Ruby: Add rb/tainted-format-string query

ruby/ql/src/queries/security/cwe-134/TaintedFormatString.qhelp

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+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+<overview>
+<p>
+Methods like <code>Kernel.printf</code> accept a format string that is used to format
+the remaining arguments by providing inline format specifiers. If the format string
+contains unsanitized input from an untrusted source, then that string may contain
+unexpected format specifiers that cause garbled output or throw an exception.
+</p>
+</overview>
+
+<recommendation>
+<p>
+Either sanitize the input before including it in the format string, or use a
+<code>%s</code> specifier in the format string, and pass the untrusted data as corresponding
+argument.
+</p>
+</recommendation>
+
+<example>
+<p>
+The following program snippet logs information about an unauthorized access attempt. The
+log message includes the user name, and the user's IP address is passed as an additional
+argument to <code>Kernel.printf</code> to be appended to the message:
+</p>
+<sample src="examples/tainted_format_string_bad.rb"/>
+<p>
+However, if a malicious user provides a format specified such as <code>%s</code> as their
+user name, <code>Kernel.printf</code> throw an exception that there are too few arguments
+to satisfy the format. This can result in denial of service or leaking of internal
+information to the attacker via a stack trace.
+</p>
+<p>
+Instead, the user name should be included using the <code>%s</code> specifier:
+</p>
+<sample src="examples/tainted_format_string_good.rb"/>
+
+<p>
+Alternatively, a method such as <code>Kernel.puts</code> should be used, which does not
+apply string formatting to its arguments.
+</p>
+</example>
+
+<references>
+<li>Ruby documentation for <a href="https://docs.ruby-lang.org/en/3.1/Kernel.html#method-i-sprintf">format strings</a>.</li>
+<li>Common Weakness Enumeration: <a href="https://cwe.mitre.org/data/definitions/134.html">CWE-134</a>.</li>
+</references>
+</qhelp>

ruby/ql/src/queries/security/cwe-134/TaintedFormatString.ql

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+/**
+ * @name Use of externally-controlled format string
+ * @description Using external input in format strings can lead to garbled output.
+ * @kind path-problem
+ * @problem.severity warning
+ * @security-severity 7.3
+ * @precision high
+ * @id rb/tainted-format-string
+ * @tags security
+ *       external/cwe/cwe-134
+ */
+
+import ruby
+import codeql.ruby.security.TaintedFormatStringQuery
+import DataFlow::PathGraph
+
+from Configuration cfg, DataFlow::PathNode source, DataFlow::PathNode sink
+where cfg.hasFlowPath(source, sink)
+select sink.getNode(), source, sink, "$@ flows here and is used in a format string.",
+  source.getNode(), "User-provided value"

ruby/ql/src/queries/security/cwe-134/examples/tainted_format_string_bad.rb

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+class UsersController < ActionController::Base
+  def index
+    printf("Unauthorised access attempt by #{params[:user]}: %s", request.ip)
+  end
+end

ruby/ql/src/queries/security/cwe-134/examples/tainted_format_string_good.rb

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+class UsersController < ActionController::Base
+  def index
+    printf("Unauthorised access attempt by %s: %s", params[:user], request.ip)
+  end
+end