5 June 2025
(Image credit: Shutterstock - monticello)
A serious vulnerability has been found in OneDrive's file selector system that puts the security of millions of users worldwide at risk. The problem, discovered by researchers at Oasis Security, involves the mechanism by which web applications request access to files stored on Microsoft's cloud service. Contrary to what one would expect from an individual file selector, the system grants external applications full read access to all content in the user's OneDrive account.
The scope of the problem is significant and involves some of the world's most widely used platforms. Affected applications include OpenAI's ChatGPT, Slack, Trello, ClickUp and hundreds of other services that implement OneDrive's File Picker to simplify document uploads. Users who used these features may have unknowingly granted the apps access to all of their personal and business files, far beyond what was required for the specific operation requested.
The flaw originates in the implementation of the OAuth scopes used by OneDrive. Microsoft has not provided granular scopes that allow applications to request access only to user-selected files, and according to Oasis Security, the File Picker requires read access to the entire drive anyway, even when the operation involves a single document. The situation is exacerbated by the vague and unclear language used in the consent prompts shown to users, according to the security firm: Microsoft does not clearly communicate the true scope of permissions granted, leaving users unable to distinguish between legitimate requests and potential data exfiltration attempts. Without specific OAuth scopes, even applications with legitimate intentions are forced to request excessive permissions.
File Picker version 8.0 presents additional critical issues related to the storage of security tokens: developers must independently manage authentication using Microsoft Authentication Library (MSAL), which stores sensitive tokens in the browser session store in plain text format. The authorization flow can also extend access for indefinite periods through update tokens, amplifying potential security risks. The consequences of this vulnerability can be severe for business organizations, with potential breaches of regulatory compliance and leaks of sensitive data that could compromise confidential customer information. Individual users, for their part, could see personal documents, photos and other private content exposed without being aware of it.
Oasis Security recommends that users immediately review all access permissions previously granted to third-party applications. For personal accounts, users can access the privacy settings of their Microsoft account and review the list of authorized applications. Corporate administrators can use Entra's administration interface to check delegated permissions at the organizational level. Microsoft has confirmed that it has received the vulnerability report and is considering future improvements to the service. However, no specific timeline for resolving the issue is currently available.
https://www.oasis.security/blog/onedrive-file-picker-security-flaw-oasis-research
