CVE-2026-59884: Pyasn1 Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-m4p7-r5rc-7g4j
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CVSS 7.5
CWE-400
Published
Advisory Summary

CVE-2026-59884 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting Pyasn1 libraries in the PyPI ecosystem. The issue involves pyasn1 ber/cer/der decoder denial of service via unbounded long-form tag ids. Remediation is available by upgrading affected packages to version 0.6.4.

CVE-2026-59884 at a Glance

CVE Identifier CVE-2026-59884
GHSA Identifier GHSA-m4p7-r5rc-7g4j
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-400: Uncontrolled Resource Consumption
Affected Product Pyasn1
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2026-59884

To remediate this vulnerability, upgrade all affected Pyasn1 dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

pyasn1 fixed: 0.6.4 
pip install pyasn1>=0.6.4
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show pyasn1

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-59884, inspect your installed packages using pip:

pip show pyasn1

Analyze the terminal output to locate any version of Pyasn1 in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 0.6.4), your application is affected.

Note: The pip show pyasn1 command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. In Pyasn1, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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.

CVE-2026-59884 Affected Pyasn1 Versions

1 component(s)
Fixed Versions:
0.6.4
Product / Package Ecosystem Affected Versions Fixed Version
pyasn1
pypi/pyasn1
PyPI
< 0.6.4
0.6.4

References

GitHub Advisory / Wiki ADVISORY
https://github.com/pyasn1/pyasn1/security/advisories/GHSA-m4p7-r5rc-7g4j
NVD — CVE-2026-59884 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-59884
Security Fix Commit FIX COMMIT
https://github.com/pyasn1/pyasn1/commit/628e36ecbb5277a3f01572ce418ef54271b165a5
Pyasn1 Source Repository REPOSITORY
https://github.com/pyasn1/pyasn1
Pyasn1 Release RELEASE
https://github.com/pyasn1/pyasn1/releases/tag/v0.6.4
Pyasn1 Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/pyasn1/PYSEC-2026-3455.yaml

Frequently Asked Questions

What is CVE-2026-59884?
CVE-2026-59884 is a security vulnerability in Pyasn1 involving pyasn1 BER/CER/DER decoder denial of service via unbounded long-form tag IDs under CWE-400: Uncontrolled Resource Consumption.
Which Pyasn1 versions are affected?
Affected versions include releases prior to 0.6.4 across affected packages: pyasn1.
What version fixes CVE-2026-59884?
Upgrading to 0.6.4 (or the latest patched release for your respective branch) resolves the vulnerability.
Is pyasn1 affected?
Yes, pyasn1 is affected in releases prior to 0.6.4. Upgrade your dependency to 0.6.4 or later.
What is the CVSS score of CVE-2026-59884?
CVE-2026-59884 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2026-59884?
CVE-2026-59884 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).
How can I check PyPI dependencies for CVE-2026-59884?
Run the dependency tree command: 'pip show pyasn1'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-m4p7-r5rc-7g4j",
  "modified" : "2026-07-31T21:55:42Z",
  "published" : "2026-07-21T19:10:11Z",
  "aliases" : [ "CVE-2026-59884" ],
  "summary" : "pyasn1 BER/CER/DER decoder denial of service via unbounded long-form tag IDs",
  "details" : "### Impact\nThe 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.\n\nAny application decoding untrusted BER/CER/DER input is affected.\n\n### Affected components\n- `pyasn1.codec.ber.decoder` — `decode()` and `StreamingDecoder`\n- `pyasn1.codec.cer.decoder` and `pyasn1.codec.der.decoder`, which inherit\n  the same tag parsing\n- `pyasn1.type.tag` — `Tag`/`TagSet` reprs could raise `ValueError` when\n  rendering oversized tag IDs (reachable through decoder error paths)\n\nThe encoders and the `pyasn1.codec.native` codec are not affected.\n\n### Patches\nFixed 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.\n\n### Workarounds\nBound the size of untrusted input passed to `decode()` before calling it.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "pyasn1"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.6.4"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/pyasn1/pyasn1/security/advisories/GHSA-m4p7-r5rc-7g4j"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-59884"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pyasn1/pyasn1/commit/628e36ecbb5277a3f01572ce418ef54271b165a5"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/pyasn1/pyasn1"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pyasn1/pyasn1/releases/tag/v0.6.4"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/pyasn1/PYSEC-2026-3455.yaml"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-21T19:10:11Z",
    "nvd_published_at" : "2026-07-14T17:17:14Z"
  }
}