Prompt injection cleanup: trace retrieval copies

Contain prompt injection in a retrieval system by tracing the suspect source into every derived record the application can still use. Restrict the affected retrieval path, preserve bounded investigation evidence, remove or quarantine the relevant records through an approved process, then verify retrieval and ordinary work before restoring access.

Deleting a source file leaves a separate question about the search index. An assistant may receive extracted text from that index without reopening the original. A useful recovery plan therefore identifies the source, its derived records and the retrieval path that supplies model context.

This guide proposes a controlled exercise. Microsoft documentation provides a concrete indexing example; no production incident, tenant test or universal deletion guarantee is claimed.

Map the records behind a source

Start with one suspect document identifier and its revision. Ask the application owner which ingestion jobs read it, which index stores its extracted text, how record keys are assigned and whether the application maintains other retrieval or response caches.

Record those relationships before changing anything. If a document becomes several chunks, retain the parent-to-child mapping. If ingestion creates a second representation, give that representation its own removal check. Keep investigation evidence under restricted access and avoid copying hostile text into general support tickets.

OWASP's 2025 vector and embedding guidance describes poisoning risks in retrieval systems and recommends source validation, access controls and retrieval monitoring. Those controls inform the investigation. The application's actual storage and indexing design determines the cleanup procedure.

Check deletion detection before relying on it

Microsoft's Azure AI Search deletion-detection documentation distinguishes automatic change detection from separately configured deletion detection. It warns that records deleted before a deletion policy was established can remain in the index, even after adding the policy and resetting the indexer.

For native blob soft delete, the indexer must process the blob while it remains in the soft-deleted state. Microsoft recommends a retention interval comfortably longer than the indexing schedule, allowing for delays and large workloads. Its documented prerequisites include suitable key mapping and disabled blob versioning; this method does not support Azure Files.

The custom-metadata method has a defined order: mark the source, let the indexer remove the search record, then delete the physical file. An incident procedure needs authority and evidence for each step. Obtain the owning team's approval before destructive changes.

Treat one-to-many indexing separately

The same Microsoft page excludes its two soft-delete strategies from the documented one-to-many indexing scenarios and directs readers to explicit index deletion. Check the exact ingestion mechanism in use before choosing a procedure for chunked content.

A filename alone may be insufficient to identify every derived record. Ask for the stored relationship between the source and its index keys, then reconcile the expected set with the records actually removed. Keep unmatched keys visible for investigation.

An indexer's successful run is one piece of evidence. A direct query for the affected record identifiers provides another. The application retrieval path needs its own check, including any relevant cache. This separates an operation completing from the content becoming unavailable to the assistant.

Exercise removal with harmless material

Use an isolated test collection and a fictional document with a distinctive, harmless phrase. Establish that the document is indexed and retrievable before starting. Capture its source revision, derived record identifiers and the relevant job state.

Apply the approved quarantine or removal process. Observe the source state, indexer outcome and direct record lookup independently. Repeat the application query in a fresh test session after the documented refresh interval. Keep an unaffected document as a control so that a broken retrieval service cannot masquerade as selective removal.

If old context can remain in an existing conversation, test that path separately and document the supported recovery action. A fresh query cannot establish what an already assembled context contains. Any retained cache or conversation store needs a named owner and an explicit investigation boundary.

Restore useful work with a bounded claim

Recovery should establish which source revision and derived records were handled, which paths were tested, when they refreshed and what remains unresolved. A missing search hit for one phrase gives limited evidence; combine it with identifier-level checks and representative application queries.

Restore the affected workflow only after the responsible owner accepts the evidence and any remaining exposure. Retest a legitimate task, retain the controlled regression case and check that routine ingestion cannot reintroduce the quarantined source.

Our production LLM security guide places retrieval recovery within the wider control programme. The prompt injection introduction explains the instruction boundary that source cleanup helps protect.

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