Environment variable injection query documentation

Author: egregius313

java/ql/src/Security/CWE/CWE-078/ExecTaintedEnvironment.qhelp

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+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+<overview>
+<p>Passing unvalidated user input into the environment variables of a subprocess can allow an attacker to execute malicious code.</p>
+</overview>
+
+<recommendation>
+<p>If possible, use hard-coded string literals to specify the environment variable or its value. 
+Instead of passing the user input directly to the
+process or library function, examine the user input and then choose
+among hard-coded string literals.</p>
+
+<p>If the applicable environment variables cannot be determined at
+compile time, then add code to verify that the user input string is
+safe before using it.</p>
+</recommendation>
+
+<example>
+<p>In the following (BAD) example, the environment variable <code>PATH</code> is set to the value of the user input <code>path</code> without validation.</p>
+
+<sample src="ExecTaintedEnvironmentValue.java" />
+
+<p>In the following (BAD) example, an environment variable is set with a name that is derived from the user input <code>var</code> without validation.</p>
+
+<sample src="ExecTaintedEnvironmentName.java" />
+
+</example>
+</qhelp>

java/ql/src/Security/CWE/CWE-078/ExecTaintedEnvironmentName.java

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+public void doGet(HttpServletRequest request, HttpServletResponse response) {
+    String attr = request.getParameter("attribute");
+    String value = request.getParameter("value");
+
+    Map<String, String> env = processBuilder.environment();
+    env.put(attribute, value);
+
+    processBuilder.start();
+}

java/ql/src/Security/CWE/CWE-078/ExecTaintedEnvironmentValue.java

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+public void doGet(HttpServletRequest request, HttpServletResponse response) {
+    String path = request.getParameter("path");
+
+    Map<String, String> env = processBuilder.environment();
+    env.put("PATH", path);
+
+    processBuilder.start();
+}