pyasn1 versions 0.6.4 and earlier contain a denial of service vulnerability due to quadratic time complexity in decoding binary-encoded ASN.1 REAL values with large mantissas. This issue affects BER, CER, and DER decoders that use the vulnerable RealPayloadDecoder. An attacker supplying crafted ASN.1 data with large REAL mantissas can cause excessive CPU consumption during decoding. The vulnerability is fixed in pyasn1 version 0.6.5.