A Study to Evaluate ALN-CIDEB in Adult Participants With Metabolic Dysfunction-Associated Steatotic Liver Disease or With Metabolic Dysfunction-Associated Steatohepatitis (MASLD/MASH)

Key Inclusion Criteria: 1. Part A: 18 to 55 years at Screening Visit 1 with MASLD, at Screening Visit 1 Part B: 18 to 65 years at Screening Visit 1 with a diagnosis of MASH, at Screening Visit 1 2. Body Mass Index (BMI) ≥30 kg/m2 and ≤40 kg/m2 at Screening Visit 1 3. Controlled-Attenuation Parameter (CAP) ≥285 dB/m by FibroScan during screening as described in the protocol 4. Liver fat content ≥8.5% by MRI-PDFF during screening 5. If on anti-hypertensive and/or lipid lowering medications and/or glucose lowering medications, must be on generally stable dose(s) for at least 12 weeks prior to screening and no changes to the dose(s) are anticipated during the study 6. Part B: A diagnosis of MASH documented in the participant's medical history, or a clinical suspicion of MASH based on non-invasive biomarkers (eg, evidence of fatty liver on imaging and elevated liver enzymes) and clinical risk factors, including having a history of 2 or more elements of metabolic syndrome, as defined in the protocol 7. Part B: Screening percutaneous liver biopsy NAFLD Activity Score (NAS) ≥3 and fibrosis stage, as defined in the protocol Key Exclusion Criteria: 1. Known historical or current diagnosis of portal hypertension or cirrhosis based on clinical assessment, imaging, and/or liver biopsy 2. Known historical or current diagnosis of other forms of chronic liver disease, as defined in the protocol 3. Prior or current suspected or known drug-induced liver injury within 1 year prior to screening 4. History of liver transplant, current placement on a liver transplant list, or Model for End-stage Liver Disease (MELD) score \>12 5. Contraindication to MRI examinations, such as persons with cardiac pacemaker and implants made of metal, severe claustrophobia, size restrictions, or other contraindications for MRI 6. Liver stiffness measurement, laboratory parameter assessment, estimated Glomerular Filtration Rate (GFR), and evidence of uncontrolled hypertension, as defined in the protocol 7. Evidence of Human Immunodeficiency Virus (HIV) infection, Hepatitis B Virus (HBV) infection, or Hepatitis C Virus (HCV) infection during screening, as described in the protocol 8. History of Type 1 Diabetes 9. Bariatric surgery, including any procedures to revise, reverse, or remove any previous bariatric surgery interventions, within approximately 5 years prior to randomization or planned during the study period NOTE: Other protocol-defined inclusion/exclusion criteria apply.