diff --git a/README.md b/README.md
index 28a8b80..5648015 100644
--- a/README.md
+++ b/README.md
@@ -16,11 +16,12 @@ Coupa PO emails are received at `amazon_po@int.seahaven.com`, parsed by Claude H
 1. Coupa sends a PO email (new, revision, or cancellation).
 2. SES (`INBOUND_MAIL` rule set) drops the raw MIME into `s3://po-ingest-emails-{AccountId}/inbound/`.
 3. S3 `ObjectCreated` triggers the `po-email-processor` Lambda.
-4. Claude extracts structured JSON (PO number, status, supplier, site code, trade classification, line items, fiscal year).
-5. Conditional write to DynamoDB:
-   - `new_po` — idempotent insert (no-op if PO exists)
-   - `revision` — unconditional overwrite
-   - `cancellation` — marks existing row `Cancelled`
+4. The processor rejects the email unless the verified sender domain is on the allowlist (see Security) — anyone can email the public address, but only legitimate Coupa/Amazon senders may write data.
+5. Claude extracts structured JSON (PO number, status, supplier, site code, trade classification, line items, fiscal year).
+6. Merge write to DynamoDB:
+   - `new_po` — merge insert. Creates the PO, or backfills data into a pre-existing `Cancelled` skeleton left by an out-of-order cancellation (preserving the `Cancelled` status). No longer silently dropped when a record already exists.
+   - `revision` — field-level merge (`update_item` SETs only the fields present in the revision). A revision that omits `line_items`/`supplier` no longer deletes them. Will not un-cancel a `Cancelled` PO.
+   - `cancellation` — marks the row `Cancelled` (creating a minimal skeleton if the cancellation arrives before the `new_po`).
 6. DynamoDB Streams feeds downstream consumers:
    - **LedgerFlow** (`seahaven-slack-bot/po-sync`) — daily KB sync
    - **Site extractor** (`po-ingest-site-extractor`) — real-time site address extraction into `verified-sites` table
@@ -69,8 +70,18 @@ All Lambdas: Python 3.12, ARM64, 60-day log retention.
 - `po-ingest/anthropic-api-key` — Anthropic API key for PO parsing
 - `workorder-ingest/anthropic-api-key` — Anthropic API key for WO parsing
 
+The email processors **require** `ANTHROPIC_API_KEY_SECRET_ARN` to be set and resolve the key from Secrets Manager at runtime. There is no plaintext `ANTHROPIC_API_KEY` env fallback — a deploy missing the ARN fails loudly instead of silently running on an unmanaged key.
+
 **SES:** Both stacks add rules to the shared `INBOUND_MAIL` receipt rule set on `int.seahaven.com`.
 
+## Security
+
+**Sender authorization (email processors).** The public receipt addresses (`amazon_po@`, `apm@`) accept mail from anyone, so each processor validates the verified sender domain against an allowlist before any DynamoDB write. The allowlist is configurable per function via the `ALLOWED_SENDER_DOMAINS` env var (comma-separated; a domain matches itself or any subdomain). Defaults: PO = `coupahost.com,amazon.com`; WO = `amazon.com,hxgnsmartcloud.com,hexagon.com`. When SES receipt-rule scanning is enabled, the processors additionally drop any email whose stored object shows an explicit SPF/DKIM/spam/virus `fail`.
+
+**Web UI auth (defense-in-depth).** The `po-web-ui` / `workorder-web-ui` handlers refuse unauthenticated requests even though their public Function URLs were removed (INFRA-74). Each requires a shared secret in the `X-Auth-Token` header (or `Authorization: Bearer <token>`), compared in constant time against the `WEB_UI_AUTH_TOKEN` env var. The handler **fails closed** if the token is unset (denies all). The CDK wires `WEB_UI_AUTH_TOKEN` from the SSM String parameter `/procurement-ingest/web-ui-auth-token` — create it before deploy. (A plaintext String, not a SecureString, is used because `value_for_string_parameter` only resolves String params into Lambda env vars at deploy; this is a defense-in-depth floor for a detached URL, not primary auth.)
+
+**Output escaping.** All caller-influenced values (including prompt-injectable strings Claude may return for `total_amount`/line-item amounts) are HTML-escaped before interpolation to prevent stored XSS.
+
 **Failure handling (INFRA-41):** Each email-processor is async-invoked (S3 → Lambda). Both have a CDK-managed SQS dead-letter queue (`dead_letter_queue=`, 14-day retention, SSL-enforced) so a failed parse is captured rather than silently dropped after Lambda's retries, plus an ALARM-only CloudWatch `Errors` alarm (Sum, threshold > 0) wired to the shared `site-alerts` SNS topic.
 
 ## Shared Resources
@@ -107,13 +118,18 @@ Branch protection on `main` — all changes through PR.
    aws secretsmanager create-secret --name po-ingest/anthropic-api-key --secret-string "sk-ant-..."
    aws secretsmanager create-secret --name workorder-ingest/anthropic-api-key --secret-string "sk-ant-..."
    ```
-3. Deploy both stacks:
+3. Store the web UI shared secret (the web-ui Lambdas fail closed without it):
+   ```bash
+   aws ssm put-parameter --name /procurement-ingest/web-ui-auth-token --type String --value "$(openssl rand -hex 32)"
+   ```
+4. Deploy both stacks:
    ```bash
    cd cdk
    pip install -r requirements.txt
    cdk deploy --all
    ```
-4. CloudFormation outputs include `WebUIUrl` for each stack's dashboard.
+
+The web UI Lambdas have no public Function URL (INFRA-74); invoke them only through an authenticated path that forwards the `X-Auth-Token` header.
 
 ## Scripts