Evolution is a GNOME groupware application.
Ulf Harnhammar from Secunia Research discovered a format string error in the emf_multipart_encrypted() function in the file mail/em-format.c when reading certain data (e.g. the "Version:" field) from an encrypted e-mail.
A remote attacker could entice a user to open a specially crafted encrypted e-mail, potentially resulting in the execution of arbitrary code with the privileges of the user running Evolution.
There is no known workaround at this time.
All Evolution users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=mail-client/evolution-2.12.3-r1"