Pierluigi Paganini December 16, 2018

Researcher Terence Eden discovered that the permissions dialog when authorizing certain apps to Twitter could expose direct messages to the third-party.

The flaw is triggered when apps that require a PIN to complete the authorization process instead of the using the OAuth protocol. The expert discovered that some permissions such as that to access direct messages, remained hidden to the Twitter user.

Terence Eden was awarded $2,940 for reporting the bug to Twitter under the bug bounty program operated through the HackerOne platform. According to Eden, the bug resides in the way the official Twitter API handles keys and secrets that could be accessed by app developers even without the service’s authorization.

“Many years ago the official Twitter API keys were leaked. This means that app authors who can’t get their app approved by Twitter are still able to access the Twitter API.” wrote Eden.

“For some reason, Twitter’s OAuth screen says that these apps do not have access to Direct Messages. But they do!

In short, users could be tricked into allowing access to their DMs.”

Twitter implemented some restrictions, the most important one is restricting callback addresses. After successful login, the apps will only return to a predefined URL preventing the abuse of the official Twitter keys to send the user to your app.

The problem is that there are some apps that haven’t a URL or don’t support callbacks. For these apps, Twitter has implemented an alternative authorisation mechanism, users log in, it provides a PIN, they type the PIN into their app.

In this alternative scenario, Eden discovered that apps did not show the correct OAuth details to the user, in particular, that the app was not able to access user direct messages.

Below the bug timeline:

• 2018-11-06 Submitted via HackerOne
• 2018-11-06 Provided clarification and PoC. Issue accepted.
• 2018-11-15 Proposed publication date of 30th November rejected due to US holidays.
• 2018-11-16 Bug Bounty of $2,940 offered. Filled in the W2 form to say I’m not a US taxpayer.
• 2018-11-17 Drank a fair amount of cider.
• 2018-11-21 £2,287.05 deposited in my UK bank account. There was also the option of receiving it via PayPal.
• 2018-12-06 Twitter fixed the issue and published the bounty payout. They let me know I was clear to publish.
• 2018-12-14 Published this report.

Pierluigi Paganini

(Security Affairs –Twitter,  hacking)

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