FOSSBilling is a free, open-source billing and client management system. Versions 0.6.0 through 0.7.2 have a SQL injection vulnerability in the Massmailer module filter functionality. An authenticated administrator can supply crafted filter values when updating a mass email message, causing untrusted input to be interpolated directly into SQL in the recipient selection query. Version 0.8.0 patches the issue. Some workarounds are available. Restrict administrator access to trusted users only, disable the Massmailer module if it is not required, audit existing records in the mod_massmailer table for suspicious filter values, and/or review administrator activity related to Massmailer message updates.