Bug 2530105 (CVE-2026-19872)

Summary: CVE-2026-19872 perl-HTML-FormHandler: HTML::FormHandler: Cross-site scripting via unescaped error messages
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in HTML::FormHandler. A remote attacker could exploit this vulnerability by submitting malicious input that is then rendered unescaped in error messages. This allows the attacker to execute arbitrary script code in the victim's browser, potentially leading to information disclosure or session hijacking.
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Bug Depends On: 2533273    
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Description OSIDB Bzimport 2026-09-08 20:25:11 UTC 
HTML::FormHandler versions before 0.410000 for Perl allow cross-site scripting via a submitted value rendered unescaped in an error message.

The wrappers and renderers that emit a form's errors interpolate the error string straight into HTML with no escaping. Two of the library's own messages, no_match and not_allowed, splice the submitted value into that string, and a failing type constraint puts the rejected value into the message it builds, which _apply_actions hands to add_error.

A field declared with a check regexp, a check list or a type constraint reaches those messages, with no custom validator and no non-default configuration. Errors rendered through an application's own escaping template layer rather than the library's rendering roles are not affected.

A request over the network that submits markup to such a field gets it back live inside the error span, running script in the victim's origin. Re-rendering a rejected value later gives the stored variant.