Affected: pyasn1

pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.5, constructed BIT STRING decoding repeatedly shifts and copies the complete bit value accumulated from earlier fragments. Attacker-controlled BER or CER input contains many constructed BIT STRING fragments; schema-driven DER also accepts a definite-length constructed BIT STRING. BitStringPayloadDecoder repeatedly calls BitString.fromOctetString with the prior bitString as prepend, shifting an ever-growing SizedInteger for each fragment and causing quadratic work. Large fragmented values can exhaust CPU during decode() without requiring later conversion or formatting. This issue is fixed in version 0.6.5.

Affected: PyPI: pyasn1

## Summary

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](https://github.com/advisories/GHSA-8ppf-4f7h-5ppj).

## Technical details

In the 0.6.4 [`BitStringPayloadDecoder`](https://github.com/pyasn1/pyasn1/blob/v0.6.4/pyasn1/codec/ber/decoder.py#L173-L301), both the [definite-length](https://github.com/pyasn1/pyasn1/blob/v0.6.4/pyasn1/codec/ber/decoder.py#L237-L255) and [indefinite-length](https://github.com/pyasn1/pyasn1/blob/v0.6.4/pyasn1/codec/ber/decoder.py#L275-L301) fragment loops call `BitString.fromOctetString(..., prepend=bitStri…