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

Advisory record · `GHSA-m4p7-r5rc-7g4j`

- **Identifiers:** `GHSA-m4p7-r5rc-7g4j` · `CVE-2026-59884` · `PYSEC-2026-3455`
- **State:** Confirmed
- **Severity:** CVSS 7.5 High
- **Critical:** No
- **Weakness (CWE):** [CWE-400](https://cwe.mitre.org/data/definitions/400.html)
- **Tags:** [Denial of service](https://agentprime.io/advisories/tag/dos.md)
- **Published:** 21 Jul 2026
- **Updated:** 10 Sept 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 21 Jul 2026, 19:10 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `pyasn1` (PyPI): affected `<0.6.4`; fixed in `0.6.4`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Versions of pyasn1 before 0.6.4 are affected by a denial-of-service flaw in the BER, CER and DER decoders. The decoder accepts long-form tag IDs of unbounded length, so a crafted input can consume a large amount of CPU time, and on Python 3.11 and later it can raise an unhandled ValueError instead of the documented PyAsn1Error. An attacker needs only to supply crafted BER, CER or DER input to an application that decodes untrusted data. Upgrade to pyasn1 0.6.4, which limits long-form tag IDs to 20 octets. Until then, limit the size of untrusted input before passing it to decode().

Written 4 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 7.5 High, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0062, the 50th percentile, on 15 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:29 UTC: First seen by agentprime
- 31 Oct 2026, 14:29 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-m4p7-r5rc-7g4j)

## Sources

- [GitHub Advisory Database: GHSA-m4p7-r5rc-7g4j](https://github.com/advisories/GHSA-m4p7-r5rc-7g4j)
- [OSV: GHSA-m4p7-r5rc-7g4j](https://osv.dev/vulnerability/GHSA-m4p7-r5rc-7g4j)
- <https://github.com/pyasn1/pyasn1/security/advisories/GHSA-m4p7-r5rc-7g4j>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-59884>
- <https://github.com/pyasn1/pyasn1/commit/628e36ecbb5277a3f01572ce418ef54271b165a5>
- <https://github.com/pyasn1/pyasn1>
- <https://github.com/pyasn1/pyasn1/releases/tag/v0.6.4>
- <https://github.com/pypa/advisory-database/tree/main/vulns/pyasn1/PYSEC-2026-3455.yaml>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-m4p7-r5rc-7g4j>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-m4p7-r5rc-7g4j/>. Everything for agents: <https://agentprime.io/llms.txt>.