GHSA-H39G-6X3C-7FQ9
Vulnerability from github – Published: 2026-04-18 00:55 – Updated: 2026-04-18 00:55Summary
SubFileSystem fails to confine operations to its declared sub path when the input path is /../ (or equivalents /../, /..\\). This path passes all validation but resolves to the root of the parent filesystem, allowing directory level operations outside the intended boundary.
Affected Component
Zio.UPath.ValidateAndNormalize
Zio.FileSystems.SubFileSystem
UPath.ValidateAndNormalize has a trailing slash optimisation.
if (!processParts && i + 1 == path.Length)
return path.Substring(0, path.Length - 1);
When the input ends with / or \, and processParts is still false, the function strips the trailing separator and returns immediately before the .. resolution logic runs. The input /../ triggers this path: the trailing / is the last character, processParts has not been set (because .. as the first relative segment after root is specifically exempted), so the function returns /.. with the .. segment unresolved.
The resulting UPath with FullName = "/.." is absolute, contains no control characters, and no colon so it passes FileSystem.ValidatePath without rejection.
When this path reaches SubFileSystem.ConvertPathToDelegate:
protected override UPath ConvertPathToDelegate(UPath path)
{
var safePath = path.ToRelative(); // "/..".ToRelative() = ".."
return SubPath / safePath; // "/jail" / ".." = "/" (resolved by Combine)
}
The delegate filesystem receives / (the root) instead of a path under /jail.
Proof of Concept
using Zio;
using Zio.FileSystems;
var root = new MemoryFileSystem();
root.CreateDirectory("/sandbox");
var sub = new SubFileSystem(root, "/sandbox");
Console.WriteLine(sub.DirectoryExists("/../")); // True (sees parent root)
Console.WriteLine(sub.ConvertPathToInternal("/../")); // "/" (parent root path)
Impact
The escape is limited to directory level operations because appending a filename after .. (e.g., /../file.txt) causes normal .. resolution to trigger, which correctly rejects the path as going above root. Only the bare terminal /../ (which strips to /..) survives. This means that exploitability is limited, and this vulnerability does not escalate to file read/write.
{
"affected": [
{
"database_specific": {
"last_known_affected_version_range": "\u003c= 0.22.1"
},
"package": {
"ecosystem": "NuGet",
"name": "Zio"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "0.22.2"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [],
"database_specific": {
"cwe_ids": [
"CWE-179",
"CWE-22"
],
"github_reviewed": true,
"github_reviewed_at": "2026-04-18T00:55:19Z",
"nvd_published_at": null,
"severity": "LOW"
},
"details": "# Summary\n\n`SubFileSystem` fails to confine operations to its declared sub path when the input path is `/../` (or equivalents `/../`, `/..\\\\`). This path passes all validation but resolves to the root of the parent filesystem, allowing directory level operations outside the intended boundary.\n\n# Affected Component\n\n`Zio.UPath.ValidateAndNormalize`\n`Zio.FileSystems.SubFileSystem`\n\n`UPath.ValidateAndNormalize` has a trailing slash optimisation.\n\n```csharp\nif (!processParts \u0026\u0026 i + 1 == path.Length)\n return path.Substring(0, path.Length - 1);\n```\n\nWhen the input ends with `/` or `\\`, and `processParts` is still false, the function strips the trailing separator and returns immediately before the `..` resolution logic runs. The input `/../` triggers this path: the trailing `/` is the last character, `processParts` has not been set (because `..` as the first relative segment after root is specifically exempted), so the function returns `/..` with the `..` segment unresolved.\n\nThe resulting `UPath` with `FullName = \"/..\"` is absolute, contains no control characters, and no colon so it passes `FileSystem.ValidatePath` without rejection.\n\nWhen this path reaches `SubFileSystem.ConvertPathToDelegate`:\n\n```csharp\nprotected override UPath ConvertPathToDelegate(UPath path)\n{\n var safePath = path.ToRelative(); // \"/..\".ToRelative() = \"..\"\n return SubPath / safePath; // \"/jail\" / \"..\" = \"/\" (resolved by Combine)\n}\n```\n\nThe delegate filesystem receives `/` (the root) instead of a path under `/jail`.\n\n# Proof of Concept\n\n```csharp\nusing Zio;\nusing Zio.FileSystems;\n\nvar root = new MemoryFileSystem();\nroot.CreateDirectory(\"/sandbox\");\nvar sub = new SubFileSystem(root, \"/sandbox\");\n\nConsole.WriteLine(sub.DirectoryExists(\"/../\")); // True (sees parent root)\nConsole.WriteLine(sub.ConvertPathToInternal(\"/../\")); // \"/\" (parent root path)\n```\n\n# Impact\n\nThe escape is limited to directory level operations because appending a filename after `..` (e.g., `/../file.txt`) causes normal `..` resolution to trigger, which correctly rejects the path as going above root. Only the bare terminal `/../` (which strips to `/..`) survives. This means that exploitability is limited, and this vulnerability does not escalate to file read/write.",
"id": "GHSA-h39g-6x3c-7fq9",
"modified": "2026-04-18T00:55:19Z",
"published": "2026-04-18T00:55:19Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/xoofx/zio/security/advisories/GHSA-h39g-6x3c-7fq9"
},
{
"type": "WEB",
"url": "https://github.com/xoofx/zio/commit/c8c2f5328e50c1e7ab8c5c405fe70e0bd35f4782"
},
{
"type": "PACKAGE",
"url": "https://github.com/xoofx/zio"
},
{
"type": "WEB",
"url": "https://github.com/xoofx/zio/releases/tag/0.22.2"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:N/A:N",
"type": "CVSS_V3"
}
],
"summary": "Zio has SubFileSystem Path Confinement Bypass via Unresolved `..` Segment"
}
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