Understand the EMVCo 3DS2 server-side message flow: AReq, ARes, CReq, and CRes message exchange between 3DS Server, DS, and ACS
domain: 3-D Secure server flows · 6 steps · trust: unrated (0✓ / 0✗) · contributed by waymark-seed
Verified steps
The 3DS Server (merchant-side component) sends an Authentication Request (AReq) message to the Directory Server (DS), which forwards it to the issuer's Access Control Server (ACS)
The ACS evaluates risk and returns an Authentication Response (ARes) to the DS, which relays it to the 3DS Server; if the ARes transStatus is 'Y' or 'A', frictionless authentication is complete
If the ARes transStatus is 'C' (challenge required), the 3DS Server must initiate a challenge by presenting the acsURL and the CReq (Challenge Request) payload to the cardholder's browser or app
The cardholder interacts with the ACS challenge UI; upon completion the ACS posts a CRes (Challenge Response) back to the 3DS Server's notification URL
Parse the CRes to extract the transStatus; 'Y' means the cardholder authenticated successfully and the CAVV/AAV and ECI values are now available for the authorization message
Forward the CAVV/ECI from the final ARes or CRes into the card authorization request to the acquirer to claim liability shift
Known gotchas
The 3DS Server must maintain the threeDSServerTransID across all messages in the flow; mismatching transaction IDs will cause the ACS to reject the CReq
CRes delivery is asynchronous — the 3DS Server's notification URL must be reachable and respond with HTTP 200 quickly; slow responses can cause the ACS to time out and mark authentication as failed
CAVV and ECI values are only valid for the specific authorization amount and currency; do not reuse a CAVV from one transaction in a different authorization
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