HL7 FHIR Partial Path Zip Slip due to bypass of CVE-2023-24057

Impact

Zip Slip protections implemented in CVE-2023-24057 (GHSA-jqh6-9574-5x22) can be bypassed due a partial path traversal vulnerability.

This issue allows a malicious actor to potentially break out of the TerminologyCacheManager cache directory. The impact is limited to sibling directories.

To demonstrate the vulnerability, consider userControlled.getCanonicalPath().startsWith("/usr/out") will allow an attacker to access a directory with a name like /usr/outnot.

Why?

To demonstrate this vulnerability, consider "/usr/outnot".startsWith("/usr/out"). The check is bypassed although /outnot is not under the /out directory. It's important to understand that the terminating slash may be removed when using various String representations of the File object. For example, on Linux, println(new File("/var")) will print /var, but println(new File("/var", "/") will print /var/; however, println(new File("/var", "/").getCanonicalPath()) will print /var.

The Fix

Comparing paths with the java.nio.files.Path#startsWith will adequately protect againts this vulnerability.

For example: file.getCanonicalFile().toPath().startsWith(BASE_DIRECTORY) or file.getCanonicalFile().toPath().startsWith(BASE_DIRECTORY_FILE.getCanonicalFile().toPath())

Other Examples

• CVE-2022-31159 - aws/aws-sdk-java
• CVE-2022-23457 - ESAPI/esapi-java-legacy

Vulnerability

https://github.com/hapifhir/org.hl7.fhir.core/blob/b0daf666725fa14476d147522155af1e81922aac/org.hl7.fhir.r4b/src/main/java/org/hl7/fhir/r4b/terminologies/TerminologyCacheManager.java#L99-L105

While getAbsolutePath will return a normalized path, because the string path is not slash terminated, the guard can be bypassed to write the contents of the Zip file to a sibling directory of the cache directory.