# On the Security of Practical Mail User Agents against Cache Side-Channel Attacks +
## 3 Attacks on Mail User Agents
### Analyzing Mail User Agents  
We first investigated 37 MUA programs supporting email encryption functionality and installed 13 of these on each VM, all of which are available in Ubuntu 14.04, 16.04 and 18.04. We then tested whether they allowed the installation of GnuPG 1.4.13, which is vulnerable to FLUSH+RELOAD attacks. As shown in Table 1, 10 of these MUAs allowed installation without checking the version of the crypto library, making our attack feasible. However, the other three MUAs (Thunderbird, Seamonkey Mail and Sylpheed) blocked the installation by checking the version of the third-party crypto library. Based on our experiment, these three MUAs only allowed the installation of GnuPG 2.0.14 or later.  
**Thunderbird** uses the Enigmail plugin for GnuPG. We installed Thunderbird 52.9.1 on Ubuntu 14.04 and Thunderbird 60.2.1 on Ubuntu 16.04 and 18.04. When we executed Thunderbird in all Ubuntu versions, we observed a warning message from the MUA program about the potential installation of an old version of GnuPG, which disallowed the sending or retrieving of encrypted email. Thus, the cache side-channel attack could not obtain any traces with either version of Thunderbird. **Seamonkey Mail**, and **Sylpeed** also prevented the use of all encrypted email services using a vulnerable version of GnuPG in Ubuntu 14.04, 16.04 and 18.04.  
**Alpine, Claws Mail, Evolution** and **Kmail** allowed the installation of GnuPG 1.4.13 in all three versions of Ubuntu in full support of encrypted email services using GnuPG. During the installation of  
\begin{table}
\begin{tabular}{l c c c c c} \hline \hline  & \multicolumn{2}{c}{**Ubuntu 14.04**} & \multicolumn{2}{c}{**Ubuntu 16.04**} & \multicolumn{2}{c}{**Ubuntu 18.04**} \\ \hline
**MUA** & **Version** & **GnuPG 1.4.13** & **Version** & **GnuPG 1.4.13** & **Version** & **GnuPG 1.4.13** \\ \hline Alpine & 2.10 & Allowed & 2.20 & Allowed & 2.21 & Allowed \\ Balsa & - & - & 2.5.6 & Allowed & 2.5.3 & Allowed \\ Claws Mail & 3.9.3 & Allowed & 3.13.2 & Allowed & 3.16.0 & Allowed \\ Cone & 0.89 & Allowed & 1.0 & Allowed & - & - \\ Evolution & 3.10.4 & Allowed & 3.18.5.2 & Allowed & 3.28.5 & Allowed \\ GNUmail & - & - & 1.2.2 & Allowed & - & - \\ i.Scribe & - & - & 2.2 & Allowed & 2.3 & Allowed \\ K Mail & 4.13.3 & Allowed & 5.1.3 & Allowed & 5.7.3 & Allowed \\ Thunderbird & 52.9.1 & Prevented & 60.2.1 & Prevented & 60.2.1 & Prevented \\ Seamonkey & 2.49.4 & Prevented & 2.49.4 & Prevented & 2.49.4 & Prevented \\ Sylpheed & 3.6 & Prevented & 3.5 & Prevented & 3.5.1 & Prevented \\ Trojita & 0.7 & Allowed & - & - & - & - \\ Zimbra & 7.3.1 & Allowed & - & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 1: Vulnerability Analysis of mail user agents (MUAs).  
Figure 2: Overview of our attack scenario.  
the vulnerable libraries on the MUA and the sending/retrieving of emails via the MUA, no warning messages were generated.  
**Cone** in Ubuntu 14.04 and 16.04 and **Balsa** and **i.Scribe** in Ubuntu 16.04 and 18.04 allowed the installation of GnuPG 1.4.13 and provided all of the functionality of encrypted email without warning the user about the old version of the crypto library.  
**Trojita** and **Zimbra** in Ubuntu 14.04 and **GNUmail** in Ubuntu 16.04 also allowed the installation of the vulnerable crypto library and supported full email encryption services without any warning messages or safeguard mechanisms.