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36 Network Security

Whenever Emacs establishes any network connection, it passes the established connection to the Network Security Manager (NSM). NSM is responsible for enforcing the network security under your control.

The network-security-level variable determines the security level that NSM enforces. If its value is low, no security checks are performed.

If this variable is medium (which is the default), a number of checks will be performed. If as result NSM determines that the network connection might not be trustworthy, it will make you aware of that, and will ask you what to do about the network connection.

You can decide to register a permanent security exception for an unverified connection, a temporary exception, or refuse the connection entirely.

Below is a list of the checks done on the medium level.

unable to verify a TLS certificate
If the connection is a TLS, SSL or STARTTLS connection, NSM will check whether the certificate used to establish the identity of the server we're connecting to can be verified.

While an invalid certificate is often the cause for concern (there could be a Man-in-the-Middle hijacking your network connection and stealing your password), there may be valid reasons for going ahead with the connection anyway. For instance, the server may be using a self-signed certificate, or the certificate may have expired. It's up to you to determine whether it's acceptable to continue with the connection.

a self-signed certificate has changed
If you've previously accepted a self-signed certificate, but it has now changed, that could mean that the server has just changed the certificate, but it might also mean that the network connection has been hijacked.
previously encrypted connection now unencrypted
If the connection is unencrypted, but it was encrypted in previous sessions, this might mean that there is a proxy between you and the server that strips away STARTTLS announcements, leaving the connection unencrypted. This is usually very suspicious.
talking to an unencrypted service when sending a password
When connecting to an IMAP or POP3 server, these should usually be encrypted, because it's common to send passwords over these connections. Similarly, if you're sending email via SMTP that requires a password, you usually want that connection to be encrypted. If the connection isn't encrypted, NSM will warn you.

If network-security-level is high, the following checks will be made, in addition to the above:

a validated certificate changes the public key
Servers change their keys occasionally, and that is normally nothing to be concerned about. However, if you are worried that your network connections are being hijacked by agencies who have access to pliable Certificate Authorities which issue new certificates for third-party services, you may want to keep track of these changes.
Diffie-Hellman low prime bits
When doing the public key exchange, the number of prime bits should be high to ensure that the channel can't be eavesdropped on by third parties. If this number is too low, you will be warned.
RC4 stream cipher
The RC4 stream cipher is believed to be of low quality and may allow eavesdropping by third parties.
SSL1, SSL2 and SSL3
The protocols older than TLS1.0 are believed to be vulnerable to a variety of attacks, and you may want to avoid using these if what you're doing requires higher security.

Finally, if network-security-level is paranoid, you will also be notified the first time NSM sees any new certificate. This will allow you to inspect all the certificates from all the connections that Emacs makes.

The following additional variables can be used to control details of NSM operation:

nsm-settings-file
This is the file where NSM stores details about connections. It defaults to ~/.emacs.d/network-security.data.
nsm-save-host-names
By default, host names will not be saved for non-STARTTLS connections. Instead a host/port hash is used to identify connections. This means that one can't casually read the settings file to see what servers the user has connected to. If this variable is t, NSM will also save host names in the nsm-settings-file.