Rack's multipart byte range processing allows denial of service via excessive overlapping ranges

Summary

Rack::Utils.get_byte_ranges parses the HTTP Range header without limiting the number of individual byte ranges. Although the existing fix for CVE-2024-26141 rejects ranges whose total byte coverage exceeds the file size, it does not restrict the count of ranges. An attacker can supply many small overlapping ranges such as 0-0,0-0,0-0,... to trigger disproportionate CPU, memory, I/O, and bandwidth consumption per request.

This results in a denial of service condition in Rack file-serving paths that process multipart byte range responses.

Details

Rack::Utils.get_byte_ranges accepts a comma-separated list of byte ranges and validates them based on their aggregate size, but does not impose a limit on how many individual ranges may be supplied.

As a result, a request such as:

Range: bytes=0-0,0-0,0-0,0-0,...

can contain thousands of overlapping one-byte ranges while still satisfying the total-size check added for CVE-2024-26141.

When such a header is processed by Rack’s file-serving code, each range causes additional work, including multipart response generation, per-range iteration, file seek and read operations, and temporary string allocation for response size calculation and output. This allows a relatively small request header to trigger disproportionately expensive processing and a much larger multipart response.

The issue is distinct from CVE-2024-26141. That fix prevents range sets whose total byte coverage exceeds the file size, but does not prevent a large number of overlapping ranges whose summed size remains within that limit.

Impact

Applications that expose file-serving paths with byte range support may be vulnerable to denial of service.

An unauthenticated attacker can send crafted Range headers containing many small overlapping ranges to consume excessive CPU time, memory, file I/O, and bandwidth. Repeated requests may reduce application availability and increase pressure on workers and garbage collection.

Mitigation

• Update to a patched version of Rack that limits the number of accepted byte ranges.
• Reject or normalize multipart byte range requests containing excessive range counts.
• Consider disabling multipart range support where it is not required.
• Apply request filtering or header restrictions at the reverse proxy or application boundary to limit abusive Range headers.