PYSEC-2026-4300

Impact

Shiny for Python's bookmark-restore path accepted a client-supplied _state_id_ query-string value and joined it into the server-side bookmark directory (<cwd>/shiny_bookmarks/<id>) without validating it. A value containing .. path segments, or an absolute path, could therefore cause the server to open and parse input.json and values.json from a directory outside the bookmark store.

On a default application the restore was attempted whenever the client supplied a URL query string — including when bookmarking was disabled (bookmark_store="disable") — so no opt-in configuration was required. For such applications the impact is an unauthenticated, server-side probe for the existence and JSON-validity of attacker-chosen paths, constrained to those two file names.

Applications configured with bookmark_store="server" and using ui.input_file() are affected more seriously. The file-input restore handler copies files out of the restore directory, so the contents of an attacker-selected file name within an attacker-selected directory could be read into the application.

Affected versions

1.4.0 through 1.6.3. Bookmarking was introduced in 1.4.0; earlier releases do not contain the affected code path and are not affected.

Patches

Fixed in 1.6.4. Upgrade with:

pip install --upgrade shiny

The fix validates bookmark IDs against an allowlist — a single path segment matching [A-Za-z0-9_-]+, which cannot express a path separator, a .. parent reference, or an absolute path — before the ID is used to build a filesystem path. Bookmark IDs generated by Shiny are unaffected by this restriction. Restore is additionally gated on the application's bookmark_store setting at a single point, so it no longer runs when bookmarking is disabled, and the on-disk restore path is honored only under bookmark_store="server".

Workarounds

There is no configuration change that mitigates this on an affected version: the restore path ran even with bookmark_store="disable". Deployments that cannot upgrade immediately can strip the _state_id_ parameter from incoming query strings at a reverse proxy or load balancer, which prevents the affected code path from being reached.

Credits

Reported by @0xRenSec.