# CVE-2020-26974: Use after free in Firefox

> A type confusion in Firefox's flex-basis handling leads to a heap use-after-free, allowing attackers to execute arbitrary code or cause a crash.

- **Severity:** High 8.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H`)
- **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:** 7 Jan 2021
- **Updated:** 4 Aug 2024
- **Also known as:** DEBIAN-CVE-2020-26974, GHSA-gr4h-9jxj-93vm, UBUNTU-CVE-2020-26974

## What is CVE-2020-26974?

CVE-2020-26974 is a high-severity use after free vulnerability in Mozilla Firefox, published on 7 Jan 2021. 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

When flex-basis was used on a table wrapper, a StyleGenericFlexBasis object could have been incorrectly cast to the wrong type. This resulted in a heap user-after-free, memory corruption, and a potentially exploitable crash. This vulnerability affects Firefox < 84, Thunderbird < 78.6, and Firefox ESR < 78.6.

Weakness: CWE-787 (Out-of-bounds Write)

## Which versions are affected by CVE-2020-26974?

It affects 3 products.


Products: Mozilla Firefox, Mozilla Firefox Esr, Mozilla Thunderbird.

## How do I fix CVE-2020-26974?

Upgrade to a fixed version.

## Is CVE-2020-26974 being exploited?

There is no known exploitation of CVE-2020-26974 in the wild and no public exploit code.

## Other vulnerabilities in Mozilla Firefox

- [CVE-2019-11708](https://insights.sbomify.com/CVE-2019-11708/): Mozilla Firefox and Thunderbird Sandbox Escape (critical 10.0, exploited in the wild)
- [CVE-2010-3765](https://insights.sbomify.com/CVE-2010-3765/): Mozilla Multiple Products Remote Code Execution (critical 9.8, exploited in the wild)
- [CVE-2022-26486](https://insights.sbomify.com/CVE-2022-26486/): Mozilla Firefox Use-After-Free (critical 9.6, exploited in the wild)
- [CVE-2022-26485](https://insights.sbomify.com/CVE-2022-26485/): Mozilla Firefox Use-After-Free (high 8.8, exploited in the wild)
- [CVE-2019-11707](https://insights.sbomify.com/CVE-2019-11707/): Mozilla Firefox and Thunderbird Type Confusion (high 8.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2020-26974 and fix it if it is affected.

What it is: CVE-2020-26974 is a high-severity use after free vulnerability in Mozilla Firefox, published on 7 Jan 2021. 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-2020-26974 in the wild and no public exploit code.

Affected product: Mozilla Firefox. 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-2020-26974/ (Markdown: https://insights.sbomify.com/CVE-2020-26974/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2020-26974
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2020-26974
- OSV: https://osv.dev/vulnerability/CVE-2020-26974

---
From sbomify Insights: https://insights.sbomify.com/CVE-2020-26974/
Summaries are generated and may contain mistakes; the official description above is the record.