GHSA-mhc4-qq83-fmrr axonflow-sdk-go: Webhook signing-key (HMAC-SHA256) not exposed by SDK type, preventing signature verification

• Go github.com/getaxonflow/axonflow-sdk-go/v5: < 5.7.0 → fixed in 5.7.0

Summary The AxonFlow SDK's `WebhookSubscription` (or equivalent) type did not expose the HMAC-SHA256 signing key returned by the platform's `CreateWebhook` endpoint. Without access to the secret through the typed SDK API, callers had no path to verify the `X-AxonFlow-Signature` header on incoming webhook deliveries. Affected callers had two unsatisfactory options: 1. Skip signature verification entirely — accepting any payload from any source that knew the webhook URL. 2. Hand-parse the raw HTTP JSON response to extract the secret, bypassing the type-safe SDK surface. This advisory is filed across all four AxonFlow SDKs (Go, Python, TypeScript, Java) because the same defect and the same fix landed in each. Affected versions Versions 5.6.1 and below. Impact A webhook receiver using the SDK's typed API to handle inbound deliveries had no path to authenticate the source of incoming payloads. An attacker who learned the webhook URL — through misconfiguration, log leakage, observable network traffic during setup, or any other discovery channel — could forge webhook deliveries indistinguishable from legitimate ones, causing the receiving application to act on fabricated events (e.g. simulated approval-granted callbacks, simulated policy-decision callbacks, simulated step-completion callbacks). Remediation Upgrade to the patched version listed in Vulnerabilities below. The signing key is now exposed on the `WebhookSubscription` response type returned by `CreateWebhook`. Implementations should: 1. Persist the secret returned by `CreateWebhook` securely (it is only returned once, at create time). 2. On each incoming webhook delivery, compute `HMAC-SHA256(secret, raw_body)` and compare it in constant time against the `X-AxonFlow-Signature` header. 3. Reject any delivery whose signature does not match. Credit Identified by AxonFlow internal security review during the April 2026 quality-freeze epic.