# GO-2026-4950: Authorization bypass via double-encoded paths in github.com/valyala/fasthttp

> Attackers can bypass authorization checks in Go applications using fasthttp to access protected files via double-encoded paths.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Aug 2026

## What is GO-2026-4950?

GO-2026-4950 is an authentication bypass vulnerability in github.com/valyala/fasthttp, published on 18 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

In github.com/valyala/fasthttp before 1.70.0, ServeFile and ServeFS reinterpret filesystem paths as URIs, decoding percent-encoded sequences and treating characters like '?' and '#' as URI delimiters. In applications where authorization guards inspect ctx.Path() before passing paths to file-serving handlers, double-encoded path variants (e.g., /%2561dmin/export.csv) bypass access controls while still resolving to the protected file on disk.

## Which versions are affected by GO-2026-4950?

1 open source package ship an affected version, including github.com/valyala/fasthttp.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/valyala/fasthttp` | Go | All versions | 1.70.0 |

## How do I fix GO-2026-4950?

Upgrade github.com/valyala/fasthttp to 1.70.0 or later.

## Is GO-2026-4950 being exploited?

There is no known exploitation of GO-2026-4950 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GO-2026-4950 and fix it if it is affected.

What it is: GO-2026-4950 is an authentication bypass vulnerability in github.com/valyala/fasthttp, published on 18 Aug 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of GO-2026-4950 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/valyala/fasthttp (Go): affected all versions; fixed in 1.70.0

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GO-2026-4950/ (Markdown: https://insights.sbomify.com/GO-2026-4950/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GO-2026-4950

---
From sbomify Insights: https://insights.sbomify.com/GO-2026-4950/
Summaries are generated and may contain mistakes; the official description above is the record.