{"id":"GHSA-mhc4-qq83-fmrr","summary":"axonflow-sdk-go: Webhook signing-key (HMAC-SHA256) not exposed by SDK type, preventing signature verification","details":"## Summary\n\nThe 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:\n\n1. Skip signature verification entirely — accepting any payload from any source that knew the webhook URL.\n2. Hand-parse the raw HTTP JSON response to extract the secret, bypassing the type-safe SDK surface.\n\nThis advisory is filed across all four AxonFlow SDKs (Go, Python, TypeScript, Java) because the same defect and the same fix landed in each.\n\n## Affected versions\n\nVersions 5.6.1 and below.\n\n## Impact\n\nA 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).\n\n## Remediation\n\nUpgrade to the patched version listed in Vulnerabilities below. The signing key is now exposed on the `WebhookSubscription` response type returned by `CreateWebhook`. Implementations should:\n\n1. Persist the secret returned by `CreateWebhook` securely (it is only returned once, at create time).\n2. On each incoming webhook delivery, compute `HMAC-SHA256(secret, raw_body)` and compare it in constant time against the `X-AxonFlow-Signature` header.\n3. Reject any delivery whose signature does not match.\n\n## Credit\n\nIdentified by AxonFlow internal security review during the April 2026 quality-freeze epic.","modified":"2026-05-06T23:36:40.068061Z","published":"2026-05-06T23:15:38Z","database_specific":{"cwe_ids":["CWE-345","CWE-347"],"nvd_published_at":null,"github_reviewed":true,"github_reviewed_at":"2026-05-06T23:15:38Z","severity":"MODERATE"},"references":[{"type":"WEB","url":"https://github.com/getaxonflow/axonflow-sdk-go/security/advisories/GHSA-mhc4-qq83-fmrr"},{"type":"PACKAGE","url":"https://github.com/getaxonflow/axonflow-sdk-go"}],"affected":[{"package":{"name":"github.com/getaxonflow/axonflow-sdk-go/v5","ecosystem":"Go","purl":"pkg:golang/github.com/getaxonflow/axonflow-sdk-go/v5"},"ranges":[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"5.7.0"}]}],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/05/GHSA-mhc4-qq83-fmrr/GHSA-mhc4-qq83-fmrr.json"}}],"schema_version":"1.7.5","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N"}]}