If you’re using Slack for team communication, there’s a good chance you’re sharing more than you think. Slack workspaces have become central hubs for business discussions, file sharing, and sensitive project details. But here’s the problem: many organizations unknowingly expose their internal conversations through simple configuration mistakes and careless practices. These leaks can expose customer data, internal strategies, financial information, and confidential discussions to people who should never see them.
Understanding these vulnerabilities isn’t just about security—it’s about protecting your business reputation, maintaining client trust, and avoiding potential legal issues. Let’s look at the most common ways Slack workspaces leak information and how to prevent them.
Public Channel Misconfigurations
The biggest mistake I see companies make is treating public channels like private ones. In Slack, public channels are visible to everyone in your workspace, including new members who join later. I once worked with a startup where the sales team discussed pricing strategies and client negotiations in a channel called #sales-general, thinking it was somehow restricted. When they hired a contractor for a two-week project, that person had immediate access to months of sensitive pricing discussions.
Public channels have their place, but they’re not for confidential information. Period. Review your channel list right now and check how many are set to public. Ask yourself: would you be comfortable if a new intern saw every message in that channel from the past year? If the answer is no, make it private.
Guest Access Gone Wrong
Guest accounts are incredibly useful for collaborating with external partners, consultants, or clients. But they’re also a major leak point when misconfigured. Many workspace admins don’t realize that guests can be added to multiple channels, and once they’re in, they see the entire message history.
Here’s a real scenario: a design agency added a client as a guest to review project progress. Someone accidentally invited that guest to an internal channel where the team had previously discussed the client’s budget constraints and markup percentages. The client saw everything. Awkward doesn’t begin to cover it.
Set strict policies for guest access. Create dedicated channels specifically for external collaboration and never add guests to channels with existing sensitive discussions. Better yet, use Slack Connect for external communications—it provides better isolation and control.
File Sharing Without Restrictions
Every file uploaded to Slack is stored and accessible through search, even if the original message gets deleted. People regularly upload documents containing passwords, API keys, customer lists, and financial data without realizing these files remain searchable indefinitely.
The file search function in Slack is powerful, and anyone in your workspace can use it to find documents they shouldn’t access. I’ve seen developers accidentally upload .env files with production database credentials, HR teams share salary spreadsheets in the wrong channels, and sales teams post unredacted client contracts.
Implement a clear policy: sensitive documents should never be uploaded directly to Slack. Use secure file-sharing platforms with proper access controls, and share only links in Slack. If you must upload something sensitive, use private messages and delete the file immediately after the recipient downloads it.
The Search Function Trap
Slack’s search is designed to be comprehensive, which is great for productivity but terrible for security. Any member can search across all public channels and their accessible private channels. This means that casual mention of a password, a customer complaint, or a strategic plan can be discovered months later by someone who wasn’t in the original conversation.
Think about how your team uses Slack. Do they casually mention client names, project codenames, or sensitive details thinking those messages disappear into history? They don’t. They’re all searchable, indexable, and discoverable.
Mobile App Notifications
Here’s one people forget: mobile notifications display message previews on lock screens. If someone’s phone is visible during a meeting, at a coffee shop, or on their desk, others can read incoming Slack messages. I’ve personally seen confidential acquisition discussions previewed on someone’s phone screen during a conference.
Encourage your team to disable message previews in their Slack notification settings. It’s a small change that prevents accidental exposure in public spaces.
Integration and Third-Party App Risks
Slack’s ecosystem includes thousands of integrations and bots. Each one you authorize gets access to specific channels and data. Many workspace admins approve these integrations without fully understanding what data they’re sharing.
Some integrations archive messages to external services, others analyze conversation patterns, and some export data for analytics. If that third-party service gets breached or mishandles data, your Slack messages are exposed. Always review what permissions an integration requests before installing it, and regularly audit which apps have access to your workspace.
Retention Policies and Deleted Messages
Many organizations believe deleted messages are truly gone. They’re not—at least not immediately, and sometimes not ever if you’re on certain Slack plans. Workspace owners can export all messages, including deleted ones, through Slack’s export features.
Even if you delete a message seconds after sending it, there’s a window where others might have seen it, screenshotted it, or where it was captured in an automated backup. Don’t rely on deletion as a security measure.
Common Questions About Slack Security
Can people outside my workspace see my messages? Not directly, but if you use Slack Connect or shared channels, those messages are visible to the connected workspace. Always verify who you’re sharing channels with.
Are private messages really private? Private messages between individuals are only visible to those people, but workspace admins with appropriate permissions can export all messages, including DMs, if your plan supports it.
What happens to messages when someone leaves? Their messages remain in channels and can still be searched. Their private messages with others also remain visible to those conversation participants.
Taking Action Today
Start by auditing your workspace. Check which channels are public that should be private. Review who has guest access and to which channels. Look at your installed integrations and remove any you’re not actively using. Set up clear guidelines for what can and cannot be shared in Slack.
Remember, Slack is a productivity tool, not a secure vault. Treat it accordingly. The convenience of quick communication shouldn’t come at the cost of exposing sensitive business information to unintended audiences.