The first part defines Format Injection and explains interesting but low severity bug in Duo Web SDK.

Update 20 March Authy contacted me to clarify that not everybody was vulnerable, and vulnerable API libraries were limited to Node.JS by Daniel Barnes, Authy.NET by Devin Martin and Authy OpenVPN.

Meanwhile we audited another popular 2FA provider and found a High-severity format injection in Authy API. In fact the root of the problem was default Sinatra dependency “rack-protection”! I responsibly disclosed this vulnerability to Authy on February 8 and worked with them to fix the issue that same day.

There are two API calls:

  1. The client requests new token: where AUTHY_ID is publicly available identifier associated with current user account. Expected response: {"success":true,"message":"SMS token was sent","cellphone":"+1-XXX-XXX-XX85"} with 200 status.

  2. The user sends the token back and the client verifies if the token is valid with and authenticates with second factor if API responds with 200 status (body is ignored): {"success":true,"message":"Token is valid.","token":"is valid"}

Authy-node does not encode token from user params

There was a blatant bug in authy-node (not an official library, btw another popular node library wasn’t vulnerable) - “token” supplied by the user was not URL encoded at all: this._request("get", "/protected/json/verify/" + token + "/" + id, {}, callback, qs);

Which means by typing VALID_TOKEN_FOR_OTHER_AUTHY_ID/OTHER_AUTH_ID# we would overwrite the path and make the client send /protected/json/verify/VALID_TOKEN_FOR_OTHER_AUTHY_ID/OTHER_AUTH_ID#/AUTH_ID. Anything after hash # is ignored and Authy’s response with 200 status for /protected/json/verify/VALID_TOKEN_FOR_OTHER_AUTHY_ID/OTHER_AUTH_ID?api_key=KEY let’s the attacker in.

It’s impossible to distinguish forged request from a valid one on the server side because #/AUTHY_ID is not sent.

Authy-python is vulnerable too

Then I noticed Python’s urllib.quote doesn’t escape slashes. Indeed, for some reason it escapes everything but slashes and it’s a documented feature - urllib.quote("#?&=/") returns %23%3F%26%3D/. Which means our “../sms” will not be encoded (/../ means “go one directory up”).

Web browsers parse /../, /%2e%2e/ and even /%252e%252e/ and go “one directory up”, but web servers don’t have to do it. Anyway, I tried and it worked - Authy API was removing directories before /../.

It introduces path traversal making attacker’s job much easier - you only need to type ../sms to turn /verify API call into /sms (/verify/../sms/authy_id) which will always return 200 status and will bypass 2FA.

No, wait. Everyone is vulnerable!

Few hours later I realized what made path traversal work: I recently read Daniel’s interview on Authy and recalled it runs Sinatra, which uses rack-protection by default.

It turns out even URL encoding was futile - path_traversal module in rack-protection was decoding %2f back to slashes! This literally affects every API running Sinatra and reading parameters from the path. This is also a great example how libraries or features that aim to add security actually introduce security vulnerabilities (see also CSP for evil and XSS auditor for evil)

  1. The attacker types ../sms in the SMS token field

  2. The client app encodes it as ..%2fsms and makes an API call to Authy -

  3. Path_traversal middleware decodes path to, splits by slashes and removes the directory in front of /...

  4. Actual Authy API sees modified path, simply sends another SMS to authy_id (the victim) and responds with 200 status and {"success":true,"message":"SMS token was sent","cellphone":"+1-XXX-XXX-XX85"}

  5. All Authy SDK libraries consider 200 status as a successful response and let the attacker in. Even a custom integration most likely will look for "success":true in the JSON body, and our /sms response body has it. So the only secure way to verify the response is to search for "token":"is valid" substring (which is what Authy libraries do now).

Yes, the attacker was able to bypass 2 factor authentication on any website using Authy with something as simple as ../sms in the token field!

Timeline: reported on Feb 8, the path_traversal module was patched right away and we waited for a month to let authy-node users to update.

Update 20 March Authy contacted me to clarify that not everybody was vulnerable, and vulnerable API libraries were limited to Node.JS by Daniel Barnes, Authy.NET by Devin Martin and Authy OpenVPN.

This is another example of format injection and why you need to treat URLs as a format like JSON or XML. Read our first post on format injection in Duo Security Web SDK.

Mar 15, 2015 • Egor Homakov (@homakov)