Cpanel::JSON::XS versions before 4.41 for Perl allow type confusion via duplicate object keys...

Cpanel::JSON::XS versions before 4.41 for Perl allow type confusion via duplicate object keys when dupkeys_as_arrayref is enabled.

decode_hv() collapses duplicate object keys into an array reference under dupkeys_as_arrayref. The branch reached for a duplicate key tests SvTYPE (old_value) != SVt_RV && SvTYPE (SvRV (old_value)) != SVt_PVAV, which evaluates SvRV(old_value) before establishing that old_value is a reference. When the existing value is a plain scalar rather than an array reference, a non-reference scalar is dereferenced as a reference.

A caller decoding untrusted JSON with dupkeys_as_arrayref enabled is crashed, and the incompatible access follows a pointer taken from attacker controlled scalar contents.