Go: Improve QHelp for go/unsafe-quoting.

Author: Max Schaefer

go/ql/src/Security/CWE-089/StringBreak.qhelp

@@ -5,19 +5,21 @@
 
 <overview>
 <p>
-Code that constructs a string containing a quoted substring needs to ensure that any user-provided
-data embedded in between the quotes does not itself contain a quote. Otherwise the embedded data
-could (accidentally or intentionally) change the structure of the overall string by terminating
-the quoted substring early, with potentially severe consequences. If, for example, the string is
-later interpreted as an operating-system command or database query, a malicious attacker may be
-able to craft input data that enables a command injection or SQL injection attack.
+Code that constructs a quoted string literal containing user-provided data needs to ensure that
+this data does not itself contain a quote. Otherwise the embedded data could (accidentally or
+intentionally) terminate the string literal early and thereby change the structure of the overall
+string, with potentially severe consequences. If, for example, the string is later used as
+part an operating-system command or database query, an attacker may be able to craft input data
+that injects a malicious command.
 </p>
 </overview>
 
 <recommendation>
 <p>
 Sanitize the embedded data appropriately to ensure quotes are escaped, or use an API that does
-not rely on manually constructing quoted substrings.
+not rely on manually constructing quoted substrings. Make sure to use the appropriate escaping
+mechanism, for example double quoting for SQL strings or backslash escaping for shell commands.
+When using backslash escaping, the backslash character itself must also be escaped.
 </p>
 </recommendation>
 
@@ -29,17 +31,38 @@ then embeds it into a SQL query built using the Squirrel library.
 </p>
 <sample src="StringBreak.go"/>
 <p>
-Note that while Squirrel provides a structured API for building SQL queries that mitigates against
-common causes of SQL injection vulnerabilities, this code is still vulnerable: if the JSON-encoded
-representation of <code>version</code> contains a single quote, this will prematurely close the
-surrounding string, changing the structure of the SQL expression being constructed. This could be
-exploited to mount a SQL injection attack.
+Note that JSON encoding does not escape single quotes in any way, so this code is vulnerable: any
+single-quote character in <code>version</code> will prematurely close the surrounding string literal,
+changing the structure of the SQL expression being constructed. This could be exploited to mount
+a SQL injection attack.
 </p>
 <p>
-To fix this vulnerability, use Squirrel's placeholder syntax, which avoids the need to explicitly
-construct a quoted string.
+To fix this vulnerability, use the placeholder syntax from Squirrel's structured API for building
+queries, which avoids the need to explicitly construct a quoted string.
 </p>
 <sample src="StringBreakGood.go"/>
+<p>
+In situations where a structured API is not available, make sure to escape quotes before embedding
+user-provided data into a quoted string. For example, this is how you can backslash-escape single
+quotes using <code>strings.ReplaceAll</code>:
+</p>
+<sample language="go">
+  quoted := strings.ReplaceAll(raw, `\`, `\\`)
+  quoted = strings.ReplaceAll(quoted, "'", "\\'")
+</sample>
+<p>
+Note that any existing backslash characters in the string must be escaped first, so that they do
+not interfere with the escaping of single quotes.
+</p>
+<p>
+In some cases, <code>strconv.Quote</code> is a convenient option for backslash escaping, but note
+that it has two limitations:
+</p>
+<ol>
+  <li>It only supports double quotes, not single quotes (as in the example).</li>
+  <li>It puts quotes around the entire string, so it can only be used to construct complete string
+      literals, not parts of larger string literals.</li>
+</ol>
 </example>
 
 <references>