Dozens of U.S. government websites have been rendered either insecure or inaccessible during the ongoing U.S. federal shutdown. These sites include sensitive government payment portals and remote access services, affecting the likes of NASA, the U.S. Department of Justice, and the Court of Appeals.

The DigiCert certificate used by this U.S. Court of Appeals website expired on 5 January 2019 and has not yet been renewed. The site provides links to a document filing system and PACER (Public Access to Court Electronic Records).

With around 400,000 federal employees currently furloughed, more than 80 TLS certificates used by .gov websites have so far expired without being renewed. To compound the situation, some of these abandoned websites can no longer be accessed due to strict security measures that were implemented long before the shutdown started.

One such example is https://ows2.usdoj.gov, a U.S. Department of Justice website which uses a certificate that expired in the week leading up the shutdown. The certificate has been signed by a trusted certificate authority, GoDaddy, but it has not been renewed since it expired on 17 December 2018.

All U.S. Department of Justice subdomains are covered by an HSTS policy. Combined with an expired TLS certificate, this currently makes it difficult for regular users to ignore the warnings and use the website.

In a twist of fate, the usdoj.gov domain — and all of its subdomains — are included in Chromium's HSTS preload list. This is a prudent security measure which forces modern browsers to only use secure, encrypted protocols when accessing the U.S. DoJ websites; however, it will also prevent users from visiting the HTTPS sites when an expired certificate is encountered. In these cases, modern browsers like Google Chrome and Mozilla Firefox deliberately hide the advanced option that would let the user bypass the warning and continue through to the site.

While this behaviour is bound to frustrate some users, in this case, security is arguably better than usability when you can't have both. If users were to ignore such warnings, they would be vulnerable to the type of man-in-the-middle attacks that TLS certificates were intended to combat.

However, only a few of the affected .gov sites implement correctly-functioning HSTS policies. Just a handful of the sites appear in the HSTS preload list, and only a small proportion of the rest attempt to set a policy via the Strict-Transport-Security HTTP header – but the latter policies will not be obeyed when they are served alongside an expired certificate, and so will only be effective if the user has already visited the sites before.

Consequently, most of the affected sites will display an interstitial security warning that the user will be able to bypass. This introduces some realistic security concerns, as task-oriented users are more likely to ignore these security warnings, and will therefore render themselves vulnerable to man-in-the-middle attacks.

For example, https://rockettest.nasa.gov/ is not included in the HSTS preload list, and its certificate expired on 5 January 2019. This causes browsers to display an interstitial security warning that users can ignore.

This NASA website is still using an expired certificate, but the domain does not appear on the HSTS preload list. Users can therefore ignore the browser's warnings and proceed to the site.

The following example clearly demonstrates the potential dangers of ignoring browser security warnings. The certificate used by this Berkeley Lab .gov website at https://d2l.lbl.gov expired on 8 January 2019 (although Berkeley Lab was not affected by the shutdown) and has not yet been replaced. As there is no effective HSTS policy, users can ignore the browser's warnings and proceed to the login form.

Encouraging users to ignore browser warnings could make them more susceptible to man-in-the-middle attacks. In this example, clicking next to the browser's address bar will explicitly advise the user not to enter any sensitive information, such as passwords – but anyone who really needs to use the site may foolishly end up doing so anyway.

With Donald Trump seemingly unwilling to compromise on his demands for a wall along the border with Mexico, and Democrats refusing to approve a budget containing $5.7bn for the wall, the hundreds of thousands of unpaid federal employees might not be the only ones hurting. As more and more certificates used by government websites inevitably expire over the following days, weeks — or maybe even months — there could be some realistic opportunities to undermine the security of all U.S. citizens.

Thousands of phishing sites have been finding homes in special hidden directories on compromised web servers.

In the past month alone, over 400 new phishing sites were found hosted within directories named /.well-known/; but rather than being created by fraudsters, these special directories are already present on millions of websites.

A Microsoft Excel Online phishing site hosted in the /.well-known/ directory on a compromised web server. The phishing site piggybacks on the trust instilled by the compromised site's existing SSL certificate, which has not been revoked.

The /.well-known/ directory acts as a URI path prefix for "well-known locations", as defined by IETF RFC 5785, and provides a way for both humans and automated processes to discover a website's policies and other information.

One of the most common legitimate uses of the /.well-known/ directory is to prove control over a domain. When a secure website uses the Automatic Certificate Management Environment (ACME) protocol to manage its SSL certificate, the issuer will verify ownership by checking for a unique token in /.well-known/acme-challenge/ or /.well-known/pki-validation/. Consequently, most of the phishing attacks that make use of the /.well-known/ directory have been deployed on sites that support HTTPS, using certificates issued by ACME-driven certificate authorities like Let's Encrypt and cPanel.

Due to the success of Let's Encrypt and ACME, millions of websites now have a /.well-known/ directory in their web root, although many website administrators may be oblivious to its presence – particularly if they did not create the directory themselves. The directory can also easily be overlooked, as a bare ls command will treat files or directories that start with a "." as hidden. These factors make /.well-known/ an ideal place to smuggle phish onto a compromised web server.

Around 3% of these phishing sites are mistakenly deployed in a /well-known/ directory, without a leading "." character. This mistake could stem from file system name limitations if the phishing kit was created on a Windows computer. This screenshot shows a Bank of America phishing kit that would be installed in a /well-known/ directory when unzipped.

Shared hosting platforms are particularly vulnerable to misuse if the file system permissions on the /.well-known/ directories are overly permissive, allowing one website to place content on another customer's website. Some of the individual servers involved in these attacks were hosting "well-known" phishing sites for multiple hostnames, which lends weight to this hypothesis.

Other well-known URIs

In addition to pki-validation and acme-challenge, there are 30 other widely recognised well-known URI suffixes defined by the IETF, W3C and others. For example, the EFF came up with the dnt-policy.txt suffix, which allows websites to announce their compliance with user opt-outs from tracking. The EFF's own Do Not Track Compliance Policy can be viewed at https://www.eff.org/.well-known/dnt-policy.txt.

Where multiple resources may be required, the well-known URI suffix is a directory rather than a file. For example, the IETF's Enrollment over Secure Transport RFC defines a set of resources that can be found under the /.well-known/est/ path.

Despite there being several other well-known URI directory suffixes, only pki-validation and acme-challenge have been used to host recent phishing sites. In fact, more than half of the phishing sites found under the /.well-known/ directory were planted within the subdirectories created by ACME clients (i.e. /.well-known/pki-validation/ and /.well-known/acme-challenge/), possibly making them even less likely to be noticed by the website administrators.

An Alibaba phishing site. More than half of all "well-known" phishing sites are installed in the directories used by ACME clients, although this does not necessarily mean the ACME clients are to blame.

The possible route of compromise is not always apparent in the aforementioned cases, but if there are any glaring security misconfigurations, a proposed new well-known URI suffix, security.txt, could come in handy. By placing contact details and disclosure policies in /.well-known/security.txt, website administrators can make it safer and easier for security researchers to reach out and report any problems they find.