Root CA
  ID:         d15c3e9578a612a658bb14e0e147db4f2279cf19
  Usage:      decrypt, sign, unwrap
Public Key Object; RSA 2048 bits
  label:      SSL Root CA
  ID:         d15c3e9578a612a658bb14e0e147db4f2279cf19
  Usage:      encrypt, verify, wrap

Create the root certificate

I will assume that you have configured pkcs11 in openssl.cnf (otherwise you will have to first run the engine command in openssl interactively before any other command).

# openssl req -config openssl.cnf -new -x509 -days 7300 -sha256 -extensions v3_ca -engine pkcs11 -keyform engine -key 0:d15c3e9578a612a658bb14e0e147db4f2279cf19 -out certs/ca.cert.pem
engine "pkcs11" set.
Enter PKCS#11 token PIN for HSM 2 (UserPIN):
0x8018b6000 07:41:35.523 cannot lock memory, sensitive data may be paged to disk
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [SE]:
State or Province Name []:
Locality Name []:
Organization Name [PeanOrg]:
Organizational Unit Name []:PeanOrg Certificate Authority
Common Name []:PeanOrg Root CA
Email Address []:

Create the intermediate pair

For the intermediate key pair I followed jamies guide. I need frequent access to this CA so I have decided to have the intermediate pair on file instead of HSM.

Create the intermediate certificate

I changed one thing in jamies intermediate/openssl.cnf because I dont see the point of having province set in the CAs

stateOrProvinceName     = optional

To use the Root key stored on pkcs11 to sign the intermediate certificate use this command:

# openssl ca -config openssl.cnf -extensions v3_intermediate_ca -days 3650 -notext -md sha256 -engine pkcs11 -keyform engine -keyfile 0:d15c3e9578a612a658bb14e0e147db4f2279cf19 -in intermediate/csr/intermediate.csr.pem -out intermediate/certs/intermediate.cert.pem
Using configuration from openssl.cnf
engine "pkcs11" set.
Enter PKCS#11 token PIN for HSM 2 (UserPIN):
Check that the request matches the signature
Signature ok
Certificate Details:
        Serial Number: 4096 (0x1000)
        Validity
            Not Before: Apr  7 05:54:22 2018 GMT
            Not After : Apr  4 05:54:22 2028 GMT
        Subject:
            countryName               = SE
            organizationName          = PeanOrg
            organizationalUnitName    = PeanOrg Certificate Authority
            commonName                = PeanOrg Intermediate CA 1
        X509v3 extensions:
            X509v3 Subject Key Identifier:
                77:9C:07:23:FD:40:E9:5C:7E:30:73:8F:59:28:25:F5:06:43:B4:70
            X509v3 Authority Key Identifier:
                keyid:A4:F2:DE:15:8E:9E:A8:87:B0:95:D4:21:A2:BD:4C:41:02:93:E0:8D

            X509v3 Basic Constraints: critical
                CA:TRUE, pathlen:0
            X509v3 Key Usage: critical
                Digital Signature, Certificate Sign, CRL Sign
Certificate is to be certified until Apr  4 05:54:22 2028 GMT (3650 days)
Sign the certificate? [y/n]:y


1 out of 1 certificate requests certified, commit? [y/n]y
Write out database with 1 new entries
Data Base Updated

We now have all we need to sign certificates. Just follow Jamies guide Sign server and client certificates

References

It took me a few hours to get this going because of sort of a lack of documentation on how to use OpenSSL and PKCS11 together, during my efforts I found these resources helpful

https://jamielinux.com/docs/openssl-certificate-authority/index.html
https://github.com/OpenSC/OpenSC/wiki/SmartCardHSM
https://github.com/OpenSC/libp11
https://dennis.silvrback.com/openssl-ca-with-yubikey-neo
https://developers.yubico.com/PIV/Guides/Certificate_authority.html
https://developers.yubico.com/YubiHSM2/Usage_Guides/OpenSSL_with_pkcs11_engine.html

Minimal scp only chroot in FreeBSD

Background

The other day I needed to receive a few files from a friend. And I wanted to provide an easy to use service to do so. Back in the days I did run a FTP service with anonymous login and write acces to a inbox folder (1777).  But these days I have a few more security concerns and wanted to provide a scp drop site, and exactly that. A “scp only chroot”.

Configuration

Giving chrooted ssh access is pretty straight forward in the sshd_config. But to actually get it to work you will have to fiddle around a bit. You will need to figure out exactly what files you will need inside the chroot for the service to function properly.

Create user

Just use your favourite way of creating users in your system.

sshd

The easy part is to configure your sshd_config to chroot the specific user, group or whatever you want to chroot. Since this is probably a one off for me I settled for a single user.

Match User scponly
  ChrootDirectory /home/%u
  X11Forwarding no
  AllowTcpForwarding no

This short configuration will chroot the user scponly to /home/scponly when it connects. You just need to restart sshd in order to these settings to take effect.

Setting up environment

The hardest part was to set up the environment since you will need to make all libs and so on available inside the chroot.

Since this was to be a one off solution I will not go through the hassle of loop/null mounts or stuff like that. I will simply copy the stuff thats actually needed into the chroot. To find out what libs you actually need you can use the tool ldd(1)

# ldd /usr/local/bin/scp 
/usr/local/bin/scp:
	libcrypto.so.8 => /lib/libcrypto.so.8 (0x800a00000)
	libz.so.6 => /lib/libz.so.6 (0x800e69000)
	libutil.so.9 => /lib/libutil.so.9 (0x801082000)
	libldns.so.2 => /usr/local/lib/libldns.so.2 (0x801296000)
	libcrypt.so.5 => /lib/libcrypt.so.5 (0x8014f3000)
	libc.so.7 => /lib/libc.so.7 (0x801712000)

So I just created a few directories in /home/scponly

# cd /home/scponly
# mkdir etc bin lib dev libexec

and then copied these libs to /home/scponly/lib/.

You will also need to have a shell that then can invoke scp. I copied /rescue/sh to /home/scponly/bin/ because it is statically linked and does not depend on any libraries. When all this was done I thought the whole thing was finished but I encountered a few problems.

Problems

The first one was scp complaining about missing ld-elf.so.1. So just like the other libs I just copied it in to /home/sftponly/libexec/.

Secondly scp started to complain about missing /dev/null. And my solution to this was to just create a empty file called null in /home/sftponly/dev/ and chmod to 666. Why it works with a regular file I dont know. If you have any idea please tell me in the comments.

The final problem I faced was an error when invoking scp that said “unknown user 1005”. I did some searches on the web and found several solutions that where Linux specific so no help there. But they involved putting a few more files and libs relevant to nss into the chroot.

What I finally found out was that its the /etc/master.passwd (think of shadow in linux) and its database that where missing. So I just did grep scponly /etc/master.passwd > /home/scponly/etc/master.passwd to copy only the record for scponly to the chroot.

sad:*:1005:1002::0:0:scp only user:/home/scponly:/bin/sh

Finally the actual databased needed to be created with this command

# pwd_mkdb -d /home/scponly/etc/ /home/scponly/etc/master.passwd

Thats it! Now I have a account thats basically all it can do is scp. Yes you can login and get a shell but since there is no other binaries than scp and sh you are pretty limited.

Strong authentication of OpenSSH hosts using hardware tokens

This article describes how to secure your SSH host keys using hardware tokens.

Background

I have previously written numerous posts about strong user authentication using smart cards, yubikeys and/or OpenSSH certificates. I have also described how you can use OpenSSH certificates to authentcate hosts. But let say your server where compromised in some way just for a few hours and your SSH host keys (and certificates) where stolen. This would allow the attacker to perform MITM attacks or impersonate your server in several different ways. Depending on the application this could be more or less detrimental. If the server have a lot of users this could be used to steal passwords for example.

One way to mitigate this attack is to store your host keys on hardware tokens. This way they cannot be stolen by someone not in physical contact with the server, and you would easily find out if the token was missing.

Configure the token

I will use a Yubikey 4 for this. I have already described how to use yubikeys for client keys, so a more detailed description on how to configure and use Yubikeys for SSH can be found here. You can check that everyting is working like this:

# opensc-tool -l
# Detected readers (pcsc)
Nr.  Card  Features  Name
0    Yes             Yubico Yubikey 4 OTP+U2F+CCID 00 00

What we need is basically a pkcs11 capable device i.e a smart card or “similar” and create a  RSA key pair on it.

Configure sshd and setting up a ssh-agent

Since it would be extremely impractical to enter the token PIN every time a client connects to your secure server, we will need to use a ssh-agent that will keep the pin in memory as long as the agent process is running. We also need to enter the PIN before we leave:

# ssh-agent -a /root/yubikey-agent
setenv SSH_AUTH_SOCK /root/yubikey-agent;
setenv SSH_AGENT_PID 10894;
echo Agent pid 10894;
# setenv SSH_AUTH_SOCK /root/yubikey-agent;
# ssh-add -s /usr/local/lib/opensc-pkcs11.so
Enter passphrase for PKCS#11: 
Card added: /usr/local/lib/opensc-pkcs11.so
# ssh-add -l
2048 SHA256:qBbMpdbUeabLe4PnfjrjPbGPu8zfbkbK+ni4mXOnV24 /usr/local/lib/opensc-pkcs11.so (RSA)

Now you have your RSA key available to the system using the UNIX-domain socket at /root/yubikey-agent.

In a production situation I would put this in a rc-script and enter the PIN at boot-time.

Configure OpenSSH

We need to  create a file containing the public key and also tell sshd to use this key and the newly created socket as a backend for its host keys.

# ssh-keygen -D /usr/local/lib/opensc-pkcs11.so > /etc/ssh/yubikey_host_key.pub
# echo "HostKey /etc/ssh/yubikey_host_key.pub" >> /etc/ssh/sshd_config
# echo "HostKeyAgent /root/yubikey-agent" >> /etc/ssh/sshd_config

Notice that if you have several keys on your token, ssh-keygen will output all of them. Make sure only the correct keys are added to the key file.

Verify the configuration

When you have configured sshd you will need to restart sshd and them we can verify that host keys are actually from the hardware token (in this case the yubikey). A very easy way to do this is to actually try to connect to the server and have a look at what keys it presents to you.

% ssh server
The authenticity of host 'server (172.25.0.15)' can't be established.
RSA key fingerprint is SHA256:qBbMpdbUeabLe4PnfjrjPbGPu8zfbkbK+ni4mXOnV24.
No matching host key fingerprint found in DNS.
Are you sure you want to continue connecting (yes/no)?

You can now easily verify that this fingerprint IS actually the same as the one ge got when we added the yubikey to the ssh-agent.

This is basically it. If you are going to use this in production you would probably want to add some rc-scripts to automate the setup process as much as possible.

Verify SHA256 hashes verbally

This article describes how to verify SHA256 hashes verbally more easily using sha2wordlist.

Background

If you are working with security you may have encountered situations where you wanted to verify a hash “out of band”.  This could be done in many ways. Secured secondary chat channels or https enabled websites. But sometimes this would be done verbally over the phone or to an audience.

Im pretty fascinated with the security model used to secure the private keys used to sign the root zone in DNS. They meet four times a year for a carefully scripted ceremony where they take out smart cards, HSMs and so on from safes locked in cages. During these ceremonies there is several situations where hashes need to be verified by the participants. It would be pretty hard to do this directly using the actual hash consisting of 64 random hex numbers.

PGP word list

One solution to this problem (and the one that is used in the DNSSEC ceremonies is to convert the hash into a list of words in a predictable way. When I found this out I got really interested and I tried to find a good description of this on the web. First I found out about the PGP Wordlist which is basically a smart way to turn data into words. The words in these wordlists are chosen for their phonetic distinctiveness which give less room for error. You can read more about the history of this system on Wikipedia.

sha2wordlist

Then I tried to find a small program that did this convertion but without any luck. So if you know of something like this, please tell me. But what I did find was a small program sha2wordlist that does exactly what I want. It takes data on stdin and outputs the SHA256 hash and the PGP words. This program is written by Jakob Schlyter who is a OpenSSH developer and co author of many (according to me) important RFCs. He is also one of the co authors of the original practice statement for the “Root Zone KSK Operator”.

Verify SHA256 hashes

One example run could look like this:

# echo smallamountofdata | sha2wordlist
SHA-256:    a42e35de45646a86c665b081212384a67de2083c0427afd47948daa619a69055
PGP Words:  regain coherence chopper telephone crusade getaway Geiger letterhead southward glossary ruffled inventive blackjack cannonball mural paragon klaxon tomorrow aimless crossover adrift celebrate rocker souvenir jawbone dictator surmount paragon bedlamp paragon peachy equipment

FreeBSD Ports

I couldn’t find any packaged programs like sha2wordlist or a packaged version of sha2wordlist itself for that matter. So I decided to make my own port of it and contribute it to FreeBSD.  It is now avilable to install using ports/security/sha2wordlist. Binary packages will be available soon.

Specify which CAs are allowed to issue certificates using CAA record

A Certification Authority Authorization (CAA) record is used to specify which certificate authorities (CAs) are allowed to issue certificates for your domains.

Background

When well behaved certificate authorities (CAs) issue new certificates they follow certain procedures. These procedures are documented in their Certification Practice Statement (CPS) which should be a public document. For example the CA needs to make sure that the entity that have ordered a new certificate is actually the owner of that domain. There is today different levels of assurance. For example the CA need to be “more” sure that you are actually the owner of a domain in order to issue a certificate that gives you “the green padlock”

There is now a way for domain owners to influence these procedures using a new DNS record type, CCA (Certification Authority Authorization)

The CAA record and CPS

RFC 6844 describes the format of the record and how CAs should handle it if they decide to take it into consideration.

Before issuing a certificate, a compliant CA MUST check for publication of a relevant CAA Resource Record set. If such a record set exists, a CA MUST NOT issue a certificate unless the CA determines that either (1) the certificate request is consistent with the applicable CAA Resource Record set or (2) an exception specified in the relevant Certificate Policy or Certification Practices Statement applies.

In this way the domain owner can influence the security of the CA infrastructure by publishing these resource records into DNS. If a company exclusively use one certificate provider CAA records will give the CA another tool to verify that they can issue certificates for a specific domain. Today not all big CAs obey these records, but they will pretty soon. Earlier this year The CAB  Forum voted  in favour of making it mandatory for all CAs by passing Ballot 187 – Make CAA Checking Mandatory. The motion states

[...]
As part of the issuance process, the CA must check for a CAA record for each dNSName in the subjectAltName extension of the certificate to be issued, according to the procedure in RFC 6844
[...]

This change will be in effect on 8 September 2017.

CAA record format

The structure if a CAA record is as follows

<domain>   CAA <flags> <tag> <value>
  • flags – Octet of option bits in decimal form. Only bit 0 is used today
  • tag – An ASCII string that represents the function of the record. The RFC defines three tags
    • issue – Specifies that the record concerns issuance of certificates
    • issuewild – Specifies that the records concern issuance of wilrdcard certificates
    • iodef – url where the certificate authority can report policy violations
  • value – Basically the domain of the CA allowed to issue certificates for the domain.

One example could be

# drill CAA framkant.org
;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 40373
;; flags: qr rd ra ; QUERY: 1, ANSWER: 2, AUTHORITY: 2, ADDITIONAL: 0 
;; QUESTION SECTION:
;; framkant.org.	IN	CAA

;; ANSWER SECTION:
framkant.org.	300	IN	CAA	0 issue "letsencrypt.org"
framkant.org.	300	IN	CAA	0 iodef "mailto:iodef@framkant.org"

This tells the issuing CAs that only letsencrypt are allowed to issue certificates for framkant.org and all its subdomains. The second record tells them that policy violations should be reported by email to iodef@framkant.org.

One big problem right now is that only a very few domains use this feature. To make this really powerful we need more domains to use this feature. But a big first step is to make CCA checking mandatory for all big CAs.

SSH certificates together with hardware keys (smartcard/yubikey)

We have showed how to use SSH certificates and SSH CAs, we have also showed how you can use the yubikey to store you SSH keys. This article will describe how to combine these two features.

First of all you need to have a yubikey set up with some RSA/ECDSA keys. Then find out the public part of you key:

% ssh-keygen -D /usr/local/lib/opensc-pkcs11.so
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCHn4jSqvNLn5NtUxqlAlm1Qj1tlunb0PjBsItmmesquULAM5oqVYmwJ+bmXpDlzgeeIbl1lf3aUTsXCs6My8mkIUwwN+3a5AJ8MA18Bzzx/qIpxe2N/nZ61e03ua5B6SjpfxAAC6i3DPHU6kUSy26sXhfx14y6abWlrwhXaILsTogz2sOganS44au+nSpa35xwMxG5vehkRkYe6vQvvIDFeMhy61DAJGOcGeCbXTfJB9yYwAgOEqTvHDBfbTrHhmnUu82/JV6twY4/tdgrjFxGE3/JsRnoP3lBCoLQR+Uxs3mV7pFelQj/8bZKVsjdzqH9AYWpvIQTJGuwAmyOk61V

This public key should be handed over to your systems administratior (probably yourself in this case) for signing. In return you will get a OpenSSH certificate file that looks sort of a public key but a bit longer.

Now the “tricky” part comes. When you have your public key in ~/.ssh/id_rsa.pub ssh will automatically look for a certificate file called id_rsa-cert.pub. But since we are going to use the smartcard/yubikey to handle our key it will not be visible in ~/.ssh.

First give your certificate a reasonable name like ~/.ssh/yubikey-cert.pub. Then we could tell ssh to combine the two.

Me most basic way is then to just specify the options you need on the command line:

% ssh -o PKCS11Provider=/usr/local/lib/opensc-pkcs11.so -o CertificateFile=~/.ssh/yubikey-cert.pub peter@torus
Enter PIN for 'PIV_II (PIV Card Holder pin)': 
[torus:~] peter>

If you want to use this permanently you can of course put the options in ~/.ssh/config instead it should look something like this:

PKCS11Provider=/usr/local/lib/opensc-pkcs11.so
CertificateFile=~/.ssh/yubikey-cert.pub

A third option if you are using the ssh-agent (like me) you could first add the card to your agent:

% ssh-add -s /usr/local/lib/pkcs11/opensc-pkcs11.so
Enter passphrase for PKCS#11: 
Card added: /usr/local/lib/pkcs11/opensc-pkcs11.so

and the specify the cert-file either on the command line or in ~/.ssh/config

% ssh -o CertificateFile=~/.ssh/yubikey-cert.pub peter@torus
[torus:~] peter>

Now you should be able to combine ssh certificates and yubikeys/smart cards

Securely update wordpress instance

Background

WordPress wants you to have automatic updates turned on for your installation. According to them this is the best way to securely update wordpress. While this is party true because time is key when it comes to web security. If you have patched your installation before anyone tries to exploit the vulnerabilities you might have that’s a good thing. But the problem is that many of these vulnerabilities depend on the web server having write access to your files. And in order to have automatic updates turned on you will have to grant wordpress (the web server) write access to all files that it might want to update.

Securely update wordpress

For a long time wordpress have offered another way of semi automatically updating your wordpress, this depends on you handing over your credentials for file transfers to wordpress. What wordpress does is basically a regular login using ftp or sftp and uploading the new files this way. According to me this is a vast improvement (maybe because I’ve been the victim of wordpress “hacks” that used the possibility to change my files on disk).

But what if you only have sftp/ssh access with key? Or you dont want to give your personal password to wordpress every time you want to do upgrades?

WordPress have a built in solution for this very similar to the one described above. You can actually use ssh keys to do the upgrade.  Create a RSA key pair WITH PASS PHRASE on your web server and store it somewhere safe where only you and the httpd daemon have access to read it.

Then its time to allow this key access to your account, but please restrict access from localhost or the servers own IP. This key should never be allowed to be used from outside. Your .ssh/authorized_keys could look something like this:

[..]
from="127.0.0.1" ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQC19oGbaEW7NKBQ5vn3auFbUKAasopYYPv03FxhEjbZwhoVTO44BIR0oIdMs1u1v5Y4gDH77ndCI/6fqwJQwc1D0YYH/45wUOEaB4MuPlCxlp7yxE+FyzMspi9mP8HETS+jEfzLIQ01F424yfVweQME9fxeCP0MFO+XK0SuMCk5ibdvaxYwCuRwPFkHcnyKIrDnIgGXv0D8YdC+K/RW/Ghpu9C7Rn2q0pQDbSHj7/xddO7aD+X6DPZfbHS/5ZrJnB+oWf7b9j5FmH8ldBSGBvUr6kplnDr1dKN/98bwRp1FpcxzShAX3q9nj44FwPhKV5JEOw146YJxXXks40ia1da5

Then you can configure this in wp-config.php

define('FTP_PUBKEY','/usr/local/www/ssh/updatekey.pub');
define('FTP_PRIKEY','/usr/local/www/ssh/updatekey');
define('FTP_USER','peter');
define('FTP_HOST','framkant.org');

When doing updates in the future wordpress will ask you for the pass phrase of your ssh key instead of your personal password. In this way the sensitive authentication “material” is never transferred over the internet.

There is still the possibility that malicious code in wordpress or a malicious plugin/theme could steal this key since it have read access to the private key and you give it the pass phrase. But this is still much better than giving it your password.

Scalable access control using OpenSSH Certificates

Background

I’ve been using OpenSSH certificates for some time now. They are very handy if you have a bunch of machines you want to trust, or a bunch of machines that shoud trust you.  It’s very effective to trust just one host CA in order to trust all servers with certificates signed by this CA. Or the other way around, have your personal public key signed by a user CA and then be automatically trusted by all servers that trust this CA. But if you working together with alot of people let say within an organisation this becomes problematic pretty soon. Maybe your frontend people should only have access to webservers and database people to the database servers and so on. The solution to this is the little known flags AuthorizedPrincipalsFile and AuthorizedPrincipalsCommand in sshd_config.

OpenSSH Certificates and principals

Lets start with the certificates and the principals within these certificates. In order to make this work I would suggest to use principals within the certificates that are closely tied to the person using it, their company wide username for example. If different keys have different access leves (lets say because on of them are stored on a physical secure element/smart card) it is good to include this kind of information in the principal, according to some standard you make up. Let say I use yubikeys to store my private keys I could have principals like peter_file and peter_physcial. These are easily parsable and  connected to a physical person.

Please note that there could be other access schemes where role is more important that how the key is stored, then role could be a better option for the principal suffix. But please note that if you burn the role into the certificate that person will need to have the current certificate revoked and have a new one issued if the role is ever changed. I will discuss a better way to handle this later.

Another option is to have multiple principals in the same certificate (ie peter,webmaster,root) but this also gets cumbersome when privileges and roles start to change over time.

AuthorizedPrincipalsFile

One solution to this problem is the configuration option AuthorizedPrincipalsFile in sshd_config. With this option you tell sshd where to look for a list of principals valid for a certain user. I looks something like

AuthorizedPrincipalsFile /etc/ssh/%u_principals

When someone tries to log in as peter sshd will check my certificate for validity and then look for valid principals in the file /etc/ssh/peter_principals. sshd expects this file to contain one valid principal per line and optionally preceded by extra options using the same format as the authorized_keys file. (ie from= and command=). This is flexible enough. I can now give multiple principals (or physical persons) access to a specific account by changing a file. I can also restrict access to certain hosts or create force commands for specific principals.

One use case for this could be a webserver where multiple principals(persons) should be able to use the “webmaster” account but at the same time the test/build system should only be allowed to run a certain commands to publish successful builds. Lets say that the account name is www, then the /etc/ssh/www_principals could look something like this:

peter
erik
from="buildserver.corp.com" command="/bin/publish_website" buildserver
from="guestcomputer.corp.com" guestworker

Please note: If you are really concerned with security, maybe you have given a certificate to a external partner or something, I would suggest to burn the from and command attributes into the certificate. In this way they will never be overridden by some configuration at the server side. The downside is that you will need to produce a new certificate if something changes.

If you have a pretty static setup and/or a decent configuration manager/orchestration tool this could be enough. It gives full flexibility on who should be able to access what, and how. But in the long run it could be tedious to manage all the principal files. This is where the AuthorizedPrincipalsCommand comes in to the picture.

AuthorizedPrincipalsCommand

This works exactly the same as AuthorizedPrincipalsFile but instead of a static file sshd will run a command followed by some options (basically the username that tries to log in) that will generate the principals file dynamically. This gives you a lot of options. Probably the most straight forward one is that you now can have a single ACL file for you whole environment and just let the command read it and produce a host and user specific principal file. One very simple example could look like this:

# Principal(person) user     host
erik                www      webserver.corp.com
peter               www      webserver.corp.com
guestworker         www      webserver.corp.com

peter               db       database.corp.com
erik                db       database.corp.com

Of course this file could be expanded to include more information and more options but this gives a example on how it can be done.

Other backends

But I think the real power in the AuthorizedPrincipalsCommand is that you now can use whatever backend you like and just have the principals command be a wrapper for this backend which could be some Active Directory, LDAP or whatever you might have at your organisation. This makes it possible to use existing infrastructure and still be able to use ssh certificates which I think is a real killer feature in OpenSSH.

If you have some ideas to improve this concept or an questions, please leave a comment.

Use a smart card or HSM to securely store your SSH CA keys

Depending on the use case, SSH CA keys can be extremely sensitive. Possession of the private key gives you the ability to sign new certificate for arbitrary usernames that will grant access to all machines where this CA is trusted. This is why you should keep your CA keys very safe, preferably offline and of course encrypted (password protected).

One big problem with digital encryption keys is that it is extremely difficult to find out if you lost control over they keys. If they where copied you have no way of finding out. This is where hardware tokens comes in. If you can be sure that the key will never leave the hardware token, you have a better chance of knowing when they keys are lost or stolen.

OpenSSH have support for storing keys onto hardware tokens that talk PKCS#11 such as smart cards and hardware security modules.

If you want to store your personal SSH key on a smart card you can read more about that here: Using Smart Card enabled yubikey for ssh authentication in FreeBSD

Since a SSH CA is just a regular SSH key they can also be stored on smart cards. But there is a few subtle differences in how you use it. First of all you need a token with a RSA key present. There is no option in OpenSSH to ask for a specific key stored on the token, instead we specifiy the public key that matches the private key we want to use. Lets download the public key from the token:

$ ssh-keygen -D /usr/local/lib/opensc-pkcs11.so
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQCOvhEqFq9Ualp1iYiNs0JFs3MgvGU+By/VvyW4qkymW+w/MmHAaHl+/UFnE+kgXChdYHaGEVxxGi6dQlSq+1ZKAWPJsOEbkysK6cgjgvP21gVNjL62TlQz+QfGF82mv0hfSGXQrZQR7VDs+6xJOe3S/0i1HvnnRTdR2v9QSJzd2EWNLmUcPy7+4x4rEB11G0oPt+Xyx60WaleJctwJHHhJS/jqHdvuf7HO6MS/EQn2NTnwIjChlmm2kUT7obnev/r6uEwz87NubnYJUrYImRDMafjS9taq8l7y33ydT00QHEI76kmrSSi7hTfmxUgStQWuQ2mq10YEVd8kZ2sqmC3N

We put this into a file, let say ssh_ca.pub. Now we are ready to start using the token for certificate signing.

$ ssh-keygen -s ssh_ca.pub -D /usr/local/lib/opensc-pkcs11.so -I key_id wack.pub 
Enter PIN for 'Framkant HSM Test (UserPIN)': 
Signed user key wack-cert.pub: id "key_id" serial 0 valid forever

To look at the certificate we can use ssh-keygen.

% ssh-keygen -L -f wack-cert.pub 
wack-cert.pub:
        Type: ssh-rsa-cert-v01@openssh.com user certificate
        Public key: RSA-CERT SHA256:AtPyAu1DL5cFruTo9XnsVz7tdec7xF9SbpX8DzsQrbs
        Signing CA: RSA SHA256:Pbdx6TAvXvwZTKQVMRYWsWYPomw6AxBVoqbtXgy9pfs
        Key ID: "key_id"
        Serial: 0
        Valid: forever
        Principals: (none)
        Critical Options: (none)
        Extensions: 
                permit-X11-forwarding
                permit-agent-forwarding
                permit-port-forwarding
                permit-pty
                permit-user-rc

In a real life scenario I would recommend against this kind of certificate. Give it some validity and specify principals.

Two nice affordable options for hardware tokens are the Yubikey and the Smart Card HSM