diff --git a/README.md b/README.md
index 0e5b58f..f7c3ace 100644
--- a/README.md
+++ b/README.md
@@ -86,6 +86,8 @@ Each lab README has the recipes, what the lab pins (`What it pins`) and how to r
 to download); Rust + Typst, static, needs 25.2 MiB (8.1 MiB with `xz -9`). The method and the
 platforms are in [`labs/ptbr-pdf`](labs/ptbr-pdf#benchmark-from-pandoc-to-rust).
 
+⚠ The xz figure is a measurement only; see the [xz warning](labs/ptbr-pdf/README.md#benchmark-from-pandoc-to-rust) (CVE-2024-3094 and later fixes up to 5.8.4) before you ship an `.xz` file.
+
 ## tools/
 
 One Cargo workspace. Run a crate with `nix run ./tools#<crate>`.
diff --git a/labs/ptbr-pdf/README.md b/labs/ptbr-pdf/README.md
index d746434..4450d78 100644
--- a/labs/ptbr-pdf/README.md
+++ b/labs/ptbr-pdf/README.md
@@ -32,6 +32,15 @@ and ptbr-writer's `scripts/pdf-bench.sh`.
 | 2. Rust + Typst, first build | one binary, all of `typst-assets`' fonts | 46.7 MiB static | 13.7 MiB (`xz -9`) | 17 min through Nix ⚠ |
 | 3. **Rust + Typst, slim** | six fonts, `opt-level = "z"`, `lto = "fat"`, `panic = "abort"`, stripped | **25.2 MiB static** | **8.1 MiB** (`xz -9`), 10.8 MiB (gzip) | **0.2 s** to convert a page |
 
+> **⚠ xz.** The xz sizes are a measurement only: nothing here ships an `.xz` file. If a release
+> asset is ever compressed with xz, use xz 5.8.4 or newer from the lab's pinned nixpkgs (the lock
+> has 5.8.4), never the host's. XZ Utils had a backdoor in 5.6.0 and 5.6.1
+> ([CVE-2024-3094](https://nvd.nist.gov/vuln/detail/CVE-2024-3094)) and memory-safety fixes in 5.8.1
+> ([CVE-2025-31115](https://github.com/tukaani-project/xz/security/advisories/GHSA-6cc8-p5mm-29w2)),
+> 5.8.3 ([CVE-2026-34743](https://github.com/tukaani-project/xz/security/advisories/GHSA-x872-m794-cxhv))
+> and 5.8.4 ([GHSA-5qpq-xqfv-j9pg](https://github.com/tukaani-project/xz/security/advisories/GHSA-5qpq-xqfv-j9pg)).
+> Prefer zstd or gzip for assets.
+
 - Step 1 to step 3: **29× less on disk and 25× less to download.**
 - ⚠ Through Nix, row 2 took 17 minutes on a fresh 4-core runner: the binary is not in
   cache.nixos.org, so Nix compiles Typst and 326 crates. Download the prebuilt static binary
diff --git a/labs/ptbr-tools/README.md b/labs/ptbr-tools/README.md
index 6c2433b..1301002 100644
--- a/labs/ptbr-tools/README.md
+++ b/labs/ptbr-tools/README.md
@@ -69,6 +69,15 @@ Measured on x86_64-linux, 2026-10-21, `nix build`, stripped, `opt-level = "z"`,
 | `packages.static-aarch64` (musl, static) | 528 696 bytes | 517.4 KiB |
 | `packages.default` (glibc) | 480 176 bytes | 47.4 MiB (glibc and libgcc_s from the store) |
 
+> **⚠ xz.** The xz sizes are a measurement only: nothing here ships an `.xz` file. If a release
+> asset is ever compressed with xz, use xz 5.8.4 or newer from the lab's pinned nixpkgs (the lock
+> has 5.8.4), never the host's. XZ Utils had a backdoor in 5.6.0 and 5.6.1
+> ([CVE-2024-3094](https://nvd.nist.gov/vuln/detail/CVE-2024-3094)) and memory-safety fixes in 5.8.1
+> ([CVE-2025-31115](https://github.com/tukaani-project/xz/security/advisories/GHSA-6cc8-p5mm-29w2)),
+> 5.8.3 ([CVE-2026-34743](https://github.com/tukaani-project/xz/security/advisories/GHSA-x872-m794-cxhv))
+> and 5.8.4 ([GHSA-5qpq-xqfv-j9pg](https://github.com/tukaani-project/xz/security/advisories/GHSA-5qpq-xqfv-j9pg)).
+> Prefer zstd or gzip for assets.
+
 The three separate binaries each carry their own copy of `std`. One binary that dispatches on its
 name is 2.5 times smaller in total, so the lab ships that one. `checks.size` fails if the static
 binary grows more than 10% over 570 112 bytes (a ceiling of 627 123 bytes).
diff --git a/labs/ptbr-tools/justfile b/labs/ptbr-tools/justfile
index b6a5768..0ac8653 100644
--- a/labs/ptbr-tools/justfile
+++ b/labs/ptbr-tools/justfile
@@ -14,9 +14,9 @@ test:
 build:
     nix build "{{here}}#static" --print-build-logs --out-link "{{here}}/result" && ls -lL "{{here}}/result/bin/"
 
-# The size of the static binary, its xz size and the closure a consumer downloads.
+# The size of the static binary, its xz size (xz from the lab's pinned nixpkgs, never the host's: see the README's xz warning) and the closure a consumer downloads.
 size:
-    p="$(nix build "{{here}}#static" --no-link --print-out-paths)"; stat -L -c '%s bytes %n' "$p/bin/ptbr"; xz -9 -c "$p/bin/ptbr" | wc -c; nix path-info -Sh "$p"
+    p="$(nix build "{{here}}#static" --no-link --print-out-paths)"; stat -L -c '%s bytes %n' "$p/bin/ptbr"; rev="$(nix flake metadata --json --no-update-lock-file "{{here}}" | jq -r .locks.nodes.nixpkgs.locked.rev)"; nix run "github:NixOS/nixpkgs/$rev#xz" -- -9 -c "$p/bin/ptbr" | wc -c; nix path-info -Sh "$p"
 
 # Enter the shell: rustc, cargo, clippy, rustfmt and rust-analyzer on PATH.
 shell: