# CVE-2025-1942: Disclosure of uninitialized memory when .toUpperCase() causes string to get longer

> Attackers can read uninitialized memory when string case conversion increases length, potentially exposing sensitive data in Firefox and Thunderbird.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/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:** Not needed
- **Fix available:** Yes
- **Published:** 4 Mar 2025
- **Updated:** 4 Jun 2026
- **Also known as:** GHSA-r83v-rmq7-r5m4, UBUNTU-CVE-2025-1942

## What is CVE-2025-1942?

CVE-2025-1942 is a critical vulnerability in Mozilla Firefox, published on 4 Mar 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

When String.toUpperCase() caused a string to get longer it was possible for uninitialized memory to be incorporated into the result string. This vulnerability was fixed in Firefox 136 and Thunderbird 136.

Weakness: CWE-908 (Use of Uninitialized Resource)

## Which versions are affected by CVE-2025-1942?

It affects 2 products.


Products: Mozilla Firefox, Mozilla Thunderbird.

## How do I fix CVE-2025-1942?

Upgrade to a fixed version.

## Is CVE-2025-1942 being exploited?

There is no known exploitation of CVE-2025-1942 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-2025-1942 and fix it if it is affected.

What it is: CVE-2025-1942 is a critical vulnerability in Mozilla Firefox, published on 4 Mar 2025. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2025-1942 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-2025-1942/ (Markdown: https://insights.sbomify.com/CVE-2025-1942/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-1942
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-1942
- OSV: https://osv.dev/vulnerability/CVE-2025-1942

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-1942/
Summaries are generated and may contain mistakes; the official description above is the record.