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CVE-2025-62727 — Starlette vulnerable to O(n^2) DoS via Range header merging in starlette.responses.FileResponse

Affected: Kludex starlette

Starlette is a lightweight ASGI framework/toolkit. Starting in version 0.39.0 and prior to version 0.49.1 , an unauthenticated attacker can send a crafted HTTP Range header that triggers quadratic-time processing in Starlette's FileResponse Range parsing/merging logic. This enables CPU exhaustion per request, causing denial‑of‑service for endpoints serving files (e.g., StaticFiles or any use of FileResponse). This vulnerability is fixed in 0.49.1.

GHSA-7f5h-v6xp-fcq8 — Starlette vulnerable to O(n^2) DoS via Range header merging in ``starlette.responses.FileResponse``

Affected: PyPI: starlette

### Summary An unauthenticated attacker can send a crafted HTTP Range header that triggers quadratic-time processing in Starlette's `FileResponse` Range parsing/merging logic. This enables CPU exhaustion per request, causing denial‑of‑service for endpoints serving files (e.g., `StaticFiles` or any use of `FileResponse`).

### Details Starlette parses multi-range requests in ``FileResponse._parse_range_header()``, then merges ranges using an O(n^2) algorithm.

```python # starlette/responses.py _RANGE_PATTERN = re.compile(r"(\d*)-(\d*)") # vulnerable to O(n^2) complexity ReDoS

class FileResponse(Response): @staticmethod def _parse_range_header(http_range: str, file_size: int) -> list[tuple[int, int]]: ranges: list[tuple[int, int]] = [] try: units, range_ = http_range.split("=", 1) except ValueError: raise MalformedRangeHeader()

# [...]

ranges = [ ( int(_[0]) if _[0] else file_size - int(_[1]), int(_[1]) + 1 if _[0] and _[1] and int(_[1]) < file_size else file_size, ) for _ in _RANGE_PATTERN.findall(range_) # vulnerable if _ != ("", "") ]

```

The parsing loop of ``FileResponse._parse_range_header()`` uses the regular expression which vulnerable to denial of service for its O(n^2) complexity. A crafted `Range` header can maximize its complexity.

The merge loop processes each input range by scanning the entire resul…

PYSEC-2026-1942 — Starlette vulnerable to O(n^2) DoS via Range header merging in ``starlette.responses.FileResponse``

Affected: PyPI: starlette

### Details Starlette parses multi-range requests in ``FileResponse._parse_range_header()``, then merges ranges using an O(n^2) algorithm.

```python # starlette/responses.py _RANGE_PATTERN = re.compile(r"(\d*)-(\d*)") # vulnerable to O(n^2) complexity ReDoS

# [...]

```

The merge loop processes each input range by scanning the entire resul…