Host Header Manipulation Leading to Unauthorized Access to Apple’s Internal Slack bot (Bug Bounty)

This article is a repost from my previous blog, archived here for accessibility.

Vulnerability Summary:
This article outlines a security flaw I discovered within the subdomain gsdcb.apple.com, part of an internal bot at Apple designed to protect confidential information shared on the Slack platform, which is widely used for corporate communications.

The vulnerability could have enabled an external attacker to gain unauthorized access and integrate the internal bot to any third-party Slack channel not affiliated with Apple.

Technical Details:
The vulnerability was identified through my participation in the Apple bug bounty program, which involved directory enumeration and manipulation of the HTTP Host Header. During this process, I discovered that an open endpoint on gsdcb.apple.com/X failed to adequately validate user-supplied Host Headers, thereby creating a potential for unauthorized access.

Directly accessing the URL gsdcb.apple.com/X initially resulted in the server returning a white page, indicating a default or non-responsive behavior for unrecognized requests. However, by modifying the Host Header to “localhost”, I was able to bypass this blank response. This manipulation deceived the server into treating the request as originating from a trusted internal source, thereby enabling unauthorized access.

After gaining access to the internal bot, I chose not to explore further in order to comply with the Vulnerability Disclosure Policy. Following discussions with the Apple Product Security team, it was clarified later that additional security controls were in place to disallow querying of sensitive information from the Slack bot.

Mitigation and Resolution:
Apple quickly addressed the reported vulnerability by removing the exposed endpoint and reinforcing the Slack bot’s configuration to explicitly prevent any external access.

This article has been reviewed by the Apple Security Team prior to its publication.

Timeline:
December, 12, 2020: Reported
December 15, 2020: Patched
December 16, 2020: Fix confirmed
January, 15, 2021: Bounty awarded 💰

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