An issue was discovered in KDE KMail before 19.12.3. By using the proprietary (non-RFC6068) "mailto?attach=..." parameter, a website (or other source of mailto links) can make KMail attach local files to a composed email message without showing a warning to the user, as demonstrated by an attach=.bash_history value.
KDE KMail does not encrypt attachments in emails when "automatic encryption" is enabled, which allows remote attackers to obtain sensitive information by sniffing the network.
KDE kmail before 5.5.2 and messagelib before 5.5.2, as distributed in KDE Applications before 17.04.2, do not ensure that a plugin's sign/encrypt action occurs during use of the Send Later feature, which allows remote attackers to obtain sensitive information by sniffing the network.
KMail since version 5.3.0 used a QWebEngine based viewer that had JavaScript enabled. Since the generated html is executed in the local file security context by default access to remote and local URLs was enabled.
KMail since version 5.3.0 used a QWebEngine based viewer that had JavaScript enabled. HTML Mail contents were not sanitized for JavaScript and included code was executed.