Remediate cargo-deny advisories in YAML and TLS root dependencies

Start with the serde_yaml dependency in the agentverify-contract crate and the workspace Cargo.toml, then run cargo deny check advisories to reproduce the failing advisories. Bumping webpki-roots and the other TLS and tokio crates should be done by updating the owning dependency chain and regenerating Cargo.lock, not by editing it by hand. Done means the deny check, workspace tests, clippy, and the parser parity tests all pass without new ignores in deny.toml.

Findings

The current cargo-deny advisory job fails on the existing dependency graph:

1. RUSTSEC-2024-0449 — unsafe-libyaml is unmaintained

   • Locked version: unsafe-libyaml 0.2.11
   • Dependency path: agentverify-contract -> serde_yaml 0.9.34+deprecated -> unsafe-libyaml
   • The advisory identifies the upstream repository as archived and recommends a maintained safe Rust YAML parser (for example Saphyr-based alternatives).

2. RUSTSEC-2026-0381 — webpki-roots bundled roots

   • Locked version: webpki-roots 1.0.9
   • Dependency path includes reqwest 0.12.28 -> hyper-rustls 0.27.9 -> webpki-roots.
   • The advisory states version 1.0.10+ removes the affected emSign root.

These findings are present in the current Cargo.lock and are unrelated to the documentation-only PR that exposed the failing gate.

Required remediation

• Replace deprecated serde_yaml with a maintained YAML parser/Serde integration. Prefer a safe implementation; preserve the existing contract parsing API where practical.
• Update webpki-roots to at least 1.0.10 through a compatible dependency resolution (do not hand-edit lockfile checksums).
• Regenerate and commit Cargo.lock using Cargo.
• Do not silence the advisories with deny.toml ignores unless a documented, time-bounded exception is explicitly justified and accepted.

Acceptance criteria

• Existing JSON and YAML contract parsing behavior remains compatible.
• Add/retain tests for valid YAML, malformed YAML, unknown fields, and equivalent JSON/YAML contract semantics.
• cargo test --workspace --all-features passes.
• cargo clippy --workspace --all-features -- -D warnings passes.
• cargo deny check advisories and the full deny workflow pass without new ignores.
• MSRV and supported WASM checks remain green.
• Include the dependency tree and advisory resolution in the PR description.

References

• https://rustsec.org/advisories/RUSTSEC-2024-0449
• https://rustsec.org/advisories/RUSTSEC-2026-0381
• https://github.com/dtolnay/unsafe-libyaml
• https://github.com/rustls/webpki-roots/pull/133

Candidate evaluation update — 2026-11-01

The contract parser only uses the top-level serde_yaml::from_str,
serde_yaml::to_string, and serde_yaml::Error surface; it does not
depend on serde_yaml::Value or lower-level parser internals. A practical
drop-in candidate is noyalib-serde-yaml (currently 0.0.58), declared under
the existing crate name:

serde_yaml = { package = "noyalib-serde-yaml", version = "=0.0.58" }

The published compatibility shim documents a Rust 1.86 MSRV (below this
workspace's Rust 1.88 floor), preserves the public serde_yaml 0.9 surface,
and removes the archived unsafe-libyaml implementation from the runtime
dependency graph. See the crate's https://docs.rs/crate/noyalib-serde-yaml/0.0.56
compatibility documentation and the current https://docs.rs/crate/noyalib-serde-yaml/latest
release page. Confirm the exact selected release and checksum through Cargo
when regenerating Cargo.lock.

This is a better first migration candidate than serde-saphyr for this
codebase because it preserves the current source-level API. The final PR must
still run the existing JSON/YAML parity, malformed-input, contract-example,
MSRV, WASM, and cargo-deny gates.

Advisory inventory from the 2026-11-01 PR #33 CI run

The current cargo deny check advisories failure is broader than the YAML parser:

• ctutils 0.4.2 — RUSTSEC-2026-0385 (non-constant-time mask generation / u128 comparisons), reached through the Postgres protocol dependency chain.
• rustls 0.23.45 — RUSTSEC-2026-0346, 0347, 0348, and 0349 (TLS session Debug exposure, ECH rejection handling, Brotli certificate decompression, and key-exchange downgrade behavior), reached through hyper-rustls 0.27.9 / reqwest 0.12.28.
• rustls-webpki 0.103.15 — RUSTSEC-2026-0382 (distrusted intermediate certificate path validation).
• tokio 1.53.1 — RUSTSEC-2026-0386 (Unix AsyncFd I/O safety).
• unsafe-libyaml 0.2.11 — RUSTSEC-2024-0449 (unmaintained parser implementation), used by agentverify-observe and agentverify-testkit.
• webpki-roots 1.0.9 — RUSTSEC-2026-0381 (distrusted emSign root/intermediate certificates).

Acceptance additions

• Resolve each advisory by updating the owning dependency chain and regenerating Cargo.lock; do not add blanket ignores or suppress the advisory database.
• Re-run cargo deny check advisories, full workspace tests, Clippy, MSRV, WASM, and dependency-tree review.
• For TLS updates, verify the selected reqwest/rustls feature set remains compatible with the supported targets.
• For unsafe-libyaml, migrate to the compatible maintained YAML implementation tracked above and add parser parity/failure tests.