CVE-2026-45398: Open WebUI Vulnerable to IDOR: Retrieval API Bypasses Knowledge Base Access Controls

_validate_collection_access() (PR #22109) checks the user-memory-* and file-* collection name prefixes but does not check knowledge base collections, which use raw UUIDs as collection names. Any authenticated user who knows a private knowledge base UUID can read its content through the retrieval query endpoints, even though the knowledge API correctly denies that user access. The same gap affects the retrieval write endpoints (/process/text, /process/file, /process/files/batch, /process/web, /process/youtube), allowing an attacker to inject content into or overwrite another user’s knowledge base.

Reproduced on main at commit 4d058a125 (v0.8.11) on March 26, 2026.