Ruby: document rb/log-injection

ruby/ql/src/queries/security/cwe-117/LogInjection.qhelp

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+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+<overview>
+<p>
+If unsanitized user input is written to a log entry, a malicious user may
+able to forge new log entries.
+</p>
+
+<p>
+Forgery can occur if a user provides some input with characters that are
+interpreted when the log output is displayed. If the log is displayed as a plain
+text file, then new line characters can be used by a malicious user. If the log
+is displayed as HTML, then arbitrary HTML may be included to spoof log entries.
+</p>
+</overview>
+
+<recommendation>
+<p>
+User input should be suitably sanitized before it is logged. Suitable means of
+sanitization depend on how the log entries will be displayed or consumed.
+</p>
+
+<p>
+If the log entries are in plain text then line breaks should be removed from
+user input, using <code>String#gsub</code> or similar. Care should also be
+taken that user input is clearly marked in log entries.
+</p>
+
+<p>
+For log entries that will be displayed in HTML, user input should be
+HTML-encoded before being logged, to prevent forgery and other forms of HTML
+injection.
+</p>
+</recommendation>
+
+<example>
+<p>
+In the example, a username, provided by the user, is logged using `Logger#info`.
+</p>
+
+<p>
+In the first case, it is logged without any sanitization. If a malicious user
+provides `username=Guest%0a[INFO]+User:+Admin%0a` as a username parameter, the
+log entry will be split in two different lines, where the second line will
+be `[INFO]+User:+Admin`.
+</p>
+<sample src="examples/log_injection_bad.rb" />
+
+<p>
+In the second example, <code>String#gsub</code> is used to ensure no line
+endings are present in the user input.
+</p>
+<sample src="examples/log_injection_good.rb" />
+</example>
+
+<references>
+<li>OWASP: <a href="https://www.owasp.org/index.php/Log_Injection">Log Injection</a>.</li>
+</references>
+</qhelp>

ruby/ql/src/queries/security/cwe-117/examples/log_injection_bad.rb

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+require 'logger'
+
+class UsersController < ApplicationController
+  def login
+    logger = Logger.new STDOUT
+    username = params[:username]
+
+    # BAD: log message constructed with unsanitized user input
+    logger.info "attempting to login user: " + username
+
+    # ... login logic ...
+  end
+end

ruby/ql/src/queries/security/cwe-117/examples/log_injection_good.rb

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+require 'logger'
+
+class UsersController < ApplicationController
+  def login
+    logger = Logger.new STDOUT
+    username = params[:username]
+
+    # GOOD: log message constructed with unsanitized user input
+    sanitized_username = username.gsub("\n", "")
+    logger.info "attempting to login user: " + sanitized_username
+
+    # ... login logic ...
+  end
+end