Through a malicious URL that contained a quote character it was possible to inject HTML code in KMail's plaintext viewer. Due to the parser used on the URL it was not possible to include the equal sign (=) or a space into the injected HTML, which greatly reduces the available HTML functionality. Although it is possible to include an HTML comment indicator to hide content.
http://www.securityfocus.com/bid/93360
http://www.openwall.com/lists/oss-security/2016/10/05/1
http://www.debian.org/security/2016/dsa-3697
http://lists.opensuse.org/opensuse-updates/2016-10/msg00065.html