pyasn1 has quadratic CPU consumption in constructed BIT STRING decoding

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-119196: 0.43% estimated exploitation activity in the next 30 days; percentile 36.84 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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…

Published by the source

30 Oct 2026, 23:07 UTC

Last modified by the source

30 Oct 2026, 23:15 UTC

Source last checked

31 Oct 2026, 06:15 UTC

1. Revision 2 · amended · 31 Oct 2026

   Amended from osv record GHSA-2qm8-x737-gmw5

2. Revision 1 · created · 30 Oct 2026

   Published from ghsa record GHSA-2qm8-x737-gmw5

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# Patch Next advisory brief: CVE-2026-119196

Advisory: pyasn1 has quadratic CPU consumption in constructed BIT STRING decoding
IDs: CVE-2026-119196, GHSA-2qm8-x737-gmw5
Record: https://patchnext.com/advisories/cve-2026-119196 (revision 2, source checked 31 Oct 2026, 06:15 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: pyasn1 0.6.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI pyasn1: affected < 0.6.5; fixed in 0.6.5

## Suggested inspection (read-only)
- `python -m pip show pyasn1`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on pyasn1.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-2qm8-x737-gmw5 (GitHub-reviewed advisory GHSA-2qm8-x737-gmw5 lists affected ranges: PyPI pyasn1 >=0 <=0.6.4)
- Verified: https://osv.dev/vulnerability/GHSA-2qm8-x737-gmw5 (OSV record GHSA-2qm8-x737-gmw5 lists affected ranges: PyPI pyasn1 >=0 <0.6.5 (fixed))

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