CVE-2023-27495

CVE-2023-27495 is a medium-severity cross-site request forgery (CSRF) vulnerability in @fastify/csrf-protection (npm), affecting versions < 4.1.0. It is fixed in 4.1.0, 6.3.0.

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Summary

Bypass of CSRF protection in the presence of predictable userInfo

Description

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.

Workarounds

As a workaround, developers can use a random, non-predictable userInfo parameter for each user.

Credits

Impact

A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones. Typical impact: state-changing actions performed as the victim without their consent.

CVE-2023-27495 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.1.0, 6.3.0); upgrading removes the vulnerable code path.

Affected versions

@fastify/csrf-protection (< 4.1.0) @fastify/csrf-protection (>= 5.0.0, < 6.3.0)

Security releases

@fastify/csrf-protection → 4.1.0 (npm) @fastify/csrf-protection → 6.3.0 (npm)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

This is patched in version 6.3.0 and v4.1.0.

Frequently Asked Questions

  1. What is CVE-2023-27495? CVE-2023-27495 is a medium-severity cross-site request forgery (CSRF) vulnerability in @fastify/csrf-protection (npm), affecting versions < 4.1.0. It is fixed in 4.1.0, 6.3.0. A victim's authenticated browser session is used to submit forged requests to an application that cannot distinguish them from legitimate ones.
  2. How severe is CVE-2023-27495? CVE-2023-27495 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
  3. Which versions of @fastify/csrf-protection are affected by CVE-2023-27495? @fastify/csrf-protection (npm) versions < 4.1.0 is affected.
  4. Is there a fix for CVE-2023-27495? Yes. CVE-2023-27495 is fixed in 4.1.0, 6.3.0. Upgrade to this version or later.
  5. Is CVE-2023-27495 exploitable, and should I be worried? Whether CVE-2023-27495 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
  6. What actually determines whether CVE-2023-27495 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
  7. How do I fix CVE-2023-27495?
    • Upgrade @fastify/csrf-protection to 4.1.0 or later
    • Upgrade @fastify/csrf-protection to 6.3.0 or later

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