Hex writes unverified package tarballs to standard output in mix hex.package fetch

Improper Validation of Integrity Check Value vulnerability in hexpm hex allows a package mirror to substitute the tarball that mix hex.package fetch PACKAGE VERSION --output - writes to standard output.

Hex compares a fetched tarball's inner and outer checksums with the repository's signed registry before writing it to a file or unpacking it. The standard output branch of mix hex.package fetch writes the bytes returned by Hex.SCM.fetch/3 without that comparison, and Hex.SCM.fetch/3 doesn't verify freshly downloaded tarballs itself. When a mirror configured with HEX_MIRROR_URL or mirror_url serves altered bytes, the command prints them and exits successfully, while the same fetch with --output FILE fails with a checksum mismatch.

Hex also prints diagnostics to standard output while the command runs: the body of a failed HTTP response from the update check or a registry fetch, and API warnings from the x-hex-message header. A mirror that answers one of those requests with an error whose body is a tar archive gets that archive written to standard output ahead of the verified tarball, so a pipeline that unpacks standard output extracts the mirror's files. This happens even when the checksum check then fails, because the archive is already on standard output.

This issue affects hex: from 0.20.2 before 2.5.2.

1. mix hex.package fetch PACKAGE VERSION --output - calls fetch_tarball!/3, which calls Hex.SCM.fetch/3.
2. For a fresh download, Hex.SCM.fetch/3 writes the response body to the cache and returns it without comparing it to the registry checksum.
3. The standard output branch passes those bytes to IO.binwrite/1. The file and --unpack branches call verify_inner_checksum!/4 and verify_outer_checksum!/4 first.
4. While the task runs, Hex prints messages with Hex.Shell.info/1, which writes to standard output. Hex.Utils.print_error_result/1 prints the body of an error response that way; the update checker calls it when its request for installs/hex-1.x.csv fails, and Hex.Registry.Server calls it when a registry fetch fails and the cached registry is used. Hex.HTTP prints the x-hex-message warning of each response the same way. These are printed before the tarball is written, so the mirror chooses bytes that come before the tarball on standard output.

1. Set HEX_MIRROR_URL to a server that returns modified bytes for tarballs/decimal-2.4.1.tar and proxies everything else to repo.hex.pm.
2. mix hex.package fetch decimal 2.4.1 --output - prints the modified bytes and exits 0.
3. mix hex.package fetch decimal 2.4.1 --output decimal.tar fails with Checksum mismatch against registry (outer).

Write the tarball to a file with --output FILE, which verifies the checksums, and read that file instead of standard output.

Affects mix hex.package fetch with --output - when packages are downloaded through a mirror set with HEX_MIRROR_URL or mirror_url. Downloads directly from the repository over HTTPS aren't affected unless TLS verification is broken.

Upgrade Hex to 2.5.2 or later, for example with mix local.hex. mix hex.package fetch PACKAGE VERSION --output - then verifies the registry checksum before writing the tarball to standard output and prints its messages to standard error.