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SummarizationMiddleware Bypasses PIIMiddleware And Sends Unredacted History To The Summary Model

SummarizationMiddleware invokes the summary model directly inside its before_model hook, bypassing the agent middleware pipeline. This means PIIMiddleware (including strategy='block') never inspects or redacts the summary prompt, allowing PII such as email addresses to be sent to an external summarization provider silently. The issue occurs regardless of middleware ordering in the affected versions unless external sanitization is added.

highConfidence 90%Langchain Affected V1.4.2

Origin Analysis

SummarizationMiddleware._create_summary and _acreate_summary call self._summary_model.invoke/ainvoke directly without routing the call through the agent middleware pipeline. Consequently, earlier middleware state transformations are not applied to the summary input, and later PIIMiddleware hooks cannot block the call. The summarization then replaces original messages, removing evidence of the unredacted PII.

Run the provided Python script with a RecordingSummaryModel, a history of six messages containing an email, and middleware=[SummarizationMiddleware(model=RecordingSummaryModel(), trigger=('messages',5), keep=('messages',2)), PIIMiddleware('email', strategy='block')]. Observe output: summary_model_called=1, summary_prompt_contains_email=True, pii_block_raised=False.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This is not a default protection: unless operators pass a sanitizer matching their PIIMiddleware rules, the leak persists. It only covers SummarizationMiddleware; LLMToolSelectorMiddleware and LLMToolEmulator may have similar direct model calls. The sanitizer must be thread-safe and stateless for concurrent runs and async use. Rolling back requires removing summary_input_sanitizer or setting it to None; no database or schema migration is needed.