pyasn1 versions up to 0.6.4 have a vulnerability in decoding constructed BIT STRING values where CPU consumption grows quadratically with input size. This occurs because the decoder repeatedly shifts and copies accumulated data for each fragment, leading to potential CPU exhaustion and denial of service when processing attacker-controlled ASN.1 data. The issue affects BER and CER decoders and certain schema-driven DER decoding paths. The vulnerability is fixed in version 0.6.5.