ASanity: On Bug Shadowing by Early ASan Exits

Bugs in memory-unsafe languages are a major source of critical vulnerabilities. Large-scale fuzzing campaigns, such as Google's OSS-Fuzz, can help find and fix these bugs. To find bugs faster during fuzzing, as well as to cluster and triage the bugs more easily in an automated setup, the targets are compiled with a set of sanitizers enabled, checking certain conditions at runtime. The most common sanitizer, ASan, reports common bug patterns found during a fuzzing campaign, such as out-of-bounds reads and writes or use-after-free bugs, and aborts the program early. The information also contains the type of bug the sanitizer found. During triage, out-of-bounds reads are often considered less critical than other bugs, namely out-of-bounds writes and use-after-free bugs. However, in this paper we show that these more severe vulnerabilities can remain undetected in ASan, shadowed by an earlier faulty read access. To prove this claim empirically, we conduct a large-scale study on 814 out-of-bounds read bugs reported by OSS-Fuzz. By rerunning the same testcases, but disabling ASan's early exits, we show that almost five percent of test cases lead to more critical violations later in the execution. Further, we pick the real-world target wasm3, and show how the reported out-of-bounds read covered up an exploitable out-of-bounds write, that got silently patched.