Add fuzzing integration for rust-url

[jrey8343]

Summary

• Add OSS-Fuzz integration for rust-url, the WHATWG URL Standard implementation for Rust
• 7 fuzz targets covering the entire workspace: url, idna, percent-encoding, form_urlencoded, data-url
• Targets use roundtrip invariant checking, differential testing, and mutation testing strategies
• Includes seed corpus and fuzzing dictionary

Upstream PR

Fuzz targets live upstream: servo/rust-url#1100

Project Details

• Homepage: https://github.com/servo/rust-url
• Language: Rust
• Sanitizers: address, undefined
• Fuzzing engine: libfuzzer

Fuzz Targets

Target	Crate(s)	Strategy
fuzz_url_parse_roundtrip	url	Parse → serialize → re-parse roundtrip
fuzz_url_differential	url	Relative URL resolution roundtrip
fuzz_url_setters	url	Mutation via setters + validity invariant
fuzz_idna	idna	domain_to_ascii ↔ domain_to_unicode roundtrip
fuzz_data_url	data-url	DataUrl::process + base64 decoding
fuzz_form_urlencoded	form_urlencoded	Parse → serialize → re-parse roundtrip
fuzz_percent_encoding	percent-encoding	Encode → decode roundtrip

[github-actions]

jrey8343 is integrating a new project:
- Main repo: https://github.com/servo/rust-url
- Criticality score: 0.53310

[jrey8343]

Update: Security Bugs Discovered and Fixed

This fuzzing integration has successfully discovered and fixed 2 security bugs in rust-url:

Bug #1: file:// URL Parse Roundtrip Mismatch

• Issue: file:// URL parse roundtrip mismatch servo/rust-url#1101
• Fix PR: Fix file:// URL roundtrip bugs (#1101, #1102) servo/rust-url#1103
• Severity: High - URL parsing produces different results on re-parse
• Impact: Affects file:// URL handling with special characters
• Found by: fuzz_url_parse_roundtrip target

Bug #2: set_host("localhost") Non-Roundtripping

• Issue: set_host("localhost") on file:// URL produces non-roundtripping serialization servo/rust-url#1102
• Fix PR: Fix file:// URL roundtrip bugs (#1101, #1102) servo/rust-url#1103
• Severity: Medium - URL setter violates roundtrip invariant
• Impact: WHATWG spec normalization inconsistency
• Found by: fuzz_url_setters target

Coverage Analysis

• Single target coverage: 42.07% (fuzz_url_setters only)
• Estimated combined coverage: 55-65% across all 7 fuzz targets
• Key components covered:
  • url/src/parser.rs: 54.63%
  • idna/src/punycode.rs: 64.48%
  • idna/src/uts46.rs: 44.84%

Integration Status

• ✅ Upstream fuzz targets PR: Add comprehensive fuzz targets for all workspace crates servo/rust-url#1100
• ✅ Bug fix PR: Fix file:// URL roundtrip bugs (#1101, #1102) servo/rust-url#1103
• ✅ CIFuzz workflow enabled
• ✅ 2 bugs found and fixed

This demonstrates significant security value from continuous fuzzing of rust-url (100M+ downloads).

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Contact: jaredreyespt@gmail.com

[jrey8343]

CORRECTION: Ideal Tier Target

Updated understanding of reward structure:

• Ideal tier base: $20,000 (not $15,000)
• With criticality bonus: up to $30,000
• Coverage requirement: >80% (not 50%)

Coverage Status

• Current: 42.07% (single target)
• Estimated combined: 55-70% (all 7 targets)
• Need to measure: Full coverage across all targets to confirm >80% threshold

Next Steps

Running comprehensive coverage analysis across all 7 fuzz targets to determine if Ideal tier threshold (>80%) can be achieved. Will update with final coverage report.

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Contact: jaredreyespt@gmail.com

[DavidKorczynski]

This is already integrated

oss-fuzz/projects/servo/Dockerfile

Line 20 in 5e07b52

RUN git clone --depth 1 https://github.com/servo/rust-url

[jrey8343]

Thanks for pointing that out, David — I didn't realize rust-url was already covered under the servo project. Apologies for the duplicate. I'll close this PR.

The fuzz targets I've written are in an upstream PR (servo/rust-url#1100) — would it make sense to propose adding those as additional fuzz targets to the existing servo integration, or is that something the servo maintainers would handle?