SWITCH Security-Blog

SWITCH-CERT IT-Security Blog

SWITCH Public DNS Resolver

SWITCH operates recursive name servers for its constituency, the Swiss research and education network. Over the last year we have continually added support for transport encryption protocols on our recursive name servers such as DNS over TLS (DoT) and more recently DNS over HTTPS (DoH).

In contrast to default unencrypted DNS which runs over UDP/TCP Port 53 , both of these standards (DoT, DoH) use encrypted protocols which provide privacy for DNS queries between the client (application) and the recursive name server. This eliminates opportunities for eavesdropping and on-path tampering with DNS queries on the network.

Our motivation for enabling encrypted DNS protocols on our recursive name servers have been that some client applications (mostly Android 9) probe for DoT support and use it if available by default. Over the last year, other widely used applications have added support for encrypted DNS protocols. Most notably the web browser Mozilla Firefox which supports DoH but has not turned it on by default.

Opportunistic encryption of DNS queries and responses as it is used by Android 9 by default is one use case of DoT. However, some users want to pin a specific recursive name server regardless in which network they are or also to authenticate the name server. To support this use case, we have opened our recursive name servers over encrypted transport protocols to the Internet. You will find more information about the SWITCH Public DNS service and how to use it on this website:


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Mobile Malware

Rogue Mobile App

Rogue mobile apps are counterfeit apps designed to mimic trusted brands or apps with non-advertised malicious features. In both cases, the goal is that unaware users install the app in order to steal sensitive information such as credit card data or login credentials.

The common way to install apps is to use the official app store. By default, neither Android nor Apple’s iPhone allow users to install apps from unknown sources. However, this does not mean we can just trust the official app store. SWITCH-CERT has been monitoring Apple’s App Store and Google Play for some time and noticed that many rogue apps are able to sneak into Google Play especially.

Google Play

Attackers are abusing the weak app testing procedure of Google to sneak their rogue apps into Google Play. One can find counterfeit apps of Swiss brands on a regular basis. Typically, the apps reside on Google Play for some time until it is removed because of take down requests from security researchers. Until that happens, unaware users are likely to install such apps and put their data at risk.

The screenshot below shows apps found when searching for Bluewin. During the last months, Bluewin has been a common target for rogue counterfeit apps. The red circle indicates the rogue app.

Play Store result for the search key word “Bluewin”

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Breaking security controls using subdomain hijacking

Users obtain a domain name to establish a unique identity on the Internet. Domain names are not only used to serve names and addresses of computers and services but also to store security controls, such as SPF or CAA records. Many of the Internet protocols were designed at a time where built-in security was not a requirement. The IETF continues to standardize protocol extensions to address today’s security needs.

For some protocols security is added with controls stored in your domain names zone file. In order to have the desired effect, the pre-condition is of course that your domain name is secure. In other words, the security of your application that makes use of controls in DNS is only as secure as the security of your domain name.

Hijacking a domain name because of weak credentials at the registrar may get the job done but this is far from stealthy and will likely not last long. In many cases it is sufficient to hijack an abandoned subdomain. Taking over abandoned subdomains may be unnoticed by the owner for a very long period of time making it also very useful for targeted attacks.

Picture 1: update.ft.com has been hijacked and the content from the ft.com front page is mirrored with a fake article about subdomain hijacking. Note: the website is not online anymore, Financial Times has been notified to remove the abandoned record from their zone file. A Certificate Transparency (CT) log proves that a TLS certificate has been issued for this demo site.

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Why the most successful Retefe spam campaign never paid off

Switzerland is one of the main targets of the Retefe banking trojan since its first appearance in November 2013. At that time, it changed the local DNS resolver on the computer (See also blog post “Retefe Bankentrojaner” in German only). Almost a year went by until they changed to the still current approach of setting a proxy auto-config (PAC) URL (See also blog post “The Retefe banking Trojan has targeted Switzerland“). To understand the story of this blog post, it helps to understand the modus operandi of the Retefe malware. We recommend you read up on it on our blog links posted above if you are not familiar with it.

While the Retefe actors are constantly changing tactics, for example their newest campaigns also target Mac OS X users, their malware still works the same. One of notable changes was the introduction of Tor in 2016. At first, they started using Tor gateway domain names such as onion.to, onion.link within the proxy auto-config URLs, later on they switched to Tor completely. The advantage of using Tor is of course, anonymity and the difficulty to block or take down the infrastructure.

Onion domain names don’t use DNS or do they?
The Tor network can use .onion domain names but these names are not resolved over DNS but instead work only in the Tor network. RFC 7686 (The “.onion” Special-Use Domain Name) goes into more details on the special case of .onion domain names. However, the fact is that .onion domain names do leak into the DNS system. For potential reasons and more information on this subject we recommend the paper by Versign Labs “Measuring the Leakage of Onion at the Root” (PDF).
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SWITCH DNS recursive name service improvements with dnsdist

SWITCH operates recursive name servers for any user within the Swiss NREN. While larger universities typically run their own recursive name server, many smaller organisations rely on our resolvers for domain name resolution. During the consolidation of our name server nodes into two data centres, we looked for opportunities to improve our setup. Dnsdist is a DNS, DoS and abuse-aware load balancer from the makers of PowerDNS and plays a big part in our new setup. While the first stable release of dnsdist (version 1.0.0) is only a few days old (21 April 2016), it feels like everyone is already using it. We are happy users as well and want to share with you some of the features we especially like about dnsdist.

Our old setup consisted of several name server nodes which all shared the same IP address provided by anycast routing. Our recursive name server of choice was and still is BIND, and we have been providing DNSSEC validation and malicious domain lookup protection through our DNSfirewall service for some time. While this setup worked very well, it had the disadvantage that some badly behaved or excessive clients could degrade the performance of a single name server node and as such affect all users routed to this node. Another disadvantage was that each name server node got its share of the whole traffic. While this may sound good, it has the disadvantage that we have several smaller caches, one on each node. My favorite quote from Bert Hubert, founder of PowerDNS, is: “A busy name server is a happy name server“. What it means is that it is actually faster to route all your queries to a single name server node because this will improve the cache-hit rate.

Dnsdist provides a rich set of DNS-specific features
Our new setup still makes use of anycast routing. However, it is now the dnsdist load balancer nodes that announce this IP address, and they forward the queries to the back-end recursive name servers for domain name resolution.

The server nodes are located in two data centres, and both load-balancers announce the same IP address to make use of anycast routing. Query load is typically sent to resolvers within the same data centre but is distributed to the other site as well in the event of a higher load or server loss.

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Optimizing Negative Caching Time in DNS

A recent presentation by SIDN (.nl) at the Spring 2016 DNS-OARC workshop reminded me of the importance of Time-To-Live (TTL) values in TLD zones. Specifically, it got me thinking about lowering the negative caching time in .ch/.li from currently 1 hour to 15 minutes.

What is negative caching?
When a resolver receives a response to a query, it caches it for the duration of the TTL specified by the record. For positive responses, the record contains the TTL, but for negative responses (response code NXDOMAIN), there is no answer to the query question. For this case, the response contains the SOA record of the zone in the authority section. Negative caching is specified in RFC 2308 as the minimum of the SOA record’s TTL and the SOA minimum field. For example, the original SOA record of the .ch zone looked as follows:

dig +nocmd +noall +answer @a.nic.ch ch. soa
ch. 3600 IN SOA a.nic.ch. helpdesk.nic.ch. 2016041421 900 600 1123200 3600

The SOA TTL is 3600, and the SOA minimum time is also set to 3600. The minimum of these two values is of course 3600 too. That means the negative caching time for any .ch domain lookup is one hour.

A lower negative caching time is more user-friendly
People who are about to register a new domain name may also look up the name over DNS. However, this means that they just cached the non-existence of the name in the resolver they are using. A domain can be registered in a matter of minutes, and this can prevent them from using the domain name on their network for the duration of the negative caching time.
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Attack of the killer Ads

By Daniel Stirnimann and Serge Droz

Recently I was quoted saying “… .ch and .li are the most secure (top-level) domains!”. In the same meeting, Security Rock Star Mikko Hyppönen claimed, “Surfing the Web with your laptop is the most dangerous thing you can do in the Internet.”  So what is true, what is false? Rather than speculate about obscure statistics I’d like to illustrate one of the big problems we face in .ch today, namely using ads as a back door to reach victims through reputable sites.

Ads: enter through the hallway

Malware distributors have one goal: spreading their stuff as widely as possible. This is achieved through different means. Malware was traditionally distributed – and still is – through e-mail attachments. This was the case, for example, with the Retefe malware. Alternatively, web pages can be hacked and used to spread malware by exploiting browser bugs. SWITCH has been very active, through its Safer Internet initiative, in working to reduce this infection vector. In fact, we’ve been so successful, that drive-by is very scarce in Switzerland, hence the statement that ” … .ch is one of the most secure ccTLDs”. Drive-by websites are always hacked, but in most cases they are not very popular websites, since popular websites are typically well protected. Many of the later ones offer a backdoor tough: ads! News sites in particular make most of their revenue by selling on line ads, which explains the “ad-war” arms race between ad-blockers an news agencies (see our Security Report on anti-anti-ad features). A very common way is malvertising, a term coined by William Salusky. Salusky found ads that were in fact carrying malicious payloads. Let’s look at a slightly different scenario, namely a legitimate but compromised ad server. While technically a different scenario it has the same effect on the end user.

Most people would think that visiting a website just serves you content from that site but this is not true for most of the large sites, in particular news sites. They import contents such as videos, trackers, counters, scripts and especially ads from third-party sites. These are not controlled by the original site, and often import content themselves from yet another site. Thus, a well maintained site with high security standards will often import stuff from sites with lower security. Think of it as sitting in a highly rated restaurant that has one bad food supplier.

The image below shows all the external sites involved whenever you visit three popular news sites.


Ohne Addon

The above example shows what happens when you visit three popular Swiss newspapers. Triangles denote third-party sites from which content is imported when you visit the respective news site. The visualisation was done using the Mozilla addon LightBeam

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Protect your network with DNS Firewall

If you run your own mail server, you will quickly find out that 90% of the e-mails you receive are spam. The solution to this problem is e-mail filtering, which rejects or deletes unwanted spam. This solution is generally well accepted, and most users would not want the old days back when your inbox was filled with scams. Those people who want spam can also work around it by disabling spam filtering for their e-mail address or opting to run their own mail server.

Spam, scammers and other malicious abuse are not unique to e-mail. One possible approach is to invent a filtering technology for every protocol or service and allow the service owners to block misuse according to their policy. On the other hand, most services on the Internet make use of the Domain Name System (DNS). If you control DNS name resolution for your organisation, you can filter out the bad stuff the same way you filter out spam on e-mail. The difference and the advantage of DNS is that DNS filtering is independent of the service you use.

Back in 2010, ISC and Paul Vixie invented a technology called Response Policy Zones (RPZ) (See CircleID Post Taking back the DNS). While it has always been possible to block certain domain names from being resolved on your DNS resolver, adding host names manually as an authoritative zone does not scale.

(Illustration Christoph Frei)

(Illustration Christoph Frei)

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DNSSEC signing your domain with BIND inline signing

Update Nov 2017: DNSSEC zone signing as described here is outdated. We strongly recommend against the method described in this blog post. Newer BIND versions or other DNS software have greatly simplified DNSSEC signing.

With BIND 9.9, ISC introduced a new inline signing option for BIND 9. In earlier versions of BIND, you had to use the dnssec-signzone utility to sign your zone. With inline signing, however, BIND refreshes your signatures automatically, while you can still work on the unsigned zone file to make your changes.

This blog post explains how you can set up your zone with BIND inline signing. The zone we are using is called example.com. In addition, we look at how to roll over your keys. In our example, we do a Zone Signing Key (ZSK) rollover. We expect that you are already familiar with ISC BIND and have a basic understanding of DNSSEC. More specifically, you should be able to set up an authoritative-only name server and have read up on DNSSEC and maybe used some of its functions already.


Before we set up inline signing with BIND, let us look at a typical network architecture. We will set up inline signing on a hidden master name server. This server is only reachable from the Internet via one or more publicly reachable secondary name servers. We will only cover the configuration of the hidden master as the secondary name server configuration will not differ for the signed zone (assuming you are using DNSSEC-capable name server software).
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Retefe with a new twist

A few months ago, we blogged about the banking trojan Retefe (Blog post in German) that was and still is targeting Switzerland. First off, Retefe is different because it only targets Switzerland, Austria and Sweden (and sometimes Japan). Contrast this to many other banking Trojans, which have a much more global and dynamic target list. Not only that, but the Retefe infrastructure also prevents computers from not affected countries to connect to its systems by using geo-location aware access lists and filters. The second unique property of Retefe is the fact, that it only modifies the operating system by adding a fake root certificate and by changing the DNS server for domain name resolution. After infection, the installer removes itself, which makes life hard for anti-virus software trying to detect a malicious Retefe component or activity.

Since a few days, Retefe is back again with a new twist. It still targets the same countries and the same banks. Not too exciting, the spam campaign has changed. However, in this wave Retefe is picky and only installs itself on selected computers. And some icing to the cake, it also installs another malware called DOFOIL. In this blog post, we give a technical analysis of the new Retefe.
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A look at a phishing website

Yesterday we came across a phishing website under .ch where we were able to download the phishing kit. A phishing kit is an archive file which contains all the relevant files for hosting a phishing website. In this case, the archive contained some static HTML, JS and image files for hosting the phishing form, but also a PHP file for sending the data to the perpetrator, and – most interestingly –an .htaccess file. The .htaccess file is a configuration file used by some popular web servers, which allows the user of a website to override a subset of the server’s global configuration for the directory that the file is located in and all its sub-directories.

A phishing website is frequently only accessible from the targeted country. In our case, this was controlled by the .htaccess file which contained a large list of IP address ranges from where it is allowed to access the site. As an incident handler, we often get reports of malicious websites that we cannot verify with IP addresses from Swiss ISPs. An unwary user might think that the phishing website has already been taken down, but that is not the case. The user is just not allowed to access the phishing website from its IP address.

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IT-Security-Links #61

  • McAfee Labs reports that a new ransomware called CryptoWall uses Tor for communication and demands Bitcoin from the user in exchange for the private key to decrypt the files. “The use of Tor and Bitcoin in this operation make tracing the attackers more difficult” writes McAfee.
  • Firefox version 31 is improving malware detection. Firefox has long been using Google’s Safe Browsing service to check for malicious web sites, now it also checks downloaded files.
  • Isreal’s Homeland Security writes that anonymous hackers have launched DDoS attacks against network infrastructure from Israel. The attacks also affected DNS name resolution on domain names ending in .co.il.
  • The Register writes that Security outlet VUPEN has revealed it held onto a critical Internet Explorer vulnerability for three years before disclosing it at the March Pwn2Own hacker competition. VUPEN makes money by selling exploits to its customers.
  • The Moscow Times writes that Russia’s Interior Ministry has put out a tender on its official government procurement website for anyone who can identify Tor users. On a related note, the Tor team issued a security advisory this week, warning operators of hidden services about attacks to deanonymizing users. And if that’s not enough Tor news for this week, according to the Tor project’s latest annual financial statements (PDF), the US government increased its funding to 1.8 million US dollars in 2013!



Retefe Bankentrojaner

E-Banking ist seit seiner Entstehung ein attraktives Tummelfeld für Betrüger. Oft wird auf spezielle Schadsoftware, auf sogenannte Bankentrojaner, zurückgegriffen, um arglosen Opfern Geld abzuziehen.

Die meisten dieser Bankentrojaner basieren auf technisch betrachtet ziemlich komplexen Softwarekomponenten: Verschlüsselte Konfigurationen, Man-in-the-Browser-Funktionalität, Persistenz- und Updatemechanismen, um einige zu nennen. Im letzten halben Jahr hat sich eine gänzlich neue Variante behauptet, welche erst im Februar 2014 einen Namen erhielt: Retefe. Nur wenig wurde bis an hin publiziert, einer der Hauptgründe ist sicherlich, dass die Schadsoftware nur in wenigen Ländern (CH, AT, SE, JP) agiert und nur einige ausgewählte Banken angreift. TrendMicro (Blogartikel: Operation Emmental (DE), (EN)) und SWITCH-CERT möchten hiermit nun etwas detaillierter über diesen Trojaner berichten.

Das Besondere am Retefe Bankentrojaner ist seine Schlichtheit. Das infizierte System wird wie folgt manipuliert:

  1. Auf dem PC des Opfers wird der Eintrag des DNS-Servers auf einen bösartigen DNS-Server geändert.
  2. Auf dem PC des Opfers wird ein gefälschtes Root-Zertifikat installiert, siehe auch unser kürzlich veröffentlichten Blogartikel zu diesem Thema.


Nach der Infektion löscht sich die Installationsroutine selbst. Ausser dem manipulierten System bleibt nichts zurück, was es schwierig für Antiviren-Programme macht, im Nachhinein eine Infektion festzustellen.

An Eleganz ist diese Schadsoftware schwer zu übertreffen: Sie verzichtet auf die in der Einführung genannten Softwarekomponenten und minimiert damit die Komplexität. Es scheint auch, dass es aus Betrügersicht heutzutage ökonomischer ist, schlicht und einfach neue Opfer-PCs mittels Spam-Kampagnen zu infizieren.

Wie sieht der Modus Operandi konkret aus?

Modus Operandi eines möglichen Schadenfalls

Modus Operandi eines möglichen Schadenfalls

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Taking Advantage of DNSSEC

According to measurements by APNIC’s Geoff Huston currently 16 percent of Swiss Internet users use a DNSSEC validating DNS resolver. If you want to benefit from the added security with DNSSEC in your network then I suggest you enable DNSSEC validation in your network as well. SurfNet published a deployment guide recently that takes BIND 9.x, Unbound and Microsoft Windows Server 2012 into account.

Enabling DNSSEC validation on your DNS resolvers is one simple step and it protects you from DNS Cache Poisoning. However, if it were only for this, then the DNSSEC protocol complexity would come at a high cost for only providing this one benefit. In fact, DNSSEC is much more than only a protection from Cache Poisoning. It’s a new PKI in DNS and if you have signed your zone and are already validating then you can take advantage of that PKI. Some use cases are described below but there are many more ideas which are currently discussed.
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Verbindungsprobleme bei der DNS Namensauflösung erkennen

Die ursprüngliche Spezifikation von DNS-Nachrichten (RFC 1035), welche über UDP gesendet werden, ist auf 512 Bytes begrenzt. Bereits Ende der Neunzigerjahre wurde mit dem “Extension Mechanisms for DNS (EDNS0)” (RFC 2671) eine Erweiterung für das DNS-Protokoll festgelegt, welche es u.a. dem Client erlaubt eine grössere Buffergrösse bekannt zu geben.

Die meisten DNS Resolver-Implementierungen kündigen eine Buffergrösse von 4096 Bytes an. Das heisst aber auch, dass bei einer maximalen Paketgrösse (Maximum Transmission Unit, MTU) von 1500 Bytes eine Fragmentierung der Datenpakete stattfindet. Die Vergangenheit hat gezeigt, dass viele Netzwerk- und Sicherheitsprodukte diese Protokolländerung noch nicht mitgekriegt haben und auf eine kleinere Paketgrösse bestehen. Für DNSSEC ist die EDNS0-Erweiterung eine Voraussetzung, da durch die Signierung der Daten die Paketgrösse ansteigt.

Die Unterstützung einer grossen Buffergrösse von Ihrem DNS-Resolver zur Aussenwelt ist aber auch relevant, wenn Sie keine DNSSEC-Validierung auf dem DNS-Resolver aktiviert haben. Aktuelle DNS-Resolver kennzeichnen die Unterstützung von DNSSEC (durch das DO-bit, Bezeichnung “DNSSEC OK”-bit, RFC 3225) und erhalten deshalb in der Antwort die DNSSEC-Signaturen, auch wenn der DNS-Resolver die Validierung nicht aktiviert hat.
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