CWE-1461
Weakness Allowed-with-ReviewHardware Simulation / Synthesis Mismatch
Abstraction: Base · Status: Incomplete
The hardware design contains structures that are interpreted differently during synthesis than during simulation, which cause inconsistencies in security-relevant behavior between the synthesized implementation and the simulation.
Hardware-description-language (HDL) code describes functionality which can be verified as correct using simulation tools. Synthesis tools then (automatically) convert this into a physically-realizable representation. Certain HDL code structures cannot be exactly represented, or have multiple valid representations from which the synthesis tool must choose. This can result in output which behaves differently to the (functionally-correct) HDL code. These "simulation/synthesis mismatches" may have security consequences.
Mapping guidance
This CWE entry is at the Base level of abstraction, which is a preferred level of abstraction for mapping to the root causes of vulnerabilities.
Since this entry is often resultant, there might be other CWEs that are more appropriate as the root cause, although there might be some gaps. Carefully read both the name and description to ensure that this mapping is an appropriate fit. Do not try to 'force' a mapping to a lower-level Base/Variant simply to comply with this preferred level of abstraction.
Modes of introduction
This weakness can be introduced when HDL constructs, assignment ordering, synthesis directives, or tool-dependent semantics are used whose simulation behavior may not match synthesis interpretation. These constructs may behave correctly during HDL simulation but produce different security-relevant behavior in the synthesized implementation.
This weakness can be introduced or exposed during synthesis when the synthesis tool applies assumptions, optimizations, or transformations to HDL structures that cannot be physically implemented exactly as the simulation semantics dictate, or that permit multiple (valid) hardware implementations. The resulting netlist may behave differently from the model that was verified during simulation.
Detection methods
Automated Static Analysis - Source Code
HDL linting, rule-based static analysis, and HDL checker tools can examine source code for structures that may be interpreted differently by simulation and synthesis tools. They may identify synthesis-sensitive coding patterns associated with this weakness.
Effectiveness: Moderate
This method can be useful for detecting potential coding issues, but it may not reveal whether these constructs result in observable behavioral divergence after synthesis. Moreover, a user may suppress or bypass these warnings and still produce the erroneous synthesized hardware implementation. This method can give false positives.
Manual Analysis
Synthesis warning review can examine synthesis logs and reports for tool messages associated with possible simulation/synthesis mismatch. It may identify ignored constructs, unsupported HDL semantics, synthesis assumptions, or optimizations that could affect security-relevant behavior.
Effectiveness: Limited
Can reveal potential synthesis issues but relies on engineers interpreting warnings. Moreover, a user may suppress or bypass these warnings and still produce the erroneous synthesized hardware implementation.
Formal Verification
HDL-source-to-netlist formal equivalence checking can examine the simulated model and synthesized netlist for behavioral mismatch associated with this weakness. It may identify non-equivalent outputs, failed compare-points, state-mapping issues, or counterexamples under the checked constraints.
Effectiveness: High
Formal equivalence checking can be effective at the IP level for identifying mismatches between the source-code model and synthesized implementation. However, effectiveness may decrease as design complexity grows, particularly at the SoC level, due to state-space expansion and difficulty mapping non-equivalent points. Synthesis optimizations may merge, eliminate, or rename signals, which can also produce false positives or require additional setup to account for expected transformations.
Simulation / Emulation
Cross-simulation across HDL-source and post-synthesis models can examine both abstraction levels for observable behavioral divergence associated with this weakness. It may identify mismatches when security-relevant behaviors are explicitly exercised.
Effectiveness: High
This method can be useful for detecting observable behavioral mismatches, but it may not exercise all execution scenarios and should be combined with other detection methods to improve coverage.
Fuzzing
Coverage-guided differential fuzzing can examine HDL source code and synthesized implementation models for behavioral divergence associated with this weakness. It may identify generated inputs or state conditions that expose synthesis-induced mismatches.
Effectiveness: High
This method can be useful for detecting previously unknown synthesis-induced behavioral mismatches by exploring diverse execution paths through coverage-guided input generation. However, it may not exercise every reachable design state and should be combined with other detection methods to improve coverage.
Content history
Participated in a series of meetings betwee March 2026 and September 26, in which members contributed significant feedback related to this entry as it evolved before publication in CWE 5.0.
Guided evolution of the name and description for this entry, followed by suggestions for mitigations, demonstrative examples, and detection methods. Note: some content development was AI-assisted, with expert review and editing by MITRE and members of the CWE Ad Hoc RTL WG.
0 vulnerabilities reference this CWE, most recent first.
No vulnerabilities reference this CWE.
Mitigation
Establish and enforce HDL coding and review practices that minimize differences between simulation semantics and synthesis interpretation. Avoid HDL constructs, synthesis directives, and tool-dependent behaviors known to cause simulation/synthesis mismatch, with particular attention to security-relevant logic.
Mitigation
For security-critical logic, preserve intentional redundancy that is required for security. Examples may include redundant logic, triple modular redundancy (TMR) structures, Hamming-distance state encodings, or state-machine recovery logic. Use synthesis constraints, preservation attributes, carefully controlled structural implementation, or manual implementation of selected critical blocks to prevent this redundancy from being merged, re-encoded, or optimized away.
No CAPEC attack patterns related to this CWE.