ssl – Ryan Schulze

How to check if you are vulnerable for the DROWN attack (CVE-2016-0800).

March 1, 2016March 1, 2016Ryanssl

CVE-2016-0800, also known as the DROWN attack, is an attack against servers that still support the old SSLv2 protocol. The only reason a server would still offer to use SSLv2 would be for possible compatibility reasons with 20-year-old PCs ( -> there is no reason to use or offer SSLv2 any more). From a configuration side you can disable the v2 protocol by adding -SSLv2 to the list of protocols being used.

Where and how you configure this depends on the software, but using all -SSLv2 -SSLv3 is fine with most modern servers and clients, Mozilla has a fantastic overview for configuring SSL and TLS.

If you want to check a bunch of your hosts remotely, you can use the sslv2 script included with nmap like this:

nmap --open --script sslv2 hostname

1	nmap --open --script sslv2 hostname

Where hostname would be either a FQDN, or an IP, or an IP range. You can swap out sslv2 with ssl-enum-ciphers to see all SSL /TLS ciphers and protocols the server offers.

Renewing “Let’s Encrypt” SSL certificates

December 15, 2015December 15, 2015Ryanbash, ssl

Let’s Encrypt provides free DV SSL certificates for everyone and is now in the open beta phase. I’m not going to go into the details of which of the clients are best, since that depends entirely on your use case (I use acme-tiny and a rule in varnish to intercept all calls to /.well-known/acme-challenge/).

Since the certificates are only valid for 90 days, I often see people suggesting to just renew them via cronjob every 2 months. I find this to be really awful advice, if that renewal fails for any reasons (network problems, local problems, problems with let’s encrypt) the next renewal is a month after the certificate expired. It is also pretty inflexible (what if you would rather prefer to renew them after 80 days).

I use openssl to check daily how long the certificate is still valid, and if a threshold has been reached it tries to renew the certificate (I believe the official client has this functionality too). And if the certificate isn’t renewed by a 2nd threshold, it sends an email altering the admin of the problem (for manually intervening and fixing whatever went wrong).

At the end of this posting I’ll add the complete script, but the quickest way to check how long a certificate is still valid is to use openssl x509 -in -checkend. It will return 0 if the file is still valid in x seconds, and 1 if the certificate doesn’t exist or if the certificate will be expired by then. Just multiply the number of days by 86400 and check if the certificate is still valid:

openssl x509 -noout -in "domain.cert" -checkend $( bc <<< "86400 * days")
echo $?

1 2	openssl x509 -noout -in "domain.cert" -checkend $( bc <<< "86400 * days") echo $?

The openssl binary has a few nice options for looking at certificates (both local files and remotely connecting to a server and looking at the provided certificate)
Show information about a local certificate file: openssl x509 -text -noout -in
Connect to a remote server and display the certificate provided: openssl s_client -showcerts -servername foo.bar -connect IP:PORT | openssl x509 -text -noout (–servername foo.bar is only required if you are connecting to a server and need to use SNI to request a cert for a specific domain, i.e. a webserver providing multiple domains on port 443 via SNI. It can of course be omitted if you don’t need it.)

This is the full script I use for checking and renewing certs. It basically just loops through a list of domains, checks if any of the date thresholds are met and then renews certificates/send emails.

#!/bin/bash 

# how many days before the certificate expires should it be renewed
RENEW="28"

# how many days before the certificate expires should start sending alerts to the admin
ALERT="14"
ALERT_EMAIL='admin@your.domain'

# some variables set by ansible
SSL_BASEDIR="/etc/ssl"
ACME_LETSENCRYPT_BINARY="python /usr/local/bin/acme_tiny.py"
ACME_CHALLENGE_DIR="/var/www/acme-challenges"

ACME_LETSENCRYPT_DIR="${SSL_BASEDIR}/letsencrypt"
ACME_ACCOUNT_KEY="${ACME_LETSENCRYPT_DIR}/account.key"


# download lets-encrypt-x1-cross-signed.pem if the local copy is more than 3 days old
if [[ ! -r "${ACME_LETSENCRYPT_DIR}/lets-encrypt-x1-cross-signed.pem" || $(($(date +%s) - $(date -r "${ACME_LETSENCRYPT_DIR}/lets-encrypt-x1-cross-signed.pem" +%s))) -ge 259299 ]] ; then
	curl --silent https://letsencrypt.org/certs/lets-encrypt-x1-cross-signed.pem > "${ACME_LETSENCRYPT_DIR}/lets-encrypt-x1-cross-signed.pem"
fi

for fqdn in www.domain1.foo www.domain2.foo ; do

	# check if we have to renew a certificate
	openssl x509 -noout -in "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt" -checkend $( bc <<< "86400 * ${RENEW}") 2>/dev/null
	if [[ $? -gt 0 ]] ; then
		${ACME_LETSENCRYPT_BINARY} \
			--account-key "${ACME_ACCOUNT_KEY}" \
			--csr "${ACME_LETSENCRYPT_DIR}/${fqdn}.csr" \
			--acme-dir "${ACME_CHALLENGE_DIR}" \
			> "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt"
		cat "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt" "${ACME_LETSENCRYPT_DIR}/lets-encrypt-x1-cross-signed.pem" \
		> "${ACME_LETSENCRYPT_DIR}/${fqdn}.pem"
		openssl x509 -noout -text -certopt no_header,no_version,no_pubkey -in "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt" | \
		mailx -s "[SSL] OK: ${fqdn} certificate was renewed" ${ALERT_EMAIL}
	fi

	# check if we need to alert about certificates that weren't renewed yet
	openssl x509 -noout -in "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt" -checkend $( bc <<< "86400 * ${ALERT}") 2>/dev/null
	if [[ $? -gt 0 ]] ; then
		openssl x509 -noout -text -certopt no_header,no_version,no_pubkey -in "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt" | \
		mailx -s "[SSL] ERROR: ${fqdn} certificate will expire soon and wasn't automatically renewed" ${ALERT_EMAIL}
	fi

done

#!/bin/bash

# how many days before the certificate expires should it be renewed

RENEW="28"

# how many days before the certificate expires should start sending alerts to the admin

ALERT="14"

ALERT_EMAIL='admin@your.domain'

# some variables set by ansible

SSL_BASEDIR="/etc/ssl"

ACME_LETSENCRYPT_BINARY="python /usr/local/bin/acme_tiny.py"

ACME_CHALLENGE_DIR="/var/www/acme-challenges"

ACME_LETSENCRYPT_DIR="${SSL_BASEDIR}/letsencrypt"

ACME_ACCOUNT_KEY="${ACME_LETSENCRYPT_DIR}/account.key"

# download lets-encrypt-x1-cross-signed.pem if the local copy is more than 3 days old

if [[ ! -r "${ACME_LETSENCRYPT_DIR}/lets-encrypt-x1-cross-signed.pem" || $(($(date +%s) - $(date -r "${ACME_LETSENCRYPT_DIR}/lets-encrypt-x1-cross-signed.pem" +%s))) -ge 259299 ]] ; then

curl --silent https://letsencrypt.org/certs/lets-encrypt-x1-cross-signed.pem > "${ACME_LETSENCRYPT_DIR}/lets-encrypt-x1-cross-signed.pem"

for fqdn in www.domain1.foo www.domain2.foo ; do

# check if we have to renew a certificate

openssl x509 -noout -in "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt" -checkend $( bc <<< "86400 * ${RENEW}") 2>/dev/null

if [[ $? -gt 0 ]] ; then

${ACME_LETSENCRYPT_BINARY} \

--account-key "${ACME_ACCOUNT_KEY}" \

--csr "${ACME_LETSENCRYPT_DIR}/${fqdn}.csr" \

--acme-dir "${ACME_CHALLENGE_DIR}" \

> "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt"

cat "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt" "${ACME_LETSENCRYPT_DIR}/lets-encrypt-x1-cross-signed.pem" \

> "${ACME_LETSENCRYPT_DIR}/${fqdn}.pem"

openssl x509 -noout -text -certopt no_header,no_version,no_pubkey -in "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt" | \

mailx -s "[SSL] OK: ${fqdn} certificate was renewed" ${ALERT_EMAIL}

# check if we need to alert about certificates that weren't renewed yet

openssl x509 -noout -in "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt" -checkend $( bc <<< "86400 * ${ALERT}") 2>/dev/null

if [[ $? -gt 0 ]] ; then

openssl x509 -noout -text -certopt no_header,no_version,no_pubkey -in "${ACME_LETSENCRYPT_DIR}/${fqdn}.crt" | \

mailx -s "[SSL] ERROR: ${fqdn} certificate will expire soon and wasn't automatically renewed" ${ALERT_EMAIL}

done

Weekly Update

July 20, 2008Ryanfirefox, ssl

Nicht wundern wenn ich nun hin und wieder auf deutsch poste. Ich will nur ein wenig Abwechselung hineinbringen. Sodele, was gibt es nun neues diese Woche? Ich habe für die Domain dopefish.de die automatische Weiterleitung auf https herausgenommen, da Firefox 3 so gerne motzt wenn Domains selbst signierte Zertifikate benutzen (leider auch die einzige Möglichkeit kostenlose Zertifikate zu erstellen). Natürlich funktioniert weiterhin https, nur eben jetzt ohne Zwang.

Wer Firefox 3 einsetzt, soll mal “about:robots” oben eingeben. Die Entwickler haben wohl in eine langweilige Minute was lustiges eingebaut.

Technisch hat sich gar nichts getan, ich war die Woche zu oft nicht daheim, um irgendwas besonderes am Server zu basteln. Dafür habe ich nun Kontaktlinsen die ich hin und wieder tragen werde. Ich habe die vor allem wegen den Sport geholt, da dort eine Brille teils extrem nervig sein kann (wenn z.B. beim Klettern das Seil knapp am Kopf vorbeikommt und die Brille im Weg ist).