A secret in a URL

Summary: Capability URLs are a lot more secure than people give them credit for.

These type of URLs are commonly known as capability / secret URLs.

It's meaningless to talk about security without specifying a threat model. Here are a couple that come to mind:

• 1: A passive attacker on the network (eavesdroping)
• 2: An active attacker on the network (can change packets at will, mitm, etc)
• 3: A shoulder-surfer
• 4: An attacker with physical access to your computer / elevated privileges
• 5: another user of your computer (regular privileges / remote access)
• 6: the user itself (as in protecting a API key)

Regarding network attacks (1 and 2), secret URLs are perfectly secure, provided you're using HTTPS (it's 2016, you shouldn't be using HTTP anymore!).

While the hostname of a server is sent in plaintext over the network, the actual URL is encrypted before being sent to the server - as it's part of the GET request, which only occurs after the TLS handshake.

Regarding shoulder-surfing (3), a secret URL with enough entropy is reasonably secure against a casual attack· As an example, I'll give a google docs URL:

https://docs.google.com/document/d/5BPuCpxGkVOxkjTG0QrS-JoaImEE-kNAi0Ma9DP1gy

Good luck remembering that while passing by a co-worker's screen!

Obviously, if your attacker has a camera and can take a picture without being noticed, it's an entirely different matter - you shouldn't use a secret URL in that situation.

Regarding an attacker with elevated privileges on your computer (4), a secret URL is not less secure than a long password or even a client-side TLS certificate - as all of those are actually completely insecure, and there's not much you can do about that.

An attacker with regular privileges (5), on the other hand, should not be able to learn the secret URL as well, as long as you follow good security practices for your OS. Your files (particularly browser history) should not be readable by other users.

For protecting API keys (6, which was the point of this question), a secret URL is also no-less-secure than another mechanism (such as an AJAX POST). Anyone that has an use for an API key will know how to use the browser debug mode to get the key.

It's not reasonable to send someone a secret and expect them not to look at it!

Some people have asked about the risks on the server side.

It's not reasonable to treat server-side risks by threat modelling; from a user perspective, you really have to treat the server as a trusted third party, as if your adversary has internal network access on the server side, there's really nothing you can do (very much like a privileged attacker on the client's computer, i.e. threat model 4 above).

Instead of modelling attacks, I'll outline common risks of unintentional secret exposure.

The most common concern with using secret URLs on the server side is that both HTTP server and reverse proxies keep logs, and the URL is very often included.

Another possibility is that the secret URLs could be generated in a predictable way - either because of a flawed implementation, a insecure PRNG, or giving insufficient entropy when seeding it.

There are also many caveats that have to be taken into consideration when designing a site that uses secret URLs. This page by W3C TAG covers many of them.

In practice, for sites with dynamic content, it's quite hard to get everything done securely - both Google and Dropbox botched it in the past, as mentioned on this answer

Finally, secret URLs have a couple of advantages over other authentication methods:

• They are extremely easy to use (just click the link, as opposed to entering your email and password)
• They don't require the server / service to securely store sensitive user credentials
• They are easily shareable without risks, unlike sharing you password (which you reuse for 50 other sites).