CVE-2026-119196: pyasn1 has quadratic CPU consumption in constructed BIT STRING decoding

In pyasn1 through 0.6.4, decoding a constructed BIT STRING with many fragments repeatedly shifts and copies the entire value accumulated so far. For fixed-size fragments, CPU time grows quadratically with the encoded input size. An application that passes sufficiently large, attacker-controlled ASN.1 data to an affected decoder can suffer CPU exhaustion during decode(); no conversion or formatting of the decoded value is required.

The BER and CER decoders reach this path for constructed BIT STRING values, with or without an asn1Spec. The DER decoder rejects constructed BIT STRING in schema-less decoding, but its schema-driven path accepts a definite-length constructed BIT STRING when given a matching asn1Spec and reaches the same vulnerable code. Constructed BIT STRING is not valid DER; this DER exposure is due to decoder permissiveness. DER rejects the indefinite-length form.

This is a separate decoder path from the OBJECT IDENTIFIER accumulation fixed in CVE-2026-59885.