Vulnerability-Lookup

CVE-2026-102265 (GCVE-0-2026-102265)

Vulnerability from cvelistv5 – Published: 2026-09-28 20:48 – Updated: 2026-09-28 20:48

Title

PyJWT: Uncaught RecursionError in jwt.decode() on deeply nested token header

Summary

PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, PyJWS._load in jwt/api_jws.py is affected because parser catches ValueError but not RecursionError. This occurs when a deeply nested token header reaches json.loads. As a result, RecursionError escapes the documented PyJWT error hierarchy. Consequently, an unauthenticated malformed token can cause a request-level failure and HTTP 500. This issue is fixed in version 2.14.0.

Severity

5.3 (Medium)


                        
                          CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CWE

CWE-674 - Uncontrolled Recursion

Assigner

GitHub_M mitre root Scorecard

References

3 references

URL	Tags
https://github.com/jpadilla/pyjwt/security/adviso…	x_refsource_CONFIRM
https://github.com/jpadilla/pyjwt/commit/06573692…	x_refsource_MISC
https://github.com/jpadilla/pyjwt/releases/tag/2.14.0	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version	CPE status
jpadilla	pyjwt	Affected: >= 2.13.0, < 2.14.0	guessed

Show details on NVD website

JSON

To clipboard

{
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          "vendor": "jpadilla",
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            "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
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          "descriptions": [
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              "cweId": "CWE-674",
              "description": "CWE-674: Uncontrolled Recursion",
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            }
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      "providerMetadata": {
        "dateUpdated": "2026-09-28T20:48:04.664Z",
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        "advisory": "GHSA-8wjv-2p76-3863",
        "discovery": "UNKNOWN"
      },
      "title": "PyJWT: Uncaught RecursionError in jwt.decode() on deeply nested token header"
    }
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  "cveMetadata": {
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    "datePublished": "2026-09-28T20:48:04.664Z",
    "dateReserved": "2026-09-28T20:11:16.658Z",
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          }
        ],
        "id": "CVE-2026-102265",
        "lastModified": "2026-09-30T19:38:27.293",
        "metrics": {
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                "confidentialityImpact": "NONE",
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                "privilegesRequired": "NONE",
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                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
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      "cve": "CVE-2026-102265",
      "id": "CVE-2026-102265",
      "initial_release_date": "2026-09-28T20:48:04.664000+00:00",
      "product_status:known_affected": "199",
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      "source": "Red Hat CSAF VEX",
      "status": "final",
      "title": "pyjwt: pyjwt: Denial of Service via deeply nested token headers",
      "url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-102265.json",
      "version": "3"
    }
  }
}

GHSA-8WJV-2P76-3863

Vulnerability from github – Published: 2026-09-29 23:43 – Updated: 2026-09-29 23:43

Summary

PyJWT: Uncaught RecursionError in jwt.decode() on deeply nested token header

Details

Package

pyjwt (PyPI)

Affected versions

tested & verified on: 2.13.0. Every version whose PyJWS._load() translates only ValueError is affected

Description

PyJWS._looad() (jwt/api_jws.py:337) splits the compact token, base64url decodes the header segment and hands it to json.loads() before _verify_signature() runs. The parse is wrapped in except ValueError as e: raise DecodeError(...). That is what makes the documented contract sufficient for untrusted input: except jwt.PyJWTError (or InvalidTokenError) around jwt.decode(token, key, algorithms=[...]).

CPython's JSON decoder raises RecursionError (a RuntimeError subclass, not a ValueError) once nesting crosses the native stack guard. That exception leaves _load(), then jwt.decode(), and matches no PyJWTError handlerr at all. No key and no valid signature are needed: the header is parsed first, so a single unauthenticated request with an unsigned token suffices.

Scope: the recursion threshold is ~65,000 nesting levels; below it the same input yields a clean DecodeError (a 346,800-byte flat header is rejected normally, so depth, not size, is the trigger). A token this large cannot ride in an Authorization header (rejected at HTTP field limits,431 under gunicorn), but body transport (RFC 7662 introspection style) is a standard pattern. Under Flask 3.1.3 / gunicorn 26.2.0 each crafted request returns HTTP 500 instead of 401 and writes a full traceback to the error log; the sync worker survives.

Proof of concept

Against any endpoint that validates a token from its JSON body with the documented pattern:

import base64, json, http.client

def b64url(b): return base64.urlsafe_b64encode(b).rstrip(b"=")

# unsigned token: the header segment is 200,000 nested '[' (266,681 bytes total)
token = (b64url(b"[" * 200_000) + b"." + b64url(b'{"a":1}') + b"." + b64url(b"x")).decode()

conn = http.client.HTTPConnection("127.0.0.1", 8125, timeout=30)
conn.request("POST", "/api/protected", body=json.dumps({"token": token}),
             headers={"Content-Type": "application/json"})
print(conn.getresponse().status)   # 500, expected 401

Determinism: 20/20 crafted requests returned 500 against the reference container (PyJWT 2.13.0, Flask 3.1.3, gunicorn 26.2.0, Python 3.14.7) in 0.12 s (about 6 ms per request in a tight loop, 14 ms for a cold request), each leaving a RecursionError: Stack overflow ... while decoding a JSON array traceback in the error log.

Impact

An unauthenticated attacker can turn every request to a JWT validating endpoint into a server error with a full traceback logged per request, a cheap and repeatable denial of service on the authentication path (no crash, no data exposure).

Fix

Catch RecursionError alongside ValueError in _load() and raise DecodeError, or parse the header with a nonrecursive, depthbounded parser.

Reproduction

A standalone package (docker-compose.yml, Dockerfile, requirements.txt, target app, driver) accompanies this report:

cd <package dir>
docker compose up -d --build
python3 poc_check_then_attack.py
docker compose down -v

poc_pyjwt_recursion.zip

Exit 0= reproduced (control checks green, exactly one crafted request returns 500, traceback verified in docker logs, service alive afterwards); 1 = not reproduced; 2 = checks failed, no attack sent. The package's run_log.txt documents a complete proof run.

Credits

@Nivid42

Maintainer update — 2026-09-09

We reproduced the reported behavior on PyJWT 2.13.0: a deeply nested JSON array in the untrusted compact-JWS header reaches json.loads() before signature verification and raises RecursionError, which is not a PyJWTError. The same input is accepted by the public jwt.decode() path and can escape an application's normal except PyJWTError handling.

The fix is committed as 06573692ebcdec8831c3927513b3e87c31fbbb62. PyJWS._load() now translates both ValueError and RecursionError from header JSON parsing into DecodeError. A regression test covers the deeply nested-header path before signature verification. The full suite passes with 373 tests and 4 intentional cryptography-environment skips.

No release containing the fix has been published yet, so the patched version remains unset pending release planning. The advisory remains medium severity and unpublished.

Maintainer update — 2026-09-11

The verified fix for this advisory is included in PyJWT 2.14.0, released on 2026-09-11 and available on PyPI. PyJWT 2.14.0 is the first release containing the fix. This advisory is now published with 2.14.0 recorded as the patched version.

Severity

5.3 (Medium)


                  
                    CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "pyJWT"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.13.0"
            },
            {
              "fixed": "2.14.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-102265"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-674"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-29T23:43:06Z",
    "nvd_published_at": "2026-09-28T21:17:14Z",
    "severity": "MODERATE"
  },
  "details": "## Package\n\npyjwt (PyPI)\n\n## Affected versions\n\ntested \u0026 verified on: 2.13.0. Every version whose `PyJWS._load()` translates only `ValueError` is affected\n\n## Description\n\n`PyJWS._looad()` (`jwt/api_jws.py:337`) splits the compact token, base64url decodes the header segment and hands it to `json.loads()` before `_verify_signature()` runs. The parse is wrapped in `except ValueError as e: raise DecodeError(...)`. That is what makes the documented contract sufficient for untrusted input: `except jwt.PyJWTError` (or `InvalidTokenError`) around `jwt.decode(token, key, algorithms=[...])`.\n\nCPython\u0027s JSON decoder raises `RecursionError` (a `RuntimeError` subclass, not a `ValueError`) once nesting crosses the native stack guard. That exception leaves `_load()`, then `jwt.decode()`, and matches no `PyJWTError` handlerr at all. No key and no valid signature are needed: the header is parsed first, so a single unauthenticated request with an unsigned token suffices.\n\nScope: the recursion threshold is ~65,000 nesting levels; below it the same input yields a clean `DecodeError` (a 346,800-byte flat header is rejected normally, so depth, not size, is the trigger). A token this large cannot ride in an `Authorization` header (rejected at HTTP field limits,431 under gunicorn), but body transport (RFC 7662 introspection style) is a standard pattern. Under Flask 3.1.3 / gunicorn 26.2.0 each crafted request returns HTTP 500 instead of 401 and writes a full traceback to the error log; the sync worker survives.\n\n## Proof of concept\n\nAgainst any endpoint that validates a token from its JSON body with the documented pattern:\n\n```python\nimport base64, json, http.client\n\ndef b64url(b): return base64.urlsafe_b64encode(b).rstrip(b\"=\")\n\n# unsigned token: the header segment is 200,000 nested \u0027[\u0027 (266,681 bytes total)\ntoken = (b64url(b\"[\" * 200_000) + b\".\" + b64url(b\u0027{\"a\":1}\u0027) + b\".\" + b64url(b\"x\")).decode()\n\nconn = http.client.HTTPConnection(\"127.0.0.1\", 8125, timeout=30)\nconn.request(\"POST\", \"/api/protected\", body=json.dumps({\"token\": token}),\n             headers={\"Content-Type\": \"application/json\"})\nprint(conn.getresponse().status)   # 500, expected 401\n```\n\nDeterminism: 20/20 crafted requests returned 500 against the reference container (PyJWT 2.13.0, Flask 3.1.3, gunicorn 26.2.0, Python 3.14.7) in 0.12 s (about 6 ms per request in a tight loop, 14 ms for a cold request), each leaving a `RecursionError: Stack overflow ... while decoding a JSON array` traceback in the error log.\n\n## Impact\n\nAn unauthenticated attacker can turn every request to a JWT validating endpoint into a server error with a full traceback logged per request, a cheap and repeatable denial of service on the authentication path (no crash, no data exposure).\n\n## Fix\n\nCatch `RecursionError` alongside `ValueError` in `_load()` and raise `DecodeError`, or parse the header with a nonrecursive, depthbounded parser.\n\n## Reproduction\n\nA standalone package (docker-compose.yml, Dockerfile, requirements.txt, target app, driver) accompanies this report:\n\n```\ncd \u003cpackage dir\u003e\ndocker compose up -d --build\npython3 poc_check_then_attack.py\ndocker compose down -v\n```\n[poc_pyjwt_recursion.zip](https://github.com/user-attachments/files/31614987/poc_pyjwt_recursion.zip)\n\nExit 0= reproduced (control checks green, exactly one crafted request returns 500, traceback verified in `docker logs`, service alive afterwards); 1 = not reproduced; 2 = checks failed, no attack sent. The package\u0027s `run_log.txt` documents a complete proof run.\n\n## Credits\n\n- @Nivid42\n\n## Maintainer update \u2014 2026-09-09\n\nWe reproduced the reported behavior on PyJWT 2.13.0: a deeply nested JSON array in the untrusted compact-JWS header reaches `json.loads()` before signature verification and raises `RecursionError`, which is not a `PyJWTError`. The same input is accepted by the public `jwt.decode()` path and can escape an application\u0027s normal `except PyJWTError` handling.\n\nThe fix is committed as `06573692ebcdec8831c3927513b3e87c31fbbb62`. `PyJWS._load()` now translates both `ValueError` and `RecursionError` from header JSON parsing into `DecodeError`. A regression test covers the deeply nested-header path before signature verification. The full suite passes with 373 tests and 4 intentional cryptography-environment skips.\n\nNo release containing the fix has been published yet, so the patched version remains unset pending release planning. The advisory remains medium severity and unpublished.\n\n## Maintainer update \u2014 2026-09-11\n\nThe verified fix for this advisory is included in PyJWT 2.14.0, released on 2026-09-11 and available on PyPI. PyJWT 2.14.0 is the first release containing the fix. This advisory is now published with 2.14.0 recorded as the patched version.",
  "id": "GHSA-8wjv-2p76-3863",
  "modified": "2026-09-29T23:43:06Z",
  "published": "2026-09-29T23:43:06Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/jpadilla/pyjwt/security/advisories/GHSA-8wjv-2p76-3863"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-102265"
    },
    {
      "type": "WEB",
      "url": "https://github.com/jpadilla/pyjwt/commit/06573692ebcdec8831c3927513b3e87c31fbbb62"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/jpadilla/pyjwt"
    },
    {
      "type": "WEB",
      "url": "https://github.com/jpadilla/pyjwt/releases/tag/2.14.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "PyJWT: Uncaught RecursionError in jwt.decode() on deeply nested token header"
}

UBUNTU-CVE-2026-102265 (CVE-2026-102265)

Vulnerability from osv_ubuntu – Published: 2026-09-29 00:00 – Updated: 2026-09-29 00:00 – Source website

Details

Severity

5.3 (Medium)


                    
                      CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

N/A (UNKNOWN)


                    
                      medium

References

URL

Type

	https://ubuntu.com/security/CVE-2026-102265	REPORT
	https://www.cve.org/CVERecord?id=CVE-2026-102265	REPORT
	https://github.com/jpadilla/pyjwt/security/adviso…	REPORT
	https://github.com/jpadilla/pyjwt/commit/06573692…	REPORT
	https://github.com/jpadilla/pyjwt/releases/tag/2.14.0	REPORT

JSON

To clipboard

{
  "affected": [
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python-jwt",
            "binary_version": "1.3.0-1ubuntu0.1+esm2"
          },
          {
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            "binary_version": "1.3.0-1ubuntu0.1+esm2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:Pro:16.04:LTS",
        "name": "pyjwt",
        "purl": "pkg:deb/ubuntu/pyjwt@1.3.0-1ubuntu0.1+esm2?arch=source\u0026distro=esm-infra-legacy/xenial"
      },
      "ranges": [
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          "events": [
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          ],
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      ]
    },
    {
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          }
        ]
      },
      "package": {
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        "name": "pyjwt",
        "purl": "pkg:deb/ubuntu/pyjwt@1.5.3+ds1-1ubuntu0.1+esm2?arch=source\u0026distro=esm-infra/bionic"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
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        "1.5.3+ds1-1",
        "1.5.3+ds1-1ubuntu0.1",
        "1.5.3+ds1-1ubuntu0.1+esm1",
        "1.5.3+ds1-1ubuntu0.1+esm2"
      ]
    },
    {
      "ecosystem_specific": {
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      },
      "package": {
        "ecosystem": "Ubuntu:Pro:20.04:LTS",
        "name": "pyjwt",
        "purl": "pkg:deb/ubuntu/pyjwt@1.7.1-2ubuntu2.1+esm2?arch=source\u0026distro=esm-infra/focal"
      },
      "ranges": [
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          "events": [
            {
              "introduced": "0"
            }
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          "type": "ECOSYSTEM"
        }
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        "1.7.1-1",
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        "1.7.1-2ubuntu2.1+esm2"
      ]
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            "binary_name": "python3-jwt",
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          }
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      "package": {
        "ecosystem": "Ubuntu:22.04:LTS",
        "name": "pyjwt",
        "purl": "pkg:deb/ubuntu/pyjwt@2.3.0-1ubuntu0.4?arch=source\u0026distro=jammy"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "1.7.1-2ubuntu2",
        "2.1.0-1",
        "2.3.0-1",
        "2.3.0-1ubuntu0.1",
        "2.3.0-1ubuntu0.2",
        "2.3.0-1ubuntu0.3",
        "2.3.0-1ubuntu0.4"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-jwt",
            "binary_version": "2.7.0-1ubuntu0.2"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:24.04:LTS",
        "name": "pyjwt",
        "purl": "pkg:deb/ubuntu/pyjwt@2.7.0-1ubuntu0.2?arch=source\u0026distro=noble"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "2.7.0-1",
        "2.7.0-1ubuntu0.1",
        "2.7.0-1ubuntu0.2"
      ]
    },
    {
      "ecosystem_specific": {
        "binaries": [
          {
            "binary_name": "python3-jwt",
            "binary_version": "2.10.1-4ubuntu1.1"
          }
        ]
      },
      "package": {
        "ecosystem": "Ubuntu:26.04:LTS",
        "name": "pyjwt",
        "purl": "pkg:deb/ubuntu/pyjwt@2.10.1-4ubuntu1.1?arch=source\u0026distro=resolute"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "2.10.1-2",
        "2.10.1-3",
        "2.10.1-3build1",
        "2.10.1-4",
        "2.10.1-4ubuntu1",
        "2.10.1-4ubuntu1.1"
      ]
    }
  ],
  "aliases": [],
  "details": "PyJWT is a Python implementation of JSON Web Token standards. From 2.13.0 until 2.14.0, PyJWS._load in jwt/api_jws.py is affected because parser catches ValueError but not RecursionError. This occurs when a deeply nested token header reaches json.loads. As a result, RecursionError escapes the documented PyJWT error hierarchy. Consequently, an unauthenticated malformed token can cause a request-level failure and HTTP 500. This issue is fixed in version 2.14.0.",
  "id": "UBUNTU-CVE-2026-102265",
  "modified": "2026-09-29T00:00:00Z",
  "published": "2026-09-29T00:00:00Z",
  "references": [
    {
      "type": "REPORT",
      "url": "https://ubuntu.com/security/CVE-2026-102265"
    },
    {
      "type": "REPORT",
      "url": "https://www.cve.org/CVERecord?id=CVE-2026-102265"
    },
    {
      "type": "REPORT",
      "url": "https://github.com/jpadilla/pyjwt/security/advisories/GHSA-8wjv-2p76-3863"
    },
    {
      "type": "REPORT",
      "url": "https://github.com/jpadilla/pyjwt/commit/06573692ebcdec8831c3927513b3e87c31fbbb62"
    },
    {
      "type": "REPORT",
      "url": "https://github.com/jpadilla/pyjwt/releases/tag/2.14.0"
    }
  ],
  "related": [],
  "schema_version": "1.7.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
      "type": "CVSS_V3"
    },
    {
      "score": "medium",
      "type": "Ubuntu"
    }
  ],
  "upstream": [
    "CVE-2026-102265"
  ]
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

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Confirmed: The vulnerability has been validated from an analyst's perspective.
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Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.
The approach is described in our paper Mapping CVEs to MITRE ATT&CK Techniques: A Curated Gold-Set Classifier and the Limits of LLM-Assisted Label Expansion.
Browse all ATT&CK techniques and the vulnerabilities related to each.

Related by attack behaviour

Vulnerabilities whose description is nearest to this one in the vector space of the CIRCL/vulnerability-attack-technique-biencoder model. This is a similarity search over the bi-encoder space (plain cosine), not a classification, and it has no measured accuracy.

Action not permitted

CVE-2026-102265 (GCVE-0-2026-102265)

GHSA-8WJV-2P76-3863

Package

Affected versions

Description

Proof of concept

Impact

Fix

Reproduction

Credits

Maintainer update — 2026-09-09

Maintainer update — 2026-09-11

UBUNTU-CVE-2026-102265 (CVE-2026-102265)

Tags

Sightings

Nomenclature

Related by attack behaviour