{"id":"GHSA-h39g-6x3c-7fq9","summary":"Zio has SubFileSystem Path Confinement Bypass via Unresolved `..` Segment","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 && 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.","modified":"2026-04-18T01:06:07.990905Z","published":"2026-04-18T00:55:19Z","database_specific":{"cwe_ids":["CWE-179","CWE-22"],"severity":"LOW","github_reviewed":true,"github_reviewed_at":"2026-04-18T00:55:19Z","nvd_published_at":null},"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"}],"affected":[{"package":{"name":"Zio","ecosystem":"NuGet","purl":"pkg:nuget/Zio"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"0.22.2"}]}],"versions":["0.1.0","0.10.0","0.11.0","0.12.0","0.13.0","0.14.0","0.15.0","0.16.0","0.16.1","0.16.2","0.17.0","0.17.1","0.18.0","0.18.1","0.19.0","0.19.1","0.19.2","0.2.0","0.20.0","0.21.0","0.21.1","0.21.2","0.21.3","0.22.0","0.22.1","0.3.0","0.3.1","0.3.2","0.3.3","0.3.4","0.3.5","0.3.6","0.4.0","0.5.0","0.6.0","0.7.0","0.7.1","0.7.2","0.7.3","0.7.4","0.7.5","0.7.6","0.8.0","0.9.0","0.9.1"],"database_specific":{"last_known_affected_version_range":"\u003c= 0.22.1","source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/04/GHSA-h39g-6x3c-7fq9/GHSA-h39g-6x3c-7fq9.json"}}],"schema_version":"1.9.0","severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:N/A:N"}]}