Partly as a consequence of the US Government shutdown, there are presently more than two hundred .gov websites using expired SSL certificates. Although the shutdown is expected to be a short term measure, the widespread use of expired certificates on .gov sites may cause long term harm. The US Government is effectively training its citizens and employees to click through SSL warnings, and once the users of a website treat SSL error messages as normal, attackers may be able to perform otherwise difficult man-in-the-middle attacks.
The situation is exacerbated by the behaviour of some mainstream browsers which do not faithfully warn the user of the most serious problem in scenarios where two or more errors are present.
An SSL error message presented on EV-enabled www.usaspending.gov in Google Chrome.
When an SSL error occurs, some browsers only display a single error message, sometimes not the most serious, or even a generic error message for all types of SSL error. An attacker can exploit this vulnerable browser behaviour on SSL sites with expired certificates to perform an almost seamless man-in-the-middle attack.
By signing his own expired SSL certificate for a US government website, the SSL error message displayed for the attacker's SSL certificate is indistinguishable (in some browsers) from the error message produced by the real SSL certificate belonging to the US Government. Citizens accustomed to seeing the "expired" error message will happily proceed with a connection using the attacker's expired (and untrusted) certificate, unwittingly communicating with the attacker instead of the US Government.
By testing an expired certificate signed by an expired untrusted issuer, Netcraft found that whilst some browsers are vulnerable, Internet Explorer is not as it correctly displays both error messages. Google Chrome on Windows and OS X displays the more serious error message but does not display a warning about the expiry. All other tested browsers displayed either a generic error message or did not mention that the issuing CA is not widely trusted. Generic error messages are dangerous if they hide the severity of the SSL error from the user: a change in the type of the SSL error (from expiry to an untrusted issuer) will not be noticed. The tested website contained in the screenshots below is not on a .gov domain, but demonstrates browser behaviour with an untrusted and expired CA certificate with an expired end-entity certificate.
Google Chrome displaying an error message for an expired SSL certificate issued by an untrusted CA. From left to right: Windows, Mac OS X, Linux, and Android.
Google Chrome's behaviour is not consistent across its supported platforms: on Windows and Mac OS X it displays the most serious SSL error message, namely that an untrusted issuer has signed this SSL certificate. On Linux and Android, however, Google Chrome displays an error message about the expired certificate and does not mention the untrusted issuer. By reading the error message and accepting the risks of trusting an expired certificate, a user may unwittingly trust an SSL certificate that was not issued by a widely trusted CA.
Internet Explorer and Opera displaying an error message for an expired SSL certificate issued by an untrusted CA.
On Windows, Internet Explorer correctly presented both applicable error messages. Opera presented the more serious error message though only after viewing an additional dialogue box. Once a user is accustomed to accepting Opera's generic error message, any other type of SSL error on the same website is unlikely to be noticed. Internet Explorer, Google Chrome, and Opera all use Microsoft's CryptoAPI on the Windows platform which may explain their similar behaviour.
Firefox displaying an error message for an expired SSL certificate issued by an untrusted CA.
Firefox, which displays a generic error message for most SSL errors, has further information hidden by default. For an expired certificate issued by an untrusted and expired CA, Firefox's error message refers only to expired certificates (both the CA and end-entity certificates) and does not make any mention of the issuer not being a widely-trusted CA. Hidden details mean that a user having seen the same error message on the .gov website may not notice a change in the category of the SSL error message.
Safari (on OS X and iOS) displaying an error message for an expired SSL certificate issued by an untrusted CA.
Safari on OS X, like both Firefox and Opera displays a generic error message. If the message is expanded, Safari displays an error message based on the expired certificate and will also highlight the lack of trust in the issuer. Safari on iOS 7 displays a generic error message, "Not trusted", for many types of SSL certificate error — it is difficult to tell what is wrong with the SSL certificate without examining the certificate in detail.
Even without the "training" from the US Government, the click-through rate of different SSL messages has been demonstrated to be very high. For Firefox, which doesn't display full error messages by default, Akhawe and Porter Felt found SSL error messages were bypassed in 85% of cases: 87% for untrusted issuer messages and 81% for expired certificate errors. Paradoxically, in Google Chrome expired certificate error messages were dismissed 57% of the time whereas error messages for an untrusted issuer (the more serious problem) were dismissed in 82% of studied cases.