Overview

A partial-path traversal issue exists within the downloadDirectory method in the AWS S3 TransferManager component of the AWS SDK for Java v1. Applications using the SDK control the destinationDirectory argument, but S3 object keys are determined by the application that uploaded the objects. The downloadDirectory method allows the caller to pass a filesystem object in the object key but contained an issue in the validation logic for the key name. A knowledgeable actor could bypass the validation logic by including a UNIX double-dot in the bucket key. Under certain conditions, this could permit them to retrieve a directory from their S3 bucket that is one level up in the filesystem from their working directory.

This issue’s scope is limited to directories whose name prefix matches the destinationDirectory. E.g. for destination directory/tmp/foo, the actor can cause a download to /tmp/foo-bar, but not /tmp/bar.

Versions of the AWS Java SDK for S3 v1 before and including v1.12.260 are affected by this issue.

Impact

If com.amazonaws.services.s3.transfer.TransferManager::downloadDirectory is used to download an untrusted buckets contents, the contents of that bucket can be written outside of the intended destination directory.

Root Cause

The com.amazonaws.services.s3.transfer.TransferManager::downloadDirectory contains a partial-path traversal vulnerability.

This is due to the guard logic in leavesRoot containing an insufficient protection against partial-path traversal.

The application controls the localBaseDirectory argument, but the key comes from the AWS bucket entry (ie. can be attacker controlled). The above bit of logic can be bypassed with the following payloads:

// The following will return 'false', although the attacker value will "leave" the `/usr/foo` directory
leavesRoot(new File("/usr/foo"), "/../foo-bar/bar")

This guard is used here which should guard against path traversal, however leavesRoot is an insufficient guard:

True Root cause

If the result of parent.getCanonicalPath() is not slash terminated it allows for partial path traversal.

Consider "/usr/outnot".startsWith("/usr/out"). The check is bypassed although outnot is not under the out directory.
The terminating slash may be removed in various places. On Linux println(new File("/var/")) returns /var, but println(new File("/var", "/")) - /var/, however println(new File("/var", "/").getCanonicalPath()) - /var.
- @JarLob (Jaroslav Lobačevski)

Patches

Upgrade to the AWS SDK for Java >= 1.12.261, if you are on a version < 1.12.261.

Workarounds

When calling com.amazonaws.services.s3.transfer.TransferManager::downloadDirectory pass a KeyFilter that forbids S3ObjectSummary objects that getKey method return a string containing the substring .. .

References

Similar vulnerabilities:

• ESAPI (The OWASP Enterprise Security API) - https://nvd.nist.gov/vuln/detail/CVE-2022-23457

For more information

If you have any questions or comments about this advisory, please contact AWS's Security team.