CVE-2026-39973: Apktool: Path Traversal to Arbitrary File Write

Executive brief

A path traversal vulnerability in `brut/androlib/res/decoder/ResFileDecoder.java` allows a maliciously crafted APK to write arbitrary files to the filesystem during standard decoding (`apktool d`). This is a security regression introduced in commit [e10a045](https://github.com/iBotPeaches/Apktool/commit/e10a0450c7afcd9462c0b76bcbff0e7428b92bdd#diff-cd531ebe1014bfd18185bf21585ca5cdb16fbcb07703ebc47949a1b4e4e36bc3) ([PR #4041](https://github.com/iBotPeaches/Apktool/pull/4041), December 12, 2025), which removed the `BrutIO.sanitizePath()` call that previously prevented path traversal in resource file output paths.

An attacker can embed `../` sequences in the `resources.arsc` Type String Pool to escape the output directory and write files to arbitrary locations, including `~/.ssh/config`, `~/.bashrc`, or Windows Startup folders, escalating to RCE.

**Fix:** Re-introduce `BrutIO.sanitizePath()` in `ResFileDecoder.java` before file write operations.

Affected products

• Maven org.apktool:apktool-lib

References

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