Impact

SQL injection can occur when all of the following conditions are met:

1. The non-default simple protocol is used.
2. A placeholder for a numeric value must be immediately preceded by a minus.
3. There must be a second placeholder for a string value after the first placeholder; both
   must be on the same line.
4. Both parameter values must be user-controlled.

e.g.

Simple mode must be enabled:

// connection string includes "prefer_simple_protocol=true"
// or
// directly enabled in code
config.ConnConfig.PreferSimpleProtocol = true

Parameterized query:

SELECT * FROM example WHERE result=-$1 OR name=$2;

Parameter values:

$1 => -42
$2 => "foo\n 1 AND 1=0 UNION SELECT * FROM secrets; --"

Resulting query after preparation:

SELECT * FROM example WHERE result=--42 OR name= 'foo
1 AND 1=0 UNION SELECT * FROM secrets; --';

Patches

The problem is resolved in v4.18.2.

Workarounds

Do not use the simple protocol or do not place a minus directly before a placeholder.