Why Mutational Grammar Fuzzing Can Mislead Bug Discovery

The article explains mutational grammar fuzzing, a technique that generates structured test inputs by mutating data while preserving grammar rules, making it effective for testing complex parsers and languages. 

However, it argues the approach has important flaws. Coverage-guided fuzzing can prioritize inputs that increase code coverage without actually finding more bugs, leading to misleading results. Grammar constraints can also limit exploration, preventing the fuzzer from reaching unexpected or invalid states where vulnerabilities often exist.

The author proposes simple mitigation strategies, emphasizing that fuzzing effectiveness depends less on structure-awareness alone and more on balancing coverage, mutation diversity, and exploration beyond strict grammar boundaries. 

https://projectzero.google/2026/03/mutational-grammar-fuzzing.html

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