pyasn1 versions up to 0.6.4 contain a denial of service vulnerability in the decoding of binary ASN.1 REAL values with large mantissas. The decoding process is quadratic in the mantissa length, causing excessive CPU consumption. This affects BER, CER, and DER decoders using the RealPayloadDecoder. The issue is fixed in pyasn1 0.6.5. Applications passing attacker-controlled ASN.1 REAL values to these decoders without proper input limits are vulnerable to CPU exhaustion during decoding.