GHSA-vx2m-jpxr-xv7w Cloudreve has Broken Access Control - Revoked Share Access Still Allows Signed File URL Generation via Cached context_hint

• Go github.com/cloudreve/Cloudreve/v4: <= 4.0.0-20260606032813-26b6b1044b02 (no fix yet)

Summary Cloudreve's file-listing responses hand the client a `context_hint` (UUID) that is meant to speed up follow-up operations. When that hint is replayed on the `file/url` (and `file/thumb`) routes, DBFS caches a `shareNavigatorState` containing the already-loaded share root and share row. On a later request carrying the same hint, `shareNavigator.RestoreState` repopulates `shareRoot`, and `shareNavigator.To` then skips `Root`. `Root` is the only place that re-checks `inventory.IsValidShare` (share expiry, remaining-download count, owner status, source-file validity) and the share password. As a result, a recipient who prewarms a context hint while access is valid can keep minting signed file URLs for already-known shared file paths for up to the context-hint TTL (`5 * 60` = 300 s) after the owner deletes the share or the share expires — plus the lifetime of any signed entity URL minted in that window. This is a revocation / expiry bypass, not a way to discover unknown share contents: the attacker must already have had access to the share and must know the target file URI from a prior listing. Root cause (verified at `26b6b10`) 1. List responses leak the hint and each file URI — `service/explorer/response.go` populates `ListResponse.ContextHint` and `FileResponse.Path` (`f.Uri(false).String()`). 2. `file/url` and `file/thumb` accept the client-supplied hint — `routers/router.go:631` and `:662`: ```go file.POST("url", middleware.ContextHint(), /* ... */ controllers.FileURL) file.GET("thumb", middleware.ContextHint(), /* ... */ controllers.Thumb) ``` The `file` group's only auth gate is `middleware.RequiredScopes(types.ScopeFilesRead)` — there is no independent share-validation middleware on this route. All share validation lives inside DBFS. 3. The middleware trusts the header verbatim — `middleware/file.go:41`: ```go func ContextHint() gin.HandlerFunc { return func(c *gin.Context) { if c.GetHeader(dbfs.ContextHintHeader) != "" { // X-Cr-Context-Hint util.WithValue(c, dbfs.ContextHintCtxKey{}, uuid.FromStringOrNil(c.GetHeader(dbfs.ContextHintHeader))) } c.Next() } } ``` 4. DBFS restores cached navigator state on a hint hit — `dbfs.go:745` (`ContextHintTTL = 5 * 60`, `dbfs.go:34`). On a miss it arms `PersistState`; the closure fires in `DBFS.Recycle()` at end of request. 5. Persisted share state carries the loaded `shareRoot` + `share` row — `share_navigator.go:72`/`:85`. `RestoreState` reinstates `n.shareRoot`, `n.share`, `n.owner`, etc. 6. `Root` is the sole validity/password gate — `share_navigator.go:114` → `inventory.IsValidShare(share)` (`inventory/share.go:227`: `IsShareExpired` checks `Expires.Before(now)` and `RemainDownloads NOT nil => Root() skipped root, err := n.Root(ctx, path) ... } ... } ``` The single-file-share branch is also affected: it calls `latestSharedSingleFile`, which fetches `n.fileClient.GetByID(n.share.Edges.File.ID)` straight from the restored `share` with no revalidation (`share_navigator.go`). 8. A failed download hook does not block URL issuance — `pkg/filemanager/manager/entity.go:250`: ```go if err := m.fs.ExecuteNavigatorHooks(ctx, fs.HookTypeBeforeDownload, file); err != nil { m.l.Warning("Failed to execute navigator hooks: %s", err) // logged, NOT fatal } ``` The share's `BeforeDownload` hook is `shareClient.Downloaded()` (`UpdateOneID(share.ID).AddDownloads(1).AddRemainDownloads(-1)`). Against a deleted share this update errors, but the error is only logged and the signed URL is still minted. The signed content endpoint `file/content/:id/...` is then guarded only by `middleware.SignRequired` — it does not re-check the share. Steps to reproduce Setup: one share owner; one recipient (a second free account, or anonymous if the default anon group keeps share-download). Recipient knows the share URL (and password, if any). 1. Recipient lists the valid share: ``` GET /api/v4/file?uri= HTTP/1.1 Host: target ``` Response includes `context_hint` and each file's `path`. 2. While the share is still valid, recipient warms the cache for a known file: ``` POST /api/v4/file/url HTTP/1.1 Host: target X-Cr-Context-Hint: Content-Type: application/json {"uri":[""]} ``` (cache MISS → `Root` runs → `PersistState` armed → `Recycle` writes `shareNavigatorState` to KV under `navigator_state__share`.) 3. Owner deletes the share, or it expires / hits zero remaining downloads. 4. Within 300 s, recipient repeats the same request from step 2 (same `X-Cr-Context-Hint`, same URI). (cache HIT → `RestoreState` sets `shareRoot` → `To` skips `Root` → `IsValidShare` never runs → signed entity URL returned.) 5. The signed URL serves the file content; `file/content/:id/...` validates only the signature. Expected: step 4 returns `ErrShareNotFound` / `ErrShareLinkExpired`. Actual: step 4 returns a signed, downloadable URL. Impact A former share recipient (including an anonymous one, under default permissions) can keep minting signed download URLs for already-known shared files for up to 300 s after the owner deletes the share or after time/download-limit expiry, plus the validity window of each signed URL minted in that period. It defeats owner revocation, time expiry, and the remaining-download limit, and bypasses password revalidation on cached state. Remediation • On `RestoreState`, re-run `inventory.IsValidShare` and re-compare the current `share.Password` before trusting cached `shareRoot`; or bind the cached state to an authorization version that changes on any share edit/delete/download-limit change. • Do not store authorization-sensitive share state in context-hint cache; treat the hint as a pagination/perf token only. • Invalidate `navigator_state_*` entries when a share is edited or deleted. • Treat `HookTypeBeforeDownload` failures as blocking for share-backed downloads. • Add a regression test: list + prewarm hint, delete share, then `file/url` with the same hint must fail.