EMB ERJ 145 ER/LR
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance; Flight Crew Became Reoriented
ATC Equipment / Nav Facility / Buildings; Human Factors
While descending into ORD we were assigned altitude of 12;000 feet.  We leveled off at 12;000 feet with our altimeter set at 30.21 which is what the ATIS info from our ACARS showed for ORD.  When level ATC inquired as to our assigned altitude which we relayed was 12;000 feet then asked for our current altitude which showed 12;000 feet.  ATC then informed us that our altitude was 11;400 feet and we asked what the ORD altimeter setting was and they then informed us it was 29.52.  We reset our altimeters and climbed back up to an indicated 12;000 feet.  We told them that ATIS showed 30.21 and ATC said that the ACARS ATIS was inoperative.  There was no NOTAM for ACARS ATIS being out of service and ATC did not inform us either.The event occurred due to the ACARS ATIS not working correctly with no NOTAMS or no notice from ATC. NOTAM the ACARS ATIS or have the ACARS ATIS have a digital ATIS not available message.
An EMB-145 Captain reports obtaining an incorrect altimeter setting for ORD via ACARS; resulting in a 600 foot altitude error. ATC detects the error and issues the correct altimeter setting; stating that ATIS is out of service.