CVE-2026-59247 gleam-lang CVE debrief

Who should care

Defenders who use Gleam should verify the integrity of package contents and update to a fixed version of Gleam. They should also review compensating controls for exposed systems, check relevant monitoring, detection, and logs for exposed assets, and track exceptions and retest remediated assets. Additionally, defenders should plan vendor-supported updates or mitigations through normal change control where exposure is confirmed.

Why it matters

This vulnerability allows an adversary in the middle to substitute forged Hex package contents during dependency resolution, affecting Gleam versions from 0.18.0 before 1.18.0. Defenders should prioritize verifying the integrity of package contents and updating to a fixed version of Gleam.

• Loss of integrity of downloaded package contents
• Potential for unauthorized modifications to package contents
• Need for verification of package contents
• Priority for updating to a fixed version of Gleam

Technical summary

The vulnerability is caused by insufficient verification of data authenticity in Gleam, which allows an adversary in the middle to substitute forged Hex package contents during dependency resolution. This issue affects Gleam versions from 0.18.0 before 1.18.0. The vulnerability arises during dependency resolution when Gleam fetches package metadata from the signature-verified Hex repository and then makes a second request to the unsigned Hex API. This second request is not covered by the Hex repository signature, allowing an adversary to modify the API release response and the corresponding repository

Defensive priority

Defenders should prioritize verifying the integrity of package contents and updating to a fixed version of Gleam.

Recommended defensive actions

• Verify the integrity of package contents
• Update to a fixed version of Gleam
• Monitor for suspicious activity
• Review compensating controls for exposed systems
• Check relevant monitoring, detection, and logs for exposed assets
• Track exceptions and retest remediated assets
• Plan vendor-supported updates or mitigations through normal change control

Evidence notes

The vulnerability is caused by insufficient verification of data authenticity in Gleam, which allows an adversary in the middle to substitute forged Hex package contents during dependency resolution. The issue arises during dependency resolution when Gleam fetches package metadata from the signature-verified Hex repository and then makes a second request to the unsigned Hex API. This second request is not covered by the Hex repository signature, allowing an adversary to modify the API release response and the corresponding repository

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.