PHPMailer is prone to a cross-site scripting vulnerability because it fails to properly sanitize user-supplied input.
An attacker may leverage this issue to execute arbitrary script code in the browser of an unsuspecting user in the context of the affected site. This may allow the attacker to steal cookie-based authentication credentials and launch other attacks.
Information
PHPMailer PHPMailer 5.2.20
PHPMailer PHPMailer 5.2.19
PHPMailer PHPMailer 5.2.18
PHPMailer PHPMailer 5.2.14
PHPMailer PHPMailer 5.2.13
PHPMailer PHPMailer 1.7.3
PHPMailer PHPMailer 1.7.2
PHPMailer PHPMailer 1.7.1
PHPMailer PHPMailer 1.7
PHPMailer PHPMailer 5.2.22
PHPMailer PHPMailer 5.0.0
PHPMailer PHPMailer 1.73
PHPMailer PHPMailer 5.2.23
Exploit
Attackers can exploit these issues by enticing an unsuspecting victim to follow a malicious URI.
References: