GitHub Security Advisory: GHSA-3cx6-j9j4-54mp
Package: decidim (RUBYGEMS)
Severity: HIGH (CVSS 0.0)
CWE: CWE-200: Exposure of Sensitive Information to an Unauthorized Actor, CWE-703: Improper Check or Handling of Exceptional Conditions
Vulnerable versions: >= 0.30.0, < 0.30.4
Patched in: 0.30.4

Summary: Decidim's private data exports can lead to data leaks

Details:
### Impact
Private data exports can lead to data leaks in cases where the UUID generation causes collisions for the generated UUIDs.

The bug was introduced by #13571 and affects Decidim versions 0.30.0 or newer (currently 2025-09-23).

This issue  was discovered by running the following spec several times in a row, as it can randomly fail due to this bug:

```bash
$ cd decidim-core
$ for i in {1..10}; do bundle exec rspec spec/jobs/decidim/download_your_data_export_job_spec.rb -e "deletes the" || break ; done
```

Run the spec as many times as needed to hit a UUID that converts to `0` through `.to_i`.

The UUID to zero conversion does not cause a security issue but the security issue is demonstrated with the following example.

The following code regenerates the issue by assigning a predefined UUID that will generate a collision (example assumes there are already two existing users in the system):

```ruby
# Create the ZIP buffers to be stored
buffer1 = Zip::OutputStream.write_buffer do |out|
  out.put_next_entry("admin.txt")
  out.write "Hello, admin!"
end
buffer1.rewind
buffer2 = Zip::OutputStream.write_buffer do |out|
  out.put_next_entry("user.txt")
  out.write "Hello, user!"
end
buffer2.rewind

# Create the private exports with a predefined IDs
user1 = Decidim::User.find(1)
export = user1.private_exports.build
export.id = "0210ae70-482b-4671-b758-35e13e0097a9"
export.export_type = "download_your_data"
export.file.attach(io: buffer1, filename: "foobar.zip", content_type: "application/zip")
export.expires_at = Decidim.download_your_data_expiry_time.from_now
export.metadata = {}
export.save!


user2 = Decidim::User.find(2)
export = user2.private_exports.build
export.id = "0210d2df-a0c7-40aa-ad97-2dae5083e3b8"
export.export_type = "download_your_data"
export.file.attach(io: buffer2, filename: "foobar.zip", content_type: "application/zip")
export.expires_at = Decidim.download_your_data_expiry_time.from_now
export.metadata = {}
export.save!
```

Expect to see an error in the situation.

Now, login as user with ID 1, go to `/download_your_data`, click "Download file" from the export and expect to see the data that should be attached to user with ID 2. This is an artificially replicated situation with the predefined UUIDs but it can easily happen in real situations.

The reason for the test case failure can be replicated in case you change the export ID to `export.id = "e9540f96-9e3d-4abe-8c2a-6c338d85a684"`. This would return `0` through `.to_s`

After attaching that ID, you can test if the file is available for the export:

```ruby
user.private_exports.last.file.attached?
=> false
user.private_exports.last.file.blob
=> nil
```

Note that this fails with such UUID as shown in the example and could easily lead to collisions in case the UUID starts with a number. E.g. UUID `"0210ae70-482b-4671-b758-35e13e0097a9"` would convert to `210` through `.to_s`. Therefore, if someone else has a "private" export with the prefixes "00000210", "0000210", "000210