The essentials in 30 seconds
- Leaks to a personal cloud, Teams or Slack are ordinary actions, often made in good faith, that travel over legitimate, encrypted channels.
- The problem is not a lack of tools but the gap between the written policy and the actual behaviour of the control.
- You measure it by replaying each path with synthetic data, simple workarounds and detection delay included.
A data leak to a personal cloud is the movement of company information to a space the organisation does not control: a private storage account, a guest space in Teams or Slack, a sharing link open to anyone. It is almost never a sophisticated attack. It is an ordinary action, often made in good faith, that your controls should detect or block, and that therefore needs to be tested.
These everyday leaks are the most frequent and the least visible. They travel over legitimate, encrypted channels allowed by the proxy, and they look exactly like normal work. Here is how to spot them and, above all, how to check that your protections really hold.
Four egress paths your users take every week
Before talking about controls, the paths need naming. There are only a few of them, but each has its blind spots.
Diagram · four egress paths
Uploading to a personal account
An employee wants to finish a presentation over the weekend. They drag the file into their personal storage from the browser, or sign in to the consumer version of the service the company already uses in its business edition. The domain is the same, and so is the certificate. Only the tenant changes. If your control reasons by domain, it sees nothing.
External sharing from the collaboration tool
In a team space, a file is shared with an outside address: a contractor, a former colleague, the user’s own personal address. This last case is the most insidious, because it looks like ordinary professional sharing.
Guests and shared channels
Guest accounts in Teams and channels shared between organisations in Slack open a lasting door. A guest added for a three-month project often still has access a year later. Everything posted in the channel is visible to them, including attachments nobody would ever have sent by email.
Public links
The “anyone with the link” option is convenient. It is also a document published on the Internet, with no authentication and often no expiry date. It ends up in a ticket, a forwarded email, a third-party tool. Nobody knows any more who has access to it.
Why these leaks slip past conventional controls
The problem is not a lack of tools. Most organisations have a DLP, a CASB or the native features of their office suite. The problem is the gap between the written policy and the actual behaviour of the control.
- The traffic is legitimate and encrypted. An upload to personal storage is an HTTPS request to an allowed service. Without inspection and without tenant distinction, it is invisible (see the blind spots of TLS inspection).
- Policies rely on classification. A file that is unlabelled, renamed or compressed often triggers no rule at all.
- Sharing settings are decentralised. Team owners, site administrators and default settings contradict each other. The global configuration says “forbidden”, a legacy site says “allowed”.
- Rules fail silently. A policy in audit mode, a forgotten exception, an uncovered connector: nothing shows up in the console. This is the typical case of DLP false negatives.
A concrete scenario: an ordinary Friday evening
A sales manager is preparing a proposal. She exports the price list and an extract of the customer file, drops them into her personal storage to review them at home, then shares the folder with her private address “just in case”. On Monday, she posts the final version in a Slack channel shared with a partner, where a guest from a former project is still present.
Three exits in three days, no malice at all. The questions a CISO must ask:
- Was the upload to the personal account blocked, or at least logged with the file name?
- Did sharing to a private address trigger an alert or a refusal?
- Could the forgotten guest open the attachment? Does anyone know?
- How long did it take before the SOC saw something, if it saw anything at all?
If you cannot answer with evidence, you have an assumption, not a protection.
What to test, channel by channel
The following table summarises the useful checks. All of them are carried out with synthetic data that mimics your sensitive formats, never with real data.
| Channel | What to check | Expected result |
|---|---|---|
| Personal storage from the browser | Upload of a sensitive decoy to a non-business tenant of the same service | Block or alert, with tenant distinction |
| Sync client | Copy into a folder synchronised with a private account on the workstation | Detection on the endpoint side |
| External sharing | Sharing a labelled document to a consumer email address | Refusal or a logged request for justification |
| Guests and shared channels | Posting a decoy in a channel that includes a guest | Rule applied to the message and to the attachment |
| Public links | Creating an anonymous link on sensitive content | Creation refused or expiring link, alert to the owner |
| Simple workarounds | Same decoy renamed, compressed, pasted as text | Detection maintained |
Two points often make the difference. First, test the simple workaround, the one a user in a hurry would try without thinking: changing the extension, pasting the content into a message. Second, measure the detection delay, not just the block. An alert that arrives three days later is not worth much. For the cloud and SaaS side, the CASB and SaaS DLP guide details the method.
Next step
Do your controls see these everyday leaks?
A 30-minute guided demo, on pre-built result sets: the campaign across the 8 channels covered, the report and its detail, nothing to install. The POC then replays one standard scenario with synthetic data, from a representative machine in your network, through your real egress controls. What actually gets out through your egress paths is what the pilot shows you.
Remediation checklist
Once the gaps have been measured, the most cost-effective fixes are rarely spectacular:
- Restrict access to approved tenants rather than to domains.
- Forbid anonymous links by default, or enforce a short expiry.
- Review guests periodically and set an end date for their access.
- Align the sharing settings of legacy sites and teams with the global policy.
- Move validated rules out of audit mode, one by one.
- Retest after every change: an unverified fix is still an assumption.
The point is not to block everything. An overly rigid policy pushes teams towards even less visible tools (that is often where shadow IT begins). The aim is to know, with evidence to back it up, what gets out and what does not.
Frequently asked questions
Is a leak to a personal cloud necessarily malicious?
No. In the vast majority of cases, it is a matter of convenience. That is precisely why it has to be handled by technical controls and tested, not just by an IT charter.
Are the native DLP features of Microsoft 365 or Google Workspace enough?
They cover some of the cases, provided they are correctly configured and enforced. They have poor visibility of what happens outside their scope, such as an upload to a competing service. Only a test tells you the actual coverage.
Can these scenarios be tested in production without risk?
As far as real data is concerned, yes, provided you use only synthetic data that reproduces the form of your sensitive data: no real data leaves the organisation. The test still carries operational risks: bursts of alerts at the SOC or MSSP, a temporary load on the workstations tested. We frame them in writing: SOC notified 48 hours in advance, agreed time slots, stop possible in under 5 minutes.
How often should you retest?
After every change of policy, licence or tool, and at regular intervals in between. Sharing settings drift quickly, especially in collaborative environments.