The Permissions-Policy directive (formerly known as Feature-Policy) is a modern web security tool designed to give website owners and developers granular control over which browser features their web pages can access.

One of the more recently introduced directives within this policy is the "web-share" directive, which governs access to the Web Share API. This API allows websites to share text, links, and files with other apps on a user’s device, such as messaging apps, email clients, or social media platforms, without leaving the web page. While it offers convenience and integration between web and native applications, unrestricted access to this API can introduce security and privacy risks.

History and Origin of Permissions-Policy

The Permissions-Policy header was first introduced by Google in 2018 as Feature-Policy. Its aim was to provide developers with more control over which powerful browser features their websites could access. This approach became necessary as web applications became more complex, gaining access to device capabilities such as cameras, microphones, sensors, and file systems. Without proper restrictions, these capabilities could be exploited by malicious actors or misused by third-party content embedded within websites.

The Web Share API was added as part of this growing list of device access APIs to provide a more integrated sharing experience on the web. Developed by Google, it allows users to share text, URLs, and files directly from a web page to native applications installed on their device, such as social media platforms, messaging apps, or email clients. While this API brings many benefits for user experience, particularly on mobile devices, its misuse could lead to privacy violations or phishing attacks.

The web-share directive in Permissions-Policy was introduced to give developers control over when and how this API is used, ensuring that it is only accessible by trusted websites and content.

What Does the Web-Share Directive Do?

The "web-share" directive in the Permissions-Policy header controls whether a website or any embedded content can access the Web Share API. This API enables websites to trigger the native sharing functionality of a user’s device, allowing them to share links, text, and files directly from the web page to native applications like messaging platforms or email clients.

For example:

• Setting web-share=() disables access to the Web Share API, ensuring that the website or any embedded content cannot trigger native sharing actions.
• Setting web-share=* enables access to the Web Share API, allowing both the website and embedded content to initiate sharing requests.

This control is crucial for preventing misuse of the Web Share API by unauthorised or malicious websites, protecting user privacy and reducing the risk of phishing or deceptive actions.

Why Was It Added?

The web-share directive was added to address several key privacy and security concerns related to the use of the Web Share API:

1. Preventing Unauthorised Sharing: The Web Share API allows websites to initiate sharing actions, but without proper restrictions, this capability could be exploited by malicious websites to share sensitive data without user consent. The web-share directive was introduced to ensure that only trusted and necessary content can initiate these actions, protecting users from unauthorised sharing of their personal information, files, or links.

2. Mitigating Phishing Risks: By using the Web Share API, malicious websites could craft deceptive sharing prompts that encourage users to share sensitive information or misleading links. Attackers could exploit this to perform phishing attacks, tricking users into sharing personal data with malicious applications or services. The web-share directive helps mitigate this risk by allowing developers to restrict API access, ensuring that only trusted websites can request sharing actions.

3. Enhancing User Privacy: Sharing data between websites and native applications could expose sensitive information, especially if the shared content includes personal data or files. If unauthorised websites have access to the Web Share API, users could inadvertently share private information with unintended parties. The web-share directive allows developers to prevent unauthorised content from accessing this API, preserving user privacy.

4. Controlling Third-Party Content: Many websites embed third-party content, such as ads or widgets, that could misuse the Web Share API to initiate sharing actions without the user’s explicit consent. The web-share directive allows website owners to prevent third-party scripts from triggering sharing functionality, reducing the risk of misuse or deceptive behaviour.

Use Cases It Guards Against

The web-share directive is particularly useful for protecting against several problematic scenarios:

1. Deceptive Sharing Prompts: Malicious websites could use the Web Share API to create deceptive prompts that encourage users to share sensitive information, such as personal data or financial details, with untrustworthy services. The web-share directive helps prevent this by restricting access to the API, ensuring that only trusted websites can initiate sharing actions.

2. Unintended Data Sharing: If the Web Share API is used without proper user consent, websites could trigger sharing actions that send personal data, files, or links to third-party apps without the user’s knowledge. This could expose sensitive information to unauthorised recipients. By using the web-share directive, developers can prevent this type of misuse, protecting user privacy and ensuring that sharing actions are always initiated intentionally.

3. Phishing Attacks: Attackers could exploit the Web Share API to trick users into sharing links to phishing sites or malicious content. By crafting convincing prompts, they could deceive users into distributing harmful links to their contacts. The web-share directive helps mitigate this risk by ensuring that only trusted content can access the API, reducing the likelihood of phishing attacks through deceptive sharing prompts.

4. Third-Party Content Misuse: Many websites incorporate third-party content, such as advertisements or social media widgets, which could misuse the Web Share API to initiate unwanted sharing actions. By configuring the web-share directive, website owners can ensure that only the necessary parts of the site have access to the API, preventing third-party content from misusing this functionality.

Why Should You Set Permissions-Policy Web-Share Correctly?

There are several compelling reasons why website owners should ensure the web-share directive is configured properly:

1. Protecting User Privacy: The Web Share API provides websites with the ability to share content between the browser and native applications. If misused, this feature could result in the unintended sharing of personal information, files, or private links. By setting the web-share directive to restrict access, website owners can protect user privacy and prevent unauthorised sharing actions.

2. Preventing Phishing Attacks: Phishing attacks can be carried out through deceptive sharing prompts that trick users into sharing malicious links or sensitive data. By controlling access to the Web Share API through the web-share directive, developers can prevent malicious websites or third-party content from initiating sharing requests, reducing the risk of phishing attempts.

3. Building User Trust: Users expect that websites will respect their privacy and security, particularly when it comes to sharing content with external applications. Websites that misuse the Web Share API risk losing user trust, leading to lower engagement and higher bounce rates. By configuring the web-share directive properly, website owners can demonstrate their commitment to safeguarding user privacy and security.

4. Preventing Third-Party Content Misuse: Websites often incorporate third-party content, such as social media plugins or ads, which could misuse the Web Share API to trigger unwanted sharing actions. Properly configuring the web-share directive ensures that only trusted, first-party content can access the API, protecting users from third-party misuse and preserving a positive user experience.

5. Maintaining Compliance with Privacy Regulations: With strict data privacy regulations like GDPR, websites must take steps to protect user data and ensure that sharing actions are only carried out with explicit user consent. Misusing the Web Share API could lead to unauthorised data sharing and privacy violations. By configuring the web-share directive correctly, website owners can maintain compliance with privacy regulations and avoid legal repercussions.

Conclusion: Safeguarding User Privacy with the Web-Share Directive

The Permissions-Policy web-share directive is a powerful tool for managing access to the Web Share API, which allows websites to share text, links, and files with native applications. While this API enhances user experience and integration between web and native apps, it also introduces security and privacy risks if misused. These risks include unauthorised sharing, phishing attacks, and privacy violations.

By configuring the web-share directive correctly, website owners can prevent malicious or unauthorised content from accessing the Web Share API, protecting users from deceptive sharing prompts, unintended data sharing, and third-party misuse. Ensuring that only trusted, necessary content can access the API is crucial for maintaining user trust, protecting privacy, and complying with data protection regulations. For any website that utilises the Web Share API, setting the web-share directive appropriately is an essential step in creating a secure and trustworthy user experience.