Deno: Permission Bypass via Unicode Normalization Mismatch on macOS (APFS)
- When
- Where
- Global (internet)
- Category
- cyber_advisory · rust
## Summary Deno's permission system enforces filesystem and execution restrictions by comparing the requested path against the path supplied to `--deny-read`, `--deny-write`, `--deny-run`, or `--deny-ffi`. On macOS, that comparison was done at the raw-byte level while the APFS filesystem treats different Unicode spellings of the same name as the same file. That means a program could reach a denied path by spelling it differently than the deny rule. For example, with `--deny-read=/secrets/passwörter.txt`, a script could still read the file by opening `/secrets/passwo\u0308rter.txt` (NFD instead of NFC), or `/SECRETS/PASSWÖRTER.txt` (different case, since default APFS volumes are case-insensitive). Other forms include ligature characters (`fi` vs `fi`, `ff` vs `ff`, …) and German `ß` vs `ss`. The denied path and the requested path differed at the byte level, so Deno's permission check passed; the kernel then resolved them to the same inode and served the file anyway. The same flaw affected `--deny-write`, `--deny-run`, and `--deny-ffi`, which share the same path-comparison code. ## Am I affected? You are potentially affected if **all** of the following are true: 1. You run Deno on **macOS** (the issue is specific to APFS path-equivalence rules; Linux and Windows are not affected by this variant). 2. You rely on `--deny-read`, `--deny-write`, `--deny-run`, or `--deny-ffi` as a security boundary against less-trusted code — a dependency, plugin, or attacker-controlled input. 3. The protected path contains characters that have alternate Unicode spellings — most commonly accented characters (`é`, `ñ`, `ö`, …), German `ß`, or Latin ligatures — or you rely on case-sensitivity on a default APFS volume. If you only run fully trusted code, or your deny rules cover paths that are pure ASCII with no case-sensitive aliases, you are not exposed to this specific bypass. ## Impact A program running with broad `--allow-read` (or `--allow-write` / `--allow-run` / `--allow-ffi`) but with `--deny-*` carve-outs for specific paths could read, write, execute, or load via FFI those denied paths by referring to them through a Unicode- or case-equivalent spelling. The sandbox model on macOS was weaker than the flags suggested. ## Workaround If you cannot upgrade immediately: - Prefer `--allow-*` allowlists over `--deny-*` denylists. Allow rules match against the original specifier, so an attacker-supplied alternate spelling will not match a path you didn't explicitly grant. - Do not rely on case-sensitivity of paths on macOS for security boundaries; default APFS volumes are case-insensitive. ## Fix On macOS, Deno now normalizes both the deny-rule path and the requested path to NFC and applies Unicode case folding before comparing them. This matches how APFS resolves paths at the inode level, so byte-different but equivalent spellings are now rejected by the same deny rule.
Sources
- GitHub Advisory Database ↗ · first seen 2026-06-16 19:11 UTC
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