The benefits of aggregate signatures and certificateless cryptography are combined in certificateless aggregate signature (CLAS) methods. In particular, it reduces communication and computation costs by combining several signatures into a single aggregate signature and streamlining certificate handling without creating the key escrow issue. Although CLAS is an effective cryptographic tool, its security should be carefully examined prior to use. Here in this work, we draw attention to the insecurity of the Xu et al.’s scheme against fully chosen key attack. Here, we introduce the idea of equivalent validity and demonstrate that the proposed system does not fulfill the condition of equivalent validity since it is unable to survive fully chosen key attacks. Our cryptanalysis explains the fully chosen key attack over the CLAS technique defined by Xu et al.