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Introduced protections against "zip slip" attacks


ImportanceReview GuidanceRequires Scanning Tool
HIGHMerge Without ReviewNo

This change updates all new instances of ZipInputStream to protect against malicious entries that attempt to escape their "file root" and overwrite other files on the running filesystem.

Normally, when you're using ZipInputStream it's because you're processing zip files. That code might look like this:

File file = new File(unzipTargetDirectory, zipEntry.getName()); // use file name from zip entry
InputStream is = zip.getInputStream(zipEntry); // get the contents of the zip entry
IOUtils.copy(is, new FileOutputStream(file)); // write the contents to the provided file name

This looks fine when it encounters a normal zip entry within a zip file, looking something like this pseudo-data:

path: data/names.txt
contents: Zeus\nHelen\nLeda...

However, there's nothing to prevent an attacker from sending an evil entry in the zip that looks more like this:

path: ../../../../../etc/passwd
contents: root::0:0:root:/:/bin/sh

Yes, in the above code, which looks like every piece of zip-processing code you can find on the Internet, attackers could overwrite any files to which the application has access. This rule replaces the standard ZipInputStream with a hardened subclass which prevents access to entry paths that attempt to traverse directories above the current directory (which no normal zip file should ever do.) Our changes end up looking something like this:

+ import;
- var zip = new ZipInputStream(is, StandardCharsets.UTF_8);
+ var zip = ZipSecurity.createHardenedInputStream(is, StandardCharsets.UTF_8);


Why is this codemod marked as Merge Without Review?

We believe this change is safe and effective. The behavior of hardened ZipInputStream instances will only be different if malicious zip entries are encountered.