Transient execution attacks (TEAs), such as Spectre and Meltdown, exploit speculative execution to leak sensitive data through residual microarchitectural state.Traditional defenses often incur high performance and hardware costs by delaying speculative execution or requiring additional shadow structures and dynamic information flow tracking.In contrast, our approach models these attacks as violations of software-defined security contracts and enforces these contracts in hardware using existing features.We introduce Speculative Address Sanitization (SpecASan), which leverages ARM's Memory Tagging Extension (MTE) to extend memory safety protection from the committed path to the speculative path.When a speculative access does not pass the MTE tag comparison, this access is delayed until speculation resolves.This ensures that only validated accesses affect the microarchitectural state while preserving the performance benefits of speculation.When combined with Control-Flow Integrity (CFI) enforcement mechanisms, already supported by some hardware implementations, our evaluation shows that SpecASan effectively mitigates a broad class of transient execution attacks, including Spectre and Microarchitectural Data Sampling (MDS).Furthermore, SpecASan achieves this with low performance overhead and minimal hardware complexity, highlighting its practicality and efficiency.