Salvo Affected by Denial of Service via Unbounded Memory Allocation in Form Data Parsing

Summary

Salvo's form data parsing implementations (form_data() method and Extractible macro) do not enforce payload size limits before reading request bodies into memory. This allows attackers to cause Out-of-Memory (OOM) conditions by sending extremely large payloads, leading to service crashes and denial of service.

Details

Vulnerability Description

Three attack vectors exist in Salvo's form handling:

1. URL-encoded form data (application/x-www-form-urlencoded)

   • Request::form_data() calls BodyExt::collect(body) which reads the entire body into memory without size checking
   • Affects handlers using req.form_data().await directly

2. Multipart form data (multipart/form-data)

   • Similar unbounded memory allocation during parsing
   • Affects handlers processing multipart uploads

3. Extractible macro

   • #[derive(Extractible)] with #[salvo(extract(default_source(from = "body")))] internally calls form_data()
   • Vulnerabilities propagate to all extractors using body sources

Root Cause

The FormData::read() implementation prioritizes convenience over safety by reading entire request bodies before validation. Even when Request::payload_with_max_size() is available, it's not automatically applied in the form parsing path.

PoC

1. run Extract data from request example in readme.md in docker file with limited memory say 100mb.
2. Send application/x-www-form-urlencoded OR multipart/form-data payload to the endpoint.
3. The server process OOM-crashes, instead of returning 413 error.

Impact

Immediate Effects

• Service Unavailability: Servers crash under memory pressure
• Resource Exhaustion: Single request can consume all available memory
• Cascading Failures: In containerized environments, OOM can affect other services

Attack Characteristics

• Low Cost: Attacker needs minimal bandwidth (header only, body can be streamed)
• No Authentication: Exploitable on public endpoints
• Difficult to Rate-Limit: Traditional rate limiting may not prevent single large request
• Amplification: Small network cost → large memory consumption

Real-World Scenarios

1. Public API endpoints accepting form data
2. User registration/profile update handlers
3. File upload endpoints using multipart forms
4. Any endpoint using #[derive(Extractible)] with body sources

Suggestion: Make Multipart File Upload Handling Explicit Opt-In

Problem Statement

Currently, Salvo's multipart form data parsing automatically handles file uploads without explicit developer intent. This creates several security and usability concerns:

1. Unintended File Storage: Developers may unknowingly accept file uploads when they only intended to handle text fields
2. Disk Space Exhaustion: Automatic file buffering to disk can fill storage without proper limits
3. Resource Cleanup: Temporary files may not be properly cleaned up if handlers don't expect them
4. Attack Surface: Endpoints inadvertently become file upload targets