---
id: CVE-2026-59884
aliases:
  - GHSA-m4p7-r5rc-7g4j
  - PYSEC-2026-3455
title: pyasn1 BER/CER/DER decoder denial of service via unbounded long-form tag IDs
summary: pyasn1 BER/CER/DER decoder denial of service via unbounded long-form tag IDs
severity: high
cvss: 7.5
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H'
vendor: pyasn1
product: pyasn1
ecosystem: pip
affected:
  - pyasn1 < 0.6.4
patched:
  - pyasn1 0.6.4
published: '2026-07-21'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:50:52.562707948Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-m4p7-r5rc-7g4j'
references:
  - url: 'https://github.com/pyasn1/pyasn1/security/advisories/GHSA-m4p7-r5rc-7g4j'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-59884'
  - url: >-
      https://github.com/pyasn1/pyasn1/commit/628e36ecbb5277a3f01572ce418ef54271b165a5
  - url: 'https://github.com/pyasn1/pyasn1'
  - url: 'https://github.com/pyasn1/pyasn1/releases/tag/v0.6.4'
  - url: >-
      https://github.com/pypa/advisory-database/tree/main/vulns/pyasn1/PYSEC-2026-3455.yaml
  - url: 'https://github.com/advisories/GHSA-m4p7-r5rc-7g4j'
tags:
  - osv
  - pip
  - ghsa
epss: 0.00616
epssPercentile: 0.47299
cwe:
  - CWE-400
ingestedAt: '2026-07-21T19:53:40.133Z'
---

## Overview

### Impact
The BER decoder (shared by the CER and DER codecs) parses long-form tags by accumulating continuation octets in a loop with no upper bound on the size of the tag ID. A crafted input can force the decoder to build an arbitrarily large integer, with CPU cost growing quadratically in input size — a ~1 MB input consumes over a minute of CPU. On Python 3.11+, the oversized tag ID can also trigger an unhandled `ValueError` (integer string conversion limit) while the decoder formats error messages, violating the documented `PyAsn1Error` contract and potentially bypassing caller error handling.

Any application decoding untrusted BER/CER/DER input is affected.

### Affected components
- `pyasn1.codec.ber.decoder` — `decode()` and `StreamingDecoder`
- `pyasn1.codec.cer.decoder` and `pyasn1.codec.der.decoder`, which inherit
  the same tag parsing
- `pyasn1.type.tag` — `Tag`/`TagSet` reprs could raise `ValueError` when
  rendering oversized tag IDs (reachable through decoder error paths)

The encoders and the `pyasn1.codec.native` codec are not affected.

### Patches
Fixed in 0.6.4. Long-form tag IDs are now limited to 20 octets (140-bit tag IDs, matching the existing OID arc limit); oversized tags are rejected with `PyAsn1Error`. Tag ID rendering in reprs and error messages was additionally hardened against the interpreter's integer-to-string conversion limit.

### Workarounds
Bound the size of untrusted input passed to `decode()` before calling it.

## Affected packages

- `pyasn1 < 0.6.4`

## Remediation

Upgrade to a patched release:

- `pyasn1 0.6.4`