0dd - The Zero (0) Day Division

The Zero (0) Day Division is a group of security professionals working towards a common goal; securing open-source projects.

K9Mail XXE

The Issue

An XML External Entity attack is a type of attack against an application that parses XML input. This attack occurs when XML input containing a reference to an external entity is processed by a weakly configured XML parser. This attack may lead to the disclosure of confidential data, denial of service, server side request forgery, port scanning from the perspective of the machine where the parser is located, and other system impacts.

K9Mail insecurely parses user-supplied XML content within the WebDavStore.java file, allowing attackers to perform XXE attacks. An attacker can potentially control a malicious WebDAV server or intercept the reponse of a valid WebDAV server to perform this attack.

Where the Issue Occurred

The code below retrieves the XML content and performs insecure XML parsing (found on line 943 k-9/mail/protocols/webdav/src/main/java/com/fsck/k9/mail/store/webdav/WebDavStore.java#L943):

xr.parse(new InputSource(istream));

Disclosure Timeline

  • Issue Reported: 24th October 2018
  • Blog Post Published: 28th October 2018
  • Applied for CVE: