Oasis Security's research team uncovered a flaw in Microsoft's OneDrive File Picker that allows websites to access a user’s entire OneDrive content, rather than just the specific files selected for upload via OneDrive File Picker. Researchers estimate that hundreds of apps are affected, including ChatGPT, Slack, Trello, and ClickUp–meaning millions of users may have already granted these apps access to their OneDrive. This flaw could have severe consequences, including customer data leakage and violation of compliance regulations.

Upon discovery, Oasis reported the flaw to Microsoft and advised vendors using OneDrive File Picker of the issue. In response, Microsoft is considering future improvements, including more precise alignment between what OneDrive File Picker does and the access it requires.

Below are details of the flaw and mitigation strategies. You can read the Oasis Security Research team’s full report here.

The Flaws
Excessive Permissions in the OneDrive File Picker
The official OneDrive File Picker implementation requests read access to the entire drive – even when uploading just a single file – due to the lack of fine-grained OAuth scopes for OneDrive.

While users are prompted to provide consent before completing an upload, the prompt’s vague and unclear language does not communicate the level of access being granted, leaving users open to unexpected security risks.

The lack of fine-grained scopes makes it impossible for users to distinguish between malicious apps that target all files and legitimate apps that ask for excessive permissions simply because there is no other secure option.

Insecure Storage of Sensitive Secrets
Sensitive secrets used for this access are often stored insecurely by default.

The latest version of OneDrive File Picker (8.0) requires developers to take care of the authentication themselves, typically using the Microsoft Authentication Library (MSAL) and most likely using the Authorization Flow.

Security risks ensue:

MSAL stores sensitive Tokens in the browser’s session storage in plain text.
With Authorization Flows a Refresh Token may also be issued, which lengthens the access period, providing ongoing access to the user's data.
Notably, OpenAI uses version 8.0.

Mitigation Steps
The lack of fine-grained OAuth scopes combined with Microsoft’s vague user prompt is a dangerous combination that puts both personal and enterprise users at risk. Oasis Security recommends that individuals and technology leaders review the third-party access they’ve granted to their account to mitigate the potential risks raised by these issues.

Check Whether or Not You’ve Previously Granted Access to a Vendor
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How to for Private Accounts
Log in to your Microsoft Account.
In the left or top pane, click on "Privacy".
Under "App Access", select the list of apps that have access to your account.
Review the list of apps, and for each app, click on “Details” to view the specific scopes and permissions granted.
You can “Stop Sharing” at any time. Consider that an Access Token takes about an hour to expire regardless of when you clicked stopped sharing. This would however revoke a Refresh Token if present.

Oasis Security's research team uncovered a critical vulnerability in Microsoft's Multi-Factor Authentication (MFA) implementation, allowing attackers to bypass it and gain unauthorized access to the user’s account, including Outlook emails, OneDrive files, Teams chats, Azure Cloud, and more. Microsoft has more than 400 million paid Office 365 seats, making the consequences of this vulnerability far-reaching.

The bypass was simple: it took around an hour to execute, required no user interaction and did not generate any notification or provide the account holder with any indication of trouble.