The Advisory Said It Was Fixed. I Didn't Believe It Until I Broke It Myself.

Update: This investigation spawned a complete stress-testing toolkit. See the apktool-diagnostics repo for the full tool with security scanning, round-trip validation, and fuzzing setup. The Setup I was deep into a project stress-testing apktool — the standard tool for decompiling and rebuilding Android APKs — hunting for real bugs to fix, not hypothetical ones. While digging through recent issues and advisories, I ran across a patched vulnerability: CVE-2026-39973 , a path traversal bug that let a malicious resources.arsc make apktool write decoded files outside the intended output directory. Already fixed upstream. Advisory published. Case closed, according to everyone else. I didn't believe it until I broke it myself. Why "already patched" wasn't good enough Advisories describe what should happen. They don't prove it. And a tool like apktool has one job that matters more than any other: you point it at a file you don't trust — someone else's APK — and it processes that file on your machine. If the sanitization it depends on for that trust boundary can silently regress in a refactor (which is exactly what happened here — a PR reorganizing resource-file-path construction dropped the call to BrutIO.detectPossibleDirectoryTraversal() without anyone noticing), then "patched now" only tells you about today. It doesn't tell you the mechanism actually holds, and it doesn't tell you what "vulnerable" concretely looks like on your own machine, with your own files. So instead of writing "confirmed CVE-2026-39973, see upstream fix" and moving on, I decided to reproduce it myself, end to end. Building both sides of the fence Step one: build apktool twice from source — once at v3.0.1 (the last vulnerable tag), once at current HEAD (post-fix). Now I had a vulnerable binary and a patched binary sitting side by side, and no more reason to trust anyone's summary of what the difference actually did. Step two: I needed a malicious input. The vulnerability lived in how ResFileDecoder