Rust

SlowShield serves crates.io at https://slowshield.example.com/cargo/ as a sparse registry that replaces crates-io in Cargo's configuration. Needs Cargo 1.68 or later.

Set it up

Cargo only takes source replacement from a configuration file, not from environment variables:

[source.crates-io]
replace-with = "slowshield"

[registries.slowshield]
index = "sparse+https://slowshield.example.com/cargo/"

mkdir -p "${CARGO_HOME:-$HOME/.cargo}" && printf '%s\n' '[source.crates-io]' 'replace-with = "slowshield"' '[registries.slowshield]' 'index = "sparse+https://slowshield.example.com/cargo/"' >> "${CARGO_HOME:-$HOME/.cargo}/config.toml"

• Cargo.lock doesn't change. It keeps crates.io as the source, with crates.io's checksums, so a project builds the same with or without SlowShield.
• The official rust images set CARGO_HOME=/usr/local/cargo, where ~/.cargo/config.toml isn't read. The command writes ${CARGO_HOME:-$HOME/.cargo}/config.toml, which works in both.

What changes

• The publish time is each version's pubtime in the index, which crates.io sets and authors can't. The first one SlowShield sees is kept, so a rewritten index can't move it earlier.
• Held versions show up as yanked. Cargo resolves to the newest version that isn't, exactly as when a version is yanked upstream. When only held versions satisfy a requirement, Cargo says the version "is yanked".
• A Cargo.lock that pins a held version fails with the reason and the command to use instead:

  failed to get successful HTTP response from `…/cargo/crates/tokio/1.53.2/download`, got 403
  body:
  slowshield: tokio@1.53.2 is too new.
  It was published 2026-10-03T11:18:32Z (3.0 days ago); this proxy requires 7 days.
  It becomes available at 2026-10-10T11:18:32Z.
  Use an older version (cargo update -p tokio@1.53.2 --precise 1.53.1), or ask your SlowShield administrator for an exception.

  Copy
• Brand-new crates none of whose versions is old enough are held too: Cargo doesn't fail open, because nearly all malicious crates are brand-new typosquats and impersonations.
• Malware from OSV (including RustSec's malicious advisories) and GitHub is refused with 451, and a .crate is checked against the index's checksum and the fingerprint SlowShield recorded the first time.

The second layer

Cargo's own setting, min-publish-age, isn't stable yet, so SlowShield is the only layer.

In Docker and CI

FROM rust:1 AS build
ARG CARGO_INDEX=sparse+https://slowshield.example.com/cargo/
RUN if [ -n "$CARGO_INDEX" ]; then printf '%s\n' '[source.crates-io]' 'replace-with = "slowshield"' \
      '[registries.slowshield]' "index = \"$CARGO_INDEX\"" >> "$CARGO_HOME/config.toml"; fi
WORKDIR /src
COPY . .
RUN cargo build --release

Or run the Setup page's one-line command in a RUN step. Building images covers the base image too, and how to check that nothing in a build goes around SlowShield.

Limits

• Commands that need the crates.io web API: cargo search fails, cargo info needs --registry slowshield, and cargo publish, yank and owner need --registry crates-io.
• Not covered: git dependencies, build scripts that download things, rustup toolchains, and alternative registries.
• Block index.crates.io and static.crates.io at the firewall for machines that should only use SlowShield.

For agents: this page as Markdown · llms.txt · the SlowShield skill