ClamAV is a GPL virus scanner. Freshclam is a utility to download virus signature updates.
Ulf Harnhammar and an anonymous German researcher discovered that Freshclam fails to check the size of the header data returned by a webserver.
By enticing a user to connect to a malicious webserver an attacker could cause the execution of arbitrary code.
There is no known workaround at this time.
All ClamAV users should upgrade to the latest version:
# emerge --sync
# emerge --ask --oneshot --verbose ">=app-antivirus/clamav-0.88.2"