HSTS your curl

HTTP Strict Transport Security (HSTS) is a standard HTTP response header for sites to tell the client that for a specified period of time into the future, that host is not to be accessed with plain HTTP but only using HTTPS. Documented in RFC 6797 from 2012.

The idea is of course to reduce the risk for man-in-the-middle attacks when the server resources might be accessible via both HTTP and HTTPS, perhaps due to legacy or just as an upgrade path. Every access to the HTTP version is then a risk that you get back tampered content.

Browsers preload

These headers have been supported by the popular browsers for years already, and they also have a system setup for preloading a set of sites. Sites that exist in their preload list then never get accessed over HTTP since they know of their HSTS state already when the browser is fired up for the first time.

The entire .dev top-level domain is even in that preload list so you can in fact never access a web site on that top-level domain over HTTP with the major browsers.

With the curl tool

Starting in curl 7.74.0, curl has experimental support for HSTS. Experimental means it isn’t enabled by default and we discourage use of it in production. (Scheduled to be released in December 2020.)

You instruct curl to understand HSTS and to load/save a cache with HSTS information using --hsts <filename>. The HSTS cache saved into that file is then updated on exit and if you do repeated invokes with the same cache file, it will effectively avoid clear text HTTP accesses for as long as the HSTS headers tell it.

I envision that users will simply use a small hsts cache file for specific use cases rather than anyone ever really want to have or use a “complete” preload list of domains such as the one the browsers use, as that’s a huge list of sites and for most use cases just completely unnecessary to load and handle.

With libcurl

Possibly, this feature is more useful and appreciated by applications that use libcurl for HTTP(S) transfers. With libcurl the application can set a file name to use for loading and saving the cache but it also gets some added options for more flexibility and powers. Here’s a quick overview:

CURLOPT_HSTS – lets you set a file name to read/write the HSTS cache from/to.

CURLOPT_HSTS_CTRL – enable HSTS functionality for this transfer

CURLOPT_HSTSREADFUNCTION – this callback gets called by libcurl when it is about to start a transfer and lets the application preload HSTS entries – as if they had been read over the wire and been added to the cache.

CURLOPT_HSTSWRITEFUNCTION – this callback gets called repeatedly when libcurl flushes its in-memory cache and allows the application to save the cache somewhere and similar things.

Feedback?

I trust you understand that I’m very very keen on getting feedback on how this works, on the API and your use cases. Both negative and positive. Whatever your thoughts are really!

4 thoughts on “HSTS your curl”

  1. So cool having hsts.

    I haven’t tested 7.74 yet (sorry).
    What about CURLINFO_EFFECTIVE_URL?
    Is there a way to know when the original URL is modified because of hsts?

    1. @Friki: very good question. I hadn’t really thought about it, but I believe CURLINFO_EFFECTIVE_URL will show the HTTPS version of it. I’ll write up a test case to make sure it works like that…

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