# CVE-2026-119196: pyasn1: Quadratic CPU consumption in constructed BIT STRING decoding

Status: published. Published: 2026-10-29. Updated: 2026-11-04.

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.

## Remediation

- fix: pyasn1 (PyPI): OSV reports a fixed version 0.6.5. Check the advisory for supported release branches. (source: OSV GHSA-2qm8-x737-gmw5) — https://github.com/advisories/GHSA-2qm8-x737-gmw5

## Affected products and versions

- pyasn1 / pyasn1: < 0.6.5 (affected). Source: CNA: GitHub_M
-  / pyasn1 (pyasn1): 0.0.10a (affected, PyPI); 0.0.11a (affected, PyPI); 0.0.12a (affected, PyPI); 0.0.13 (affected, PyPI); 0.0.13a (affected, PyPI); 0.0.13b (affected, PyPI); 0.0.6a (affected, PyPI); 0.0.9a (affected, PyPI); 0.1.1 (affected, PyPI); 0.1.2 (affected, PyPI); 0.1.3 (affected, PyPI); 0.1.4 (affected, PyPI); 0.1.5 (affected, PyPI); 0.1.6 (affected, PyPI); 0.1.7 (affected, PyPI); 0.1.8 (affected, PyPI); 0.1.9 (affected, PyPI); 0.2.1 (affected, PyPI); 0.2.2 (affected, PyPI); 0.2.3 (affected, PyPI); 0.3.1 (affected, PyPI); 0.3.2 (affected, PyPI); 0.3.3 (affected, PyPI); 0.3.4 (affected, PyPI); 0.3.5 (affected, PyPI); 0.3.6 (affected, PyPI); 0.3.7 (affected, PyPI); 0.4.1 (affected, PyPI); 0.4.2 (affected, PyPI); 0.4.3 (affected, PyPI); 0.4.4 (affected, PyPI); 0.4.5 (affected, PyPI); 0.4.6 (affected, PyPI); 0.4.7 (affected, PyPI); 0.4.8 (affected, PyPI); 0.5.0 (affected, PyPI); 0.5.1 (affected, PyPI); 0.6.0 (affected, PyPI); 0.6.1 (affected, PyPI); 0.6.2 (affected, PyPI); 0.6.3 (affected, PyPI); 0.6.4 (affected, PyPI); Ecosystem version events: introduced 0; fixed 0.6.5. Source: OSV GHSA-2qm8-x737-gmw5

## Severity and prioritization

- CVSS 3.1: 7.5 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H; source: CNA: GitHub_M

- CISA KEV: No listing in collected sources

- EPSS: 0.43% as of 2026-11-03; percentile 36.84%

- SSVC: exploitation poc; automatable yes; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-407

## Exploit evidence

- exploit: https://github.com/pyasn1/pyasn1/security/advisories/GHSA-2qm8-x737-gmw5 (ADP: CISA-ADP)

## References

- https://github.com/pyasn1/pyasn1/security/advisories/GHSA-2qm8-x737-gmw5 (CNA: GitHub_M; x_refsource_CONFIRM)

- https://github.com/pyasn1/pyasn1/pull/132 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/pyasn1/pyasn1/commit/73ba7b043cc7e92cb37f16f2c955bff93b0e6f6c (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/pyasn1/pyasn1/releases/tag/v0.6.5 (CNA: GitHub_M; x_refsource_MISC)

- https://github.com/pyasn1/pyasn1/security/advisories/GHSA-2qm8-x737-gmw5 (ADP: CISA-ADP; exploit)

- https://github.com/advisories/GHSA-2qm8-x737-gmw5 (OSV / GitHub Security Advisories; advisory)

- https://osv.dev/vulnerability/GHSA-2qm8-x737-gmw5 (OSV / GitHub Security Advisories; osv)

## Provenance

- CNA: GitHub_M: https://www.cve.org/CVERecord?id=CVE-2026-119196; retrieved 2026-11-04; source updated 2026-11-04

- OSV / GitHub Security Advisories: https://osv.dev/vulnerability/GHSA-2qm8-x737-gmw5; retrieved 2026-11-04; source updated 2026-10-30

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-119196; retrieved 2026-11-04; source updated Not reported

Revision: 05ed63f4eb079862eac3db4e8b7f2c82c019cfa3a91357d42c9a80b6f83313e9

Canonical: https://y2.dev/cve/CVE-2026-119196

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