pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.5, binary REAL decoding builds a large mantissa one octet at a time by shifting a growing integer and slicing the remaining byte string. Attacker-controlled ASN.1 includes a binary-encoded REAL with a large mantissa and reaches BER, CER, or DER decode(), with or without a schema containing REAL. RealPayloadDecoder.valueDecoder repeatedly performs a left shift, OR, and suffix copy for every mantissa octet, making decode time quadratic in mantissa length. A comparatively small REAL value can hold a decoding worker on one CPU core for seconds or longer and cause availability loss. This issue is fixed in version 0.6.5.

Affected: PyPI: pyasn1

## Summary

In pyasn1 0.6.4 and earlier, decoding a binary-encoded ASN.1 REAL with a large mantissa can consume CPU time quadratic in the mantissa length. The BER, CER, and DER decoders all use the affected `RealPayloadDecoder`. An application is exposed when it passes attacker-controlled ASN.1 containing a REAL value to one of these decoders, either without an `asn1Spec` or with a schema that includes REAL. The work occurs inside `decode()`; the application does not need to convert, print, or otherwise use the resulting REAL object.

This is distinct from [CVE-2026-59886](https://github.com/advisories/GHSA-hm4w-wwcw-mr6r), which concerns converting an already-decoded REAL to a Python float. That issue was fixed in 0.6.4 and its advisory states that decoding alone does not trigger it.

## Root cause and affected components

For binary REAL values, [`RealPayloadDecoder.valueDecoder` in v0.6.4](https://github.com/pyasn1/pyasn1/blob/v0.6.4/pyasn1/codec/ber/decoder.py#L535-L603) builds the mantissa one octet at a time:

```python p = 0 while chunk: p <<= 8 p |= chunk[0] chunk = chunk[1:] ```

Each shift processes an integer that grows with the mantissa; with byte strings, each slice also copies the remaining suffix. Repeating both operations for every octet makes decoding quadratic in mantissa size. The decoder's [eight-octet length-field limit](https://github.com/pyasn1/pyasn1/blob/v0.6.4/pyasn1/codec/ber/decoder.py#L1704-L1717) does not impose a practical mantissa-…