[Chapter 22] 22.2 The Environment
Author: Bryan Costales & Eric Allman
Date: 1999-01-06T19:28:47Z

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| Chapter 22 Security |

## 22.2 The Environment

As a general rule, programs should never trust their environment. Such trust can lead to exploitation that has grave security consequences. To illustrate, consider the often misused SunOS LD_LIBRARY_PATH environment variable. Programs that use shared libraries look at this variable to determine which shared library routines they should use and in what order they should load them. One form of attack against non-suid programs (such as some delivery agents) is to modify the LD_LIBRARY_PATH variable (as in a user's ~/.forward file) to introduce Trojan horse library routines in place of the real system's library routines. Certainly, sendmail should not pass such variables to its delivery agents.

To improve security, V8 sendmail began deleting variables from its environment before passing them to its delivery agents. It removed the IFS variable to protect Bourne shell-script agents and all variables beginning with "LD`_`" to protect all delivery agents from shared library attacks.

Beginning with V8.7, sendmail now takes the opposite approach. Instead of trying to second-guess attackers, it instead constructs the delivery agent environment from scratch. In this scheme it defines the AGENT variable as `sendmail`, and the TZ variable is as appropriate (see the `TimeZoneSpec` (`t`) option, Section 34.8.69, TimeZoneSpec (t)). Also, in support of operating systems that require them, it passes the ISP and SYSTYPE variables from its own environment to the delivery agent's environment.

### 22.2.1 The E Configuration Command

When sendmail executes (runs) a delivery agent (see Section 30.6.2, "The Child"), it passes to that delivery agent an environment that includes only the items described above. Some delivery agents, however, may require additional environmental variables to function properly. For those special cases, sendmail offers the `E` configuration command to set individual environment variables that will be passed to all delivery agents:

>     Evar=value

The `var` is the environment variable that will be either defined or redefined. It is immediately followed (with no intervening space) by an equal sign and then (again with no intervening space) by the `value` that will be assigned to it.

If the `=``value` is missing, sendmail looks up the variable `var` in its environment and, if it is found, uses that value. If the `=` is present but the `value` is absent, the `var` is assigned an empty string (a single zero byte). If the `var` is missing, a variable name that is an empty string is used.

The `var` is looked up to see whether it is already a part of the delivery agent's environment. If it is, it is redefined to be the new value. If it is not, it is added to that list of variables. If that addition will cause the list to exceed MAXUSERENVIRON variables (as defined in conf.h, see Section 18.8.19, MAX...), the definition is silently ignored.

Whether or not the `var` was added to, or updated in, the delivery agent's environment, it is always added or updated to sendmail's environment with putenv(2). If this call fails, sendmail logs and prints the following message:

>     setuserenv: putenv(var=value) failed

Only one `var` may be defined per `E` command. Additional environment variables require multiple `E` commands. Each `E` command affects all delivery agents. There is no way to tune the environment on a per delivery agent basis.

For DG/UX under V8.7 sendmail you will need to declare

>     E_FORCE_MAIL_LOCAL_=yes

in your configuration file to enable /bin/mail to work properly. Beginning with V8.8 sendmail, this is already done in cf/ostype/dgux.m4.

22.1 Why root? |  | 22.3 SMTP Probes