litellm already supports Google, Microsoft and generic OIDC SSO through
fastapi-sso, which has no SAML support; AuthMethod.SAML existed only as an
unused enum value. This adds real SAML 2.0 single sign-on for the admin UI.
A new SAMLAuthHandler validates signed assertions with the OneLogin
python3-saml toolkit and maps them onto a CustomOpenID, then reuses the
shared post-login path every other provider goes through, so provisioning,
role/team mapping and the UI session JWT are unchanged. Both SP-initiated
and IdP-initiated HTTP-POST flows are supported. SP-initiated logins are
bound to the browser that started them via an HttpOnly state cookie plus a
cached AuthnRequest id, and the ACS rejects any response whose InResponseTo
doesn't match; unsolicited (IdP-initiated) responses cannot be browser-bound
so they are rejected unless SAML_ALLOW_UNSOLICITED=true. Replays are rejected
by a consumed-assertion guard whose lifetime tracks each assertion's
NotOnOrAfter, and both the replay guard and the login-state binding go
through the proxy's shared in-memory + Redis cache for multi-instance
deployments. The ACS honors DISABLE_ADMIN_UI and re-applies the
free-SSO-user Enterprise gate after the assertion is validated, so an
unvalidated POST can no longer drive the billable-user count query.
SAML is configurable from the admin UI SSO settings (IdP metadata URL or
inline XML, SP entity ID, and an allow-unsolicited toggle), which persists
the SAML_* environment variables the handler reads, exactly like the Google,
Microsoft and generic OIDC providers.
python3-saml is kept as an optional saml extra; its xmlsec and lxml wheels
bundle the native libraries so no system packages are required, and the
import is guarded so the proxy still starts without the package with the
SAML routes returning a clear 501.
Resolves LIT-4016
35dc982692