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Cloud Storage Is An Application Of Clouds That Liberates Organizations From Establishing In-house Data Storage Systems. However, Cloud Storage Gives Rise To Security Concerns. In Case Of Group-shared Data, The Data Face Both Cloud-specific And Conventional Insider Threats. Secure Data Sharing Among A Group That Counters Insider Threats Of Legitimate Yet Malicious Users Is An Important Research Issue. In This Paper, We Propose The Secure Data Sharing In Clouds (SeDaSC) Methodology That Provides: 1) Data Confidentiality And Integrity; 2) Access Control; 3) Data Sharing (forwarding) Without Using Compute-intensive Reencryption; 4) Insider Threat Security; And 5) Forward And Backward Access Control. The SeDaSC Methodology Encrypts A File With A Single Encryption Key. Two Different Key Shares For Each Of The Users Are Generated, With The User Only Getting One Share. The Possession Of A Single Share Of A Key Allows The SeDaSC Methodology To Counter The Insider Threats. The Other Key Share Is Stored By A Trusted Third Party, Which Is Called The Cryptographic Server. The SeDaSC Methodology Is Applicable To Conventional And Mobile Cloud Computing Environments. We Implement A Working Prototype Of The SeDaSC Methodology And Evaluate Its Performance Based On The Time Consumed During Various Operations. We Formally Verify The Working Of SeDaSC By Using High-level Petri Nets, The Satisfiability Modulo Theories Library, And A Z3 Solver. The Results Proved To Be Encouraging And Show That SeDaSC Has The Potential To Be Effectively Used For Secure Data Sharing In The Cloud.