pyasn1 has quadratic CPU consumption in constructed BIT STRING decoding

[ Why it matters ]Priority signals

• Public proof-of-concept exploit exists (CISA SSVC)

[ Description ]Official description

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.

[ Fixes ]Fixes and patches

• pyasn1 0.6.5Fixed version · GitHub Advisories · pip package pyasn1: upgrade to 0.6.5 or later (affected <= 0.6.4)

[ Packages ]Affected open-source packages

• pyasn1 pipAffected <= 0.6.4 · fixed in 0.6.5 · GHSA-2qm8-x737-gmw5

[ References ]References

• github.com/pyasn1/pyasn1/commit/73ba7b043cc7e92cb37f16f2c955bff93b0e6f6c
• github.com/pyasn1/pyasn1/pull/132
• github.com/pyasn1/pyasn1/releases/tag/v0.6.5
• github.com/pyasn1/pyasn1/security/advisories/GHSA-2qm8-x737-gmw5