# CVE-2026-8257: WebAssembly Binaryen BrOn wasm-ir-builder.cpp makeBrOn assertion

> A local attacker can trigger a reachable assertion in the BrOn parser, causing a denial of service in WebAssembly Binaryen.

- **Severity:** Medium 4.8 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N/E:P`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 11 May 2026
- **Updated:** 11 May 2026
- **Also known as:** DEBIAN-CVE-2026-8257, GHSA-gmmj-chcc-2f2x, JLSEC-2026-1094, UBUNTU-CVE-2026-8257

## What is CVE-2026-8257?

CVE-2026-8257 is a medium-severity denial of service vulnerability in WebAssembly Binaryen, published on 11 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

A vulnerability was detected in WebAssembly Binaryen up to 117. This issue affects the function IRBuilder::makeBrOn of the file src/wasm/wasm-ir-builder.cpp of the component BrOn Parser. Performing a manipulation results in reachable assertion. The attack needs to be approached locally. The exploit is now public and may be used. The patch is named 1251efbc1ea471c1311d2726b2bbe061ff2a291c. It is suggested to install a patch to address this issue.

Weakness: CWE-617 (Reachable Assertion)

## Which versions are affected by CVE-2026-8257?

It affects 1 product.


Products: Webassembly Binaryen.

## How do I fix CVE-2026-8257?

Apply the vendor's patch.

## Is CVE-2026-8257 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-8257 exists.

## Other vulnerabilities in WebAssembly Binaryen

- [CVE-2020-18378](https://insights.sbomify.com/CVE-2020-18378/): Null dereference in n/a (medium 6.5)
- [CVE-2020-18382](https://insights.sbomify.com/CVE-2020-18382/): Memory corruption in n/a (medium 6.5)
- [CVE-2019-15758](https://insights.sbomify.com/CVE-2019-15758/): Denial of service in n/a (medium 6.5)
- [CVE-2019-15759](https://insights.sbomify.com/CVE-2019-15759/): Null dereference in n/a (medium 6.5)
- [CVE-2019-7700](https://insights.sbomify.com/CVE-2019-7700/): Memory corruption in n/a (medium 6.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-8257 and fix it if it is affected.

What it is: CVE-2026-8257 is a medium-severity denial of service vulnerability in WebAssembly Binaryen, published on 11 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-8257 exists.

Affected product: WebAssembly Binaryen. 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-2026-8257/ (Markdown: https://insights.sbomify.com/CVE-2026-8257/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-8257
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-8257
- OSV: https://osv.dev/vulnerability/CVE-2026-8257

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-8257/
Summaries are generated and may contain mistakes; the official description above is the record.