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From an email:

We're writing to let you know that between September 2025 and January 2026, webhook secrets for webhooks you are responsible for were inadvertently included in an HTTP header on webhook deliveries. This means that any system receiving webhook payloads during this window could have logged the webhook secret from the request headers. Webhook deliveries are encrypted in transit via TLS, so the header containing the secret was only accessible to the receiving endpoint in a base64-encoded format. We have no evidence to suggest your secrets were intercepted. This issue was fixed on January 26, 2026. Please read on for more information.

User privacy and security are essential for maintaining trust, and we want to remain as transparent as possible about events like these. GitHub itself did not experience a compromise or data breach as a result of this event.

* What happened? *

On January 26, 2026, GitHub identified a bug in a new version of the webhook delivery platform where webhook secrets were included in an X-Github-Encoded-Secret HTTP header sent with webhook payloads. This header was not intended to be part of the delivery and made the webhook secret available to the receiving endpoint in a base64-encoded format. Webhook secrets are used to verify that deliveries are genuinely from GitHub, and should only be known to GitHub and the webhook owner.

The bug was limited to only a subset of webhook deliveries that were feature flagged to use this new version of the webhooks platform. The bug was present between September 11, 2025, and December 10, 2025, and briefly on January 5, 2026. The bug was fixed on January 26, 2026