GHSA-2q8v-3gqq-4f8p
Vulnerability from github
### Summary
concat built-in can write over the bounds of the memory buffer that was allocated for it and thus overwrite existing valid data. The root cause is that the build_IR for concat doesn't properly adhere to the API of copy functions (for >=0.3.2 the copy_bytes function).
A contract search was performed and no vulnerable contracts were found in production.
Tracked in issue https://github.com/vyperlang/vyper/issues/3737
Details
The build_IR allocates a new internal variable for the concatenation: https://github.com/vyperlang/vyper/blob/3b310d5292c4d1448e673d7b3adb223f9353260e/vyper/builtins/functions.py#L534-L550
Notice that the buffer is allocated for the maxlen + 1 word to actually hold the length of the array.
Later the copy_bytes function is used to copy the actual source arguments to the destination: https://github.com/vyperlang/vyper/blob/3b310d5292c4d1448e673d7b3adb223f9353260e/vyper/builtins/functions.py#L569-L572
The dst_data is defined via:
- data ptr - to skip the 1 word that holds the length
- offset - to skip the source arguments that were already written to the buffer
- the offset is increased via: ["set", ofst, ["add", ofst, arglen]], ie it is increased by the length of the source argument
Now, the copy_bytes function has multiple control flow paths, the following ones are of interest:
1) https://github.com/vyperlang/vyper/blob/3b310d5292c4d1448e673d7b3adb223f9353260e/vyper/codegen/core.py#L270-L273
2) https://github.com/vyperlang/vyper/blob/3b310d5292c4d1448e673d7b3adb223f9353260e/vyper/codegen/core.py#L301-L320
Note that the function itself contains the following note: https://github.com/vyperlang/vyper/blob/3b310d5292c4d1448e673d7b3adb223f9353260e/vyper/codegen/core.py#L245-L247
That is we can ask for a copy of 1B yet a whole word is copied.
Consider the first interesting path - if the dst_data's distance to the end of the concat data buffer is < 32B, the copy_op = STORE(dst, LOAD(src)) from copy_bytes will result in buffer overflow as it essentially will mstore to dst_data the mload of the source (mload will load whole word and the distance of the dst_data to the word boundary is <32B).
From the two mentioned paths in copy_bytes it can be seen that both sources from memory and storage can cause the corruption.
PoC
The main attack vector that was found was when the concat is inside an internal function. Suppose we have an external function that calls internal one. In such case the address space is divided such that the memory for the internal function is in lower portion of the adr space. As such the buffer overflow can overwrite valid data of the caller.
Here is a simple example: ```python
@version ^0.3.9
@internal def bar() -> uint256: sss: String[2] = concat("a", "b") return 1
@external def foo() -> int256: a: int256 = -1 b: uint256 = self.bar() return a ```
foo should clearly return -1, but it returns 452312848583266388373324160190187140051835877600158453279131187530910662655
-1 was used intentionally due to its bit structure but the value here is fairly irelevant. In this example during the second iteration of the for loop in the build_IR mload to dst+1 will be executed (because len('a') == 1), thus the function will write 1B over the bounds of the buffer. The string 'b' is stored such that its right-most byte is a zero byte. So a zero byte will be written over the bounds. So when -1 is considered it's left-most B will be overwritten to all 0. Therefore it can be seen: 452312848583266388373324160190187140051835877600158453279131187530910662655 == (2**248-1) will output True.
IR
If we look at the contract's IR (vyper --no optimize -f ir), we see: ```
Line 30
/* a: int256 = -1 */ [mstore, 320, -1 <-1>],
And for the second iteration of the loop in concat:
len,
[mload, arg],
[seq,
[with,
src,
[add, arg, 32],
[with,
dst,
[add, [add, 256 ``
So the address of theint` is 320.
The dst is defined as: [add, [add, 256 <concat destination>, 32], concat_ofst],.
In the second iteration the concat_ofst will be 1 because len('a)==1 so 256+32+1 = 289. Now this address will be mstored to - so the last mstored B will have the address 289+32=320 which clearly overlaps with the address of the int a.
PoC 2
Due to how immutables are handled, they can be corrupted too:
```python
@version ^0.3.9
i: immutable(int256)
@external def init(): i = -1 s: String[2] = concat("a", "b")
@external def foo() -> int256: return i ```
Output of calling foo() = 452312848583266388373324160190187140051835877600158453279131187530910662655.
Impact
The buffer overflow can result in the change of semantics of the contract. The overflow is length-dependent and thus it might go unnoticed during contract testing.
However, certainly not all usages of concat will result in overwritten valid data as we require it to be in an internal function and close to the return statement where other memory allocations don't occur.
Concluding remarks
The bug based on the fast path in copy_bytes was likely introduced in: 548d35d720fb6fd8efbdc0ce525bed259a73f0b9. git bisect was used between v0.3.1 and v0.3.2, forge test was run and the test asserted that the function indeed returns -1.
For the general case, 0.3.0 and 0.3.1 are also affected.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 0.3.10"
},
"package": {
"ecosystem": "PyPI",
"name": "vyper"
},
"ranges": [
{
"events": [
{
"introduced": "0.3.0"
},
{
"fixed": "0.4.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2024-22419"
],
"database_specific": {
"cwe_ids": [
"CWE-120",
"CWE-787"
],
"github_reviewed": true,
"github_reviewed_at": "2024-01-19T16:19:51Z",
"nvd_published_at": "2024-01-18T19:15:10Z",
"severity": "HIGH"
},
"details": " ### Summary\n`concat` built-in can write over the bounds of the memory buffer that was allocated for it and thus overwrite existing valid data. The root cause is that the `build_IR` for `concat` doesn\u0027t properly adhere to the API of copy functions (for `\u003e=0.3.2` the `copy_bytes` function).\n\nA contract search was performed and no vulnerable contracts were found in production.\n\nTracked in issue https://github.com/vyperlang/vyper/issues/3737\n\n### Details\nThe `build_IR` allocates a new internal variable for the concatenation: https://github.com/vyperlang/vyper/blob/3b310d5292c4d1448e673d7b3adb223f9353260e/vyper/builtins/functions.py#L534-L550\n\nNotice that the buffer is allocated for the `maxlen` + 1 word to actually hold the length of the array.\n\nLater the `copy_bytes` function is used to copy the actual source arguments to the destination: https://github.com/vyperlang/vyper/blob/3b310d5292c4d1448e673d7b3adb223f9353260e/vyper/builtins/functions.py#L569-L572\n\nThe `dst_data` is defined via:\n- `data ptr` - to skip the 1 word that holds the length\n- `offset` - to skip the source arguments that were already written to the buffer\n - the `offset` is increased via: `[\"set\", ofst, [\"add\", ofst, arglen]]`, ie it is increased by the length of the source argument\n\nNow, the `copy_bytes` function has multiple control flow paths, the following ones are of interest:\n1) https://github.com/vyperlang/vyper/blob/3b310d5292c4d1448e673d7b3adb223f9353260e/vyper/codegen/core.py#L270-L273\n2) https://github.com/vyperlang/vyper/blob/3b310d5292c4d1448e673d7b3adb223f9353260e/vyper/codegen/core.py#L301-L320\n\nNote that the function itself contains the following note: \nhttps://github.com/vyperlang/vyper/blob/3b310d5292c4d1448e673d7b3adb223f9353260e/vyper/codegen/core.py#L245-L247\n\nThat is we can ask for a copy of `1B` yet a whole word is copied.\n\nConsider the first interesting path - if the `dst_data`\u0027s distance to the end of the concat data buffer is `\u003c 32B`, the `copy_op = STORE(dst, LOAD(src))` from `copy_bytes` will result in buffer overflow as it essentially will `mstore` to `dst_data` the `mload` of the source (mload will load whole word and the distance of the `dst_data` to the word boundary is `\u003c32B`).\n\nFrom the two mentioned paths in `copy_bytes` it can be seen that both sources from memory and storage can cause the corruption.\n\n### PoC\nThe main attack vector that was found was when the `concat` is inside an `internal` function. Suppose we have an `external` function that calls `internal` one. In such case the address space is divided such that the memory for the internal function is in _lower_ portion of the adr space. As such the buffer overflow can overwrite _valid_ data of the caller.\n\nHere is a simple example:\n```python\n#@version ^0.3.9\n\n@internal\ndef bar() -\u003e uint256:\n sss: String[2] = concat(\"a\", \"b\") \n return 1\n\n\n@external\ndef foo() -\u003e int256:\n a: int256 = -1\n b: uint256 = self.bar()\n return a \n```\n\n`foo` should clearly return `-1`, but it returns `452312848583266388373324160190187140051835877600158453279131187530910662655`\n\n`-1` was used intentionally due to its bit structure but the value here is fairly irelevant. In this example during the second iteration of the for loop in the `build_IR` `mload` to `dst+1` will be executed (because len(\u0027a\u0027) == 1), thus the function will write `1B` over the bounds of the buffer. The string \u0027b\u0027 is stored such that its right-most byte is a zero byte. So a zero byte will be written over the bounds. So when `-1` is considered it\u0027s left-most B will be overwritten to all 0. Therefore it can be seen: `452312848583266388373324160190187140051835877600158453279131187530910662655 == (2**248-1)` will output `True`.\n\n#### IR\nIf we look at the contract\u0027s IR (vyper --no optimize -f ir), we see:\n```\n# Line 30\n /* a: int256 = -1 */ [mstore, 320, -1 \u003c-1\u003e],\n```\nAnd for the second iteration of the loop in concat:\n```\n len,\n [mload, arg],\n [seq,\n [with,\n src,\n [add, arg, 32],\n [with,\n dst,\n [add, [add, 256 \u003cconcat destination\u003e, 32], concat_ofst],\n [mstore, dst, [mload, src]]]],\n [set, concat_ofst, [add, concat_ofst, len]]]]],\n [mstore, 256 \u003cconcat destination\u003e, concat_ofst],\n 256 \u003cconcat destination\u003e]],\n```\nSo the address of the `int` is 320. \n\nThe `dst` is defined as: `[add, [add, 256 \u003cconcat destination\u003e, 32], concat_ofst],`.\nIn the second iteration the `concat_ofst` will be 1 because `len(\u0027a)==1` so `256+32+1 = 289`. Now this address will be `mstored` to - so the last mstored B will have the address `289+32=320` which clearly overlaps with the address of the `int a`.\n\n#### PoC 2\nDue to how `immutables` are handled, they can be corrupted too:\n```python\n#@version ^0.3.9\n\ni: immutable(int256)\n\n@external\ndef __init__():\n i = -1\n s: String[2] = concat(\"a\", \"b\")\n\n@external\ndef foo() -\u003e int256:\n return i\n```\n\nOutput of calling `foo()` = `452312848583266388373324160190187140051835877600158453279131187530910662655`.\n\n### Impact\nThe buffer overflow can result in the change of semantics of the contract. The overflow is length-dependent and thus it might go unnoticed during contract testing.\n\nHowever, certainly not all usages of `concat` will result in overwritten valid data as we require it to be in an `internal` function and close to the `return` statement where other memory allocations don\u0027t occur. \n\n### Concluding remarks\nThe bug based on the fast path in `copy_bytes` was likely introduced in: `548d35d720fb6fd8efbdc0ce525bed259a73f0b9`. `git bisect` was used between v0.3.1 and v0.3.2, `forge test` was run and the test asserted that the function indeed returns -1.\n\nFor the general case, `0.3.0` and `0.3.1` are also affected.",
"id": "GHSA-2q8v-3gqq-4f8p",
"modified": "2024-10-10T14:46:31Z",
"published": "2024-01-19T16:19:51Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/vyperlang/vyper/security/advisories/GHSA-2q8v-3gqq-4f8p"
},
{
"type": "ADVISORY",
"url": "https://nvd.nist.gov/vuln/detail/CVE-2024-22419"
},
{
"type": "WEB",
"url": "https://github.com/vyperlang/vyper/issues/3737"
},
{
"type": "WEB",
"url": "https://github.com/vyperlang/vyper/commit/55e18f6d128b2da8986adbbcccf1cd59a4b9ad6f"
},
{
"type": "WEB",
"url": "https://github.com/pypa/advisory-database/tree/main/vulns/vyper/PYSEC-2024-103.yaml"
},
{
"type": "PACKAGE",
"url": "https://github.com/vyperlang/vyper"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L",
"type": "CVSS_V3"
}
],
"summary": "concat built-in can corrupt memory in vyper"
}
- Seen: The vulnerability was mentioned, discussed, or seen somewhere by the user.
- Confirmed: The vulnerability is confirmed from an analyst perspective.
- Exploited: This vulnerability was exploited and seen by the user reporting the sighting.
- Patched: This vulnerability was successfully patched by the user reporting the sighting.
- Not exploited: This vulnerability was not exploited or seen by the user reporting the sighting.
- Not confirmed: The user expresses doubt about the veracity of the vulnerability.
- Not patched: This vulnerability was not successfully patched by the user reporting the sighting.