# Malicious module proxy can bypass checksum database in cmd/go

Advisory record · `GO-2026-4984`

- **Identifiers:** `GO-2026-4984` · `BIT-golang-2026-42501` · `CVE-2026-42501`
- **State:** Confirmed
- **Severity:** CVSS 7.5 High
- **Critical:** No
- **Weakness (CWE):** [CWE-347](https://cwe.mitre.org/data/definitions/347.html)
- **Tags:** [Weak cryptography or TLS](https://agentprime.io/advisories/tag/weak-crypto.md)
- **Published:** 7 May 2026
- **Updated:** 31 Aug 2026
- **Reviewed by a person:** No

Published by OSV at 7 May 2026, 19:21 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `toolchain` (Go): affected `<1.25.10 || >=1.26.0-0, <1.26.3`; fixed in `1.25.10`, `1.26.3`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Go toolchain versions before 1.25.10 and from 1.26.0 up to (but not including) 1.26.3 are affected. An attacker running a malicious module proxy (GOPROXY) can serve altered Go toolchains or modules, and the go command can accept them by bypassing checksum database validation, so users of an untrusted proxy may run attacker-supplied code. Upgrade to Go 1.25.10 or 1.26.3; setting GOTOOLCHAIN to a fixed version is not sufficient.

Written 6 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 7.5 High, CVSS 3.1: `CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H`, from ADP (CISA-ADP)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0023, the 14th percentile, on 12 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 7 May 2026, 20:16 UTC: [Confirmed by NVD](https://nvd.nist.gov/vuln/detail/CVE-2026-42501)
- 31 Oct 2026, 14:48 UTC: First seen by agentprime
- 31 Oct 2026, 14:48 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GO-2026-4984)

## Sources

- [OSV: GO-2026-4984](https://osv.dev/vulnerability/GO-2026-4984)
- <https://go.dev/cl/775321>
- <https://groups.google.com/g/golang-announce/c/qcCIEXso47M>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GO-2026-4984>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GO-2026-4984/>. Everything for agents: <https://agentprime.io/llms.txt>.