# CVE-2023-27495: Bypass of CSRF protection in the presence of predictable userInfo in @fastify/csrf-protection

> Attackers can bypass CSRF protection in Fastify applications if the userInfo parameter is predictable or missing, allowing unauthorized state-changing requests.

- **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 20 Apr 2023
- **Updated:** 5 Feb 2025
- **Also known as:** GHSA-qrgf-9gpc-vrxw

## What is CVE-2023-27495?

CVE-2023-27495 is a medium-severity vulnerability in fastify csrf-protection, published on 20 Apr 2023. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

@fastify/csrf-protection is a plugin which helps protect Fastify servers against CSRF attacks. The CSRF protection enforced by the @fastify/csrf-protection library in combination with @fastify/cookie can be bypassed from network and same-site attackers under certain conditions. @fastify/csrf-protection supports an optional userInfo parameter that binds the CSRF token to the user. This parameter has been introduced to prevent cookie-tossing attacks as a fix for CVE-2021-29624. Whenever userInfo parameter is missing, or its value can be predicted for the target user account, network and same-site attackers can 1. fixate a _csrf cookie in the victim's browser, and 2. forge CSRF tokens that are valid for the victim's session. This allows attackers to bypass the CSRF protection mechanism. As a fix, @fastify/csrf-protection starting from version 6.3.0 (and v4.1.0) includes a server-defined secret hmacKey that cryptographically binds the CSRF token to the value of the _csrf cookie and the userInfo parameter, making tokens non-spoofable by attackers. This protection is effective as long as the userInfo parameter is unique for each user. This is patched in versions 6.3.0 and v4.1.0. Users are advised to upgrade. Users unable to upgrade may use a random, non-predictable userInfo parameter for each user as a mitigation.

Weakness: CWE-352 (Cross-Site Request Forgery)

## Which versions are affected by CVE-2023-27495?

4 open source packages ship an affected version, including @fastify/csrf-protection. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `@fastify/csrf-protection` | npm | All versions | 4.1.0 |
| `@fastify/csrf-protection` | npm | All versions | 4.1.0 |
| `@fastify/csrf-protection` | npm | From 5.0.0 | 6.3.0 |
| `@fastify/csrf-protection` | npm | From 5.0.0 | 6.3.0 |

Products: Fastify Csrf-Protection.

## How do I fix CVE-2023-27495?

Upgrade @fastify/csrf-protection to a fixed version for your release line: 4.1.0 or 6.3.0.

## Is CVE-2023-27495 being exploited?

There is no known exploitation of CVE-2023-27495 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-27495 and fix it if it is affected.

What it is: CVE-2023-27495 is a medium-severity vulnerability in fastify csrf-protection, published on 20 Apr 2023. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2023-27495 in the wild and no public exploit code.

Affected packages and fixed versions:
- @fastify/csrf-protection (npm): affected all versions; fixed in 4.1.0
- @fastify/csrf-protection (npm): affected all versions; fixed in 4.1.0
- @fastify/csrf-protection (npm): affected from 5.0.0; fixed in 6.3.0
- @fastify/csrf-protection (npm): affected from 5.0.0; fixed in 6.3.0

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/CVE-2023-27495/ (Markdown: https://insights.sbomify.com/CVE-2023-27495/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-27495
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-27495
- OSV: https://osv.dev/vulnerability/CVE-2023-27495

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-27495/
Summaries are generated and may contain mistakes; the official description above is the record.