wisp has Allocation of Resources Without Limits or Throttling

Summary

A multipart form parsing bug allows any unauthenticated user to bypass configured request size limits and trigger a denial of service by exhausting server memory or disk.

Details

The issue is in the multipart parsing logic, specifically in multipart_body and multipart_headers.

When parsing multipart data, the implementation distinguishes between:
- chunks where a boundary is found
- chunks where more data is required

In the normal case (boundary found), the parser correctly accounts for consumed bytes by calling decrement_quota.

However, in the MoreRequiredForBody branch, the parser appends incoming data to the output but recurses without decrementing the quota. This means that any chunk that does not contain the multipart boundary is effectively “free” from a quota perspective. Only the final chunk, the one containing the boundary, is counted.

The same pattern exists in multipart_headers, where MoreRequiredForHeaders also recurses without decrementing the quota.

As a result, an attacker can send arbitrarily large multipart bodies split across many chunks that avoid the boundary. The parser will accumulate the data (in memory for form fields, on disk for file uploads) without enforcing max_body_size or max_files_size.

Impact

This is a denial of service vulnerability caused by uncontrolled resource consumption.

Any application using require_form or require_multipart_form on user-controlled input is affected. An unauthenticated attacker can send large multipart requests that bypass configured limits and cause:

• memory exhaustion (for form fields accumulated in memory)
• disk exhaustion (for file uploads written to temporary storage)

In both cases, the application may become unavailable or be terminated by the operating system.

Workaround

Deploy a reverse proxy (such as nginx or HAProxy) in front of the application and enforce request body size limits there. This ensures large multipart requests are rejected before they reach the vulnerable parser.

Resources

• Introducing commit: https://github.com/gleam-wisp/wisp/commit/d8e722e22ccb42bda9d0b6248658d37ab4e9b376
• Fix commit: https://github.com/gleam-wisp/wisp/commit/7a978748e12ab29db232c222254465890e1a4a90