GHSA-v25g-mvwr-f5fp: Medium severity go/github.com/ncarlier/webhookd vulnerability
Description Before 1.22, if the Basic Auth (htpasswd) middleware was not configured, all incoming HTTP headers were blindly forwarded to the webhook script execution environment as shell variables. While the Basic Auth middleware correctly strips the authentication header (X-WebAuthn-User) from the incoming request before conditionally re-injecting it on successful authentication, disabling Basic Auth left the system vulnerable if deployed behind an unhardened reverse proxy.
Impact If an upstream reverse proxy is not properly hardened to strip client-provided authentication headers, an attacker could manually supply these headers (e.g., X-WebAuthn-User). A webhook script relying on this forwarded header for privilege elevation or identity verification could therefore be exploited to bypass security controls and impersonate other users.
Mitigation The WHDALLOWEDUPSTREAMHEADERS configuration setting has been introduced to enforce a strict allowlist of HTTP headers that can be converted into shell variables.
Additionally, the default behavior has been changed to adhere to the principle of least privilege. It is no longer (allow all). The default allowed headers are now restricted to standard operational headers: Accept,Content-Type,Content-Length,User-Agent,X-Forwarded-For
Administrators relying on upstream authentication proxies must explicitly add their authentication headers (e.g., WHDALLOWEDUPSTREAMHEADERS="Accept,Content-Type,Content-Length,User-Agent,X-Forwarded-For,x-webauthn-user") to ensure they are passed to the scripts securely.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
go/github.com/ncarlier/webhookdto a version that resolves this vulnerability.Fixed in 1.22.0 - Configuration
Set WHD_ALLOWED_UPSTREAM_HEADERS to an explicit allowlist that includes any authentication/user identity headers your upstream proxies must forward (example given: "WHD_ALLOWED_UPSTREAM_HEADERS=\"Accept,Content-Type,Content-Length,User-Agent,X-Forwarded-For,x-webauthn-user\""), so only those headers are converted into shell variables in the webhook execution environment.
webhook script / forwarded-header handling WHD_ALLOWED_UPSTREAM_HEADERS = Accept,Content-Type,Content-Length,User-Agent,X-Forwarded-For,x-webauthn-user - Configuration
Ensure WHD_ALLOWED_UPSTREAM_HEADERS is restricted to the default set of standard operational headers "Accept,Content-Type,Content-Length,User-Agent,X-Forwarded-For" rather than allowing "*" (all).
webhook script / forwarded-header handling WHD_ALLOWED_UPSTREAM_HEADERS = Accept,Content-Type,Content-Length,User-Agent,X-Forwarded-For
Event History
Frequently Asked Questions
Which deployments are exposed to exploitation?
Deployments before 1.22 are exposed when Basic Auth is disabled, an upstream reverse proxy does not strip client-provided authentication headers, and a webhook script trusts a forwarded header for identity verification or privilege elevation.
What does an attacker need to exploit this issue?
An attacker needs to be able to send HTTP requests through the reverse proxy and supply an authentication-related header such as X-WebAuthn-User. No successful Basic Auth authentication is required when the middleware is disabled.
How can I reduce risk if I cannot update immediately?
Harden the upstream reverse proxy to remove client-supplied authentication headers before requests reach webhookd. Ensure webhook scripts do not use forwarded request headers as trusted identity or privilege signals unless those headers are protected by the proxy.
How can I determine whether an existing deployment may be affected?
Check whether the deployment is before 1.22 and has the htpasswd Basic Auth middleware disabled. Then review reverse-proxy header handling and webhook scripts for reliance on headers forwarded into the script environment, particularly X-WebAuthn-User.
What configuration change addresses header forwarding?
Use WHD_ALLOWED_UPSTREAM_HEADERS to define a strict allowlist of HTTP headers that may be converted into shell variables. In 1.22 and later, the default no longer allows all headers.