pyasn1 has quadratic CPU use in schemaless BER/CER/DER container 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-2026-119194: 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 When an application decodes attacker-controlled BER, CER, or DER data without an ASN.1 schema, a container with many distinct long tag IDs can cause quadratic CPU use. Repeated inputs can exhaust CPU and reduce service availability.

Details ConstructedPayloadDecoderBase._decodeComponentsSchemaless inserts every decoded component’s TagSet into a growing set, although it only needs to distinguish one tag set from two or more. Attacker-chosen tag IDs can have colliding TagSet hashes, making each later insertion compare against an increasing number of earlier tags. This path is present in v0.6.4 and is inherited by the CER and DER decoders. The proposed fix stops growing the classification set after it contains two distinct tag sets, while retaining every decoded component and its order. On v0.6.4 with CPython 3.12.10, increasing the collision input from 3,200 to 6,400 components raised decode CPU time from 0.438 to 1.747 seconds; a comparable control rose from 0.031 to…

Published by the source

30 Oct 2026, 23:03 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-fm9c-5rcf-2q5p

2. Revision 1 · created · 30 Oct 2026

   Published from ghsa record GHSA-fm9c-5rcf-2q5p

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# Patch Next advisory brief: CVE-2026-119194

Advisory: pyasn1 has quadratic CPU use in schemaless BER/CER/DER container decoding
IDs: CVE-2026-119194, GHSA-fm9c-5rcf-2q5p
Record: https://patchnext.com/advisories/cve-2026-119194 (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 >= 0.3.3, < 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-fm9c-5rcf-2q5p (GitHub-reviewed advisory GHSA-fm9c-5rcf-2q5p lists affected ranges: PyPI pyasn1 >=0.3.3 <0.6.5)
- Verified: https://osv.dev/vulnerability/GHSA-fm9c-5rcf-2q5p (OSV record GHSA-fm9c-5rcf-2q5p lists affected ranges: PyPI pyasn1 >=0.3.3 <0.6.5 (fixed))

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