# CVE-2023-40021: Timing Attack Reveals CSRF Tokens in oppia

> Attackers can brute-force CSRF tokens via timing attacks to perform unauthorized actions on behalf of logged-in users.

- **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:** No
- **Published:** 16 Aug 2023
- **Updated:** 3 Oct 2024
- **Also known as:** GHSA-49jp-pjc3-2532

## What is CVE-2023-40021?

CVE-2023-40021 is a medium-severity vulnerability in oppia, published on 16 Aug 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

Oppia is an online learning platform. When comparing a received CSRF token against the expected token, Oppia uses the string equality operator (`==`), which is not safe against timing attacks. By repeatedly submitting invalid tokens, an attacker can brute-force the expected CSRF token character by character. Once they have recovered the token, they can then submit a forged request on behalf of a logged-in user and execute privileged actions on that user's behalf. In particular the function to validate received CSRF tokens is at `oppia.core.controllers.base.CsrfTokenManager.is_csrf_token_valid`. An attacker who can lure a logged-in Oppia user to a malicious website can perform any change on Oppia that the user is authorized to do, including changing profile information; creating, deleting, and changing explorations; etc. Note that the attacker cannot change a user's login credentials. An attack would need to complete within 1 second because every second, the time used in computing the token changes. This issue has been addressed in commit `b89bf80837` which has been included in release `3.3.2-hotfix-2`. Users are advised to upgrade. There are no known workarounds for this vulnerability.

Weakness: CWE-208 (Observable Timing Discrepancy), CWE-203 (Observable Discrepancy)

## Which versions are affected by CVE-2023-40021?

It affects 1 product.


Products: Oppia Oppia.

## How do I fix CVE-2023-40021?

No fix has been published for CVE-2023-40021 yet. Limit exposure of the affected software until one is available.

## Is CVE-2023-40021 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2023-40021 exists.

## Other vulnerabilities in oppia

- [CVE-2021-41733](https://insights.sbomify.com/CVE-2021-41733/): Open redirect in n/a (medium 6.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-40021 and fix it if it is affected.

What it is: CVE-2023-40021 is a medium-severity vulnerability in oppia, published on 16 Aug 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: It is not confirmed as exploited in the wild, but public exploit code for CVE-2023-40021 exists.

Affected product: oppia. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2023-40021/ (Markdown: https://insights.sbomify.com/CVE-2023-40021/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-40021
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-40021
- OSV: https://osv.dev/vulnerability/CVE-2023-40021

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-40021/
Summaries are generated and may contain mistakes; the official description above is the record.