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

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:

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:

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 <= 0, plus owner-active and source-file checks) followed by the share.Password comparison.

7. To skips Root once shareRoot is set — share_navigator.go:181:

func (n *shareNavigator) To(ctx context.Context, path *fs.URI) (*File, error) {
    if n.shareRoot == nil {            // restored state => 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:

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=<share-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: <context_hint>
   Content-Type: application/json
   
   {"uri":["<known-shared-file-uri>"]}
   

   (cache MISS → Root runs → PersistState armed → Recycle writes shareNavigatorState to KV under navigator_state_<hint>_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.