Posted in Security, Tech

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

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:

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.


Posted in Internet Stuff, Tech

Renewing “Let’s Encrypt” SSL certificates

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:

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 -connect IP:PORT | openssl x509 -text -noout  (servername 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.

Posted in Me

Weekly Update

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 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).