# CVE-2026-39817: Google Go arbitrary file write in go tool pack

Source: https://junglewise.ai/threats/cve-2026-39817-google-go-arbitrary-file-write-in-go-tool-pack  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-07. Updated: 2026-05-14  
License: CC BY 4.0 with a link back

CVE-2026-39817 · Severity: medium · CVSS 5.9 · Published 2026-05-07

A vulnerability in the Go programming language's 'pack' tool could allow a malicious archive to write files to unintended locations on a computer. This tool is typically used internally by the Go compiler, but if a user is tricked into manually extracting a specially crafted archive, it could overwrite sensitive system or application files. This could lead to unauthorized system changes or a compromise of the development environment.

Technologies: [toolchain (Go)](https://junglewise.ai/threats/technologies/toolchain). Vendors: [Go](https://junglewise.ai/threats/vendors/go).

## Quick answers

**Is CVE-2026-39817 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-39817 being exploited in the wild.

**How severe is CVE-2026-39817?** Medium severity, with a CVSS score of 5.9, published 7 May 2026.

**Which products and versions are affected?** toolchain (Go) SEMVER:introduced=0,fixed=1.25.10,introduced=1.26.0-0,fixed=1.26.3.

**Is there a fix for CVE-2026-39817?** Yes. toolchain (Go): upgrade to 1.26.3 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-39817 attacked?** It is attacked with local access, in a software library.

**Am I affected by CVE-2026-39817?** Check whether you run toolchain (Go) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- toolchain (Go): upgrade to 1.26.3 or later

  ```sh
  go get toolchain@v1.26.3
  ```


## Technical details

A path traversal vulnerability exists in the 'go tool pack' subcommand, which serves as a minimal version of the Unix 'ar' utility. The tool fails to sanitize output filenames during extraction, allowing an attacker to provide a malicious archive that writes files to arbitrary locations on the filesystem. While primarily used as an internal compiler tool with trusted inputs, manual invocation on untrusted archives poses a risk. The vulnerability is addressed by modifying 'pack' to refuse extraction of files containing directory components. Patches are available in Go versions 1.25.10 and 1.26.3.

## Affected products

- Google Go < 1.25.10, >= 1.26.0 < 1.26.3

## Identifiers

- CVE-2026-39817
- GO-2026-4979

## Timeline

- 2026-04-15: disclosed: Issue opened on Go GitHub repository
- 2026-05-07: advisory: NVD and Go Project published advisory

## References

- [https://go.dev/cl/767520](https://go.dev/cl/767520)
- [https://go.dev/issue/78778](https://go.dev/issue/78778)
- [https://groups.google.com/g/golang-announce/c/qcCIEXso47M](https://groups.google.com/g/golang-announce/c/qcCIEXso47M)
- [https://pkg.go.dev/vuln/GO-2026-4979](https://pkg.go.dev/vuln/GO-2026-4979)

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