Insufficient Verification of Data Authenticity in Gleam

Publication date: 2026-07-29

Last updated on: 2026-07-29

Assigner: EEF

Description

Insufficient Verification of Data Authenticity vulnerability in Gleam allows an adversary in the middle to substitute forged Hex package contents during dependency resolution. During dependency resolution Gleam fetches package metadata from the signature-verified Hex repository, which covers each release's dependency requirements and SHA-256 outer_checksum. After resolving versions, gleam_cli::dependencies::lookup_package makes a second request to the unsigned Hex API through gleam_core::hex::get_package_release and records the outer_checksum and dependency names from that JSON response into manifest.toml, instead of the values from the verified repository metadata. The Hex repository signature does not cover the API response. An adversary in the middle who can intercept TLS with a certificate trusted by the Gleam process (for example a TLS-inspecting proxy using a CA in the operating system trust store or added through GLEAM_CACERTS_PATH), and who can modify both the API release response and the corresponding repository tarball, can supply a package archive with a matching forged checksum without the Hex repository signing key. Gleam verifies the forged tarball against the forged checksum, accepts it, and extracts it as a dependency source, resulting in loss of integrity of the downloaded package contents. Only projects that resolve or update Hex dependencies are affected, which happens when the manifest is missing, a dependency is added or updated, or dependency requirements change. Builds that reuse an unchanged, known-good manifest.toml continue to verify tarballs against its pinned checksum. This issue affects gleam: from 0.18.0 before 1.18.0.

CVSS Scores

EPSS Scores

Probability:
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Meta Information

Published

2026-07-29

Last Modified

2026-07-29

Generated

2026-07-30

AI Q&A

2026-07-30

EPSS Evaluated

N/A

NVD

CVE-2026-59247

Affected Vendors & Products

Vendor	Product	Version / Range
gleam	gleam	From 0.18.0 (inc) to 1.18.0 (exc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-345	The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.

Attack-Flow Graph

Executive Summary

This vulnerability in Gleam allows an attacker to substitute fake package contents during dependency resolution. Gleam fetches package metadata from a signature-verified repository but then uses an unsigned API response to record checksums and dependencies. An attacker intercepting TLS traffic can modify both the API response and package files to match forged checksums, bypassing integrity checks and injecting malicious dependencies.

Impact Analysis

If you use Gleam projects that resolve or update Hex dependencies, an attacker could replace legitimate packages with malicious ones. This could lead to compromised builds, data breaches, or unauthorized code execution in your projects. Only projects with unchanged manifest.toml files are unaffected.

Mitigation Strategies

Upgrade Gleam to version 1.18.0 or later to address the insufficient verification of package contents during dependency resolution.

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