kaPoW Webmail: Effective Disincentives Against Spam
Wu-chang Feng
Ed Kaiser
Portland State University Portland State University
wuchang@cs.pdx.edu
edkaiser@cs.pdx.edu
ABSTRACT
Webmail spam poses a significant threat to major web-
mail providers such as Google GMail, Yahoo! Mail, and
Microsoft Live Mail, as well as to individual companies
and universities that provide web-based interfaces to their
e-mail. Whether spammers create new accounts or hijack
existing accounts to send spam, the transmission of spam
drives up server operating costs as well as the human
costs required to identify and disable spamming accounts.
This paper presents kaPoWWebmail, a system for slow-
ing down and disincentivizing webmail spammers using
transparent, web-based proof-of-work (also known as
client puzzles). The approach identifies and forces such
clients to solve a computational puzzle for each e-mail
sent. The system employs a novel puzzle algorithm that
efficiently generates and verifies fine-grained computa-
tional puzzles that have deterministic solution-times. Un-
like prior proof-of-work systems, kaPoW Webmail also
adaptively issues puzzles of varying difficulties based on
a comprehensive set of client-specific and content-specific
measurements. The evaluation shows that the system
thwarts spammers while preserving service to legitimate
webmail clients.
Keywords: Webmail, Spam, Proof-of-Work, Client puz-
zles
1.
INTRODUCTION
As sender reputation systems for e-mail transmission
continue to improve, spammers are increasingly turning
towards web-based e-mail to continue their operations.
As a result, in 2008, the amount of webmail spam ap-
proached close to 5% of all spam sent [18]. To execute
attacks using webmail services, spammers attempt to au-
tomate the creation of new accounts at free webmail sites
such as Google GMail, Yahoo!Mail, andMicrosoft’s Live
Mail or they perform reputation hijacking by obtaining
CEAS 2010 - Seventh Annual Collaboration, Electronic messaging, Anti-
abuse and Spam Conference, July 13-14, 2010, Redm