# CVE-2026-66046: Expat Denial of Service via storeAtts() Quadratic Complexity

> Remote attackers can cause denial of service in applications using Expat by submitting large XML documents that trigger quadratic complexity in attribute processing.

- **Severity:** High 8.7 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Aug 2026
- **Updated:** 1 Oct 2026
- **Also known as:** ALPINE-CVE-2026-66046, AZL-96605, BELL-CVE-2026-66046, CLSA-2026-1790064092, CLSA-2026-1790153508, CLSA-2026-1790239193, CLSA-2026-1790239270, CLSA-2026-1790239757, DEBIAN-CVE-2026-66046, ECHO-38c1-1304-759b, GHSA-m3rp-25vq-jfcv, ROOT-OS-DEBIAN-11-CVE-2026-66046, UBUNTU-CVE-2026-66046

## What is CVE-2026-66046?

CVE-2026-66046 is a high-severity denial of service vulnerability in Libexpat Project Libexpat, published on 18 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Expat through 2.8.3 contains a denial of service vulnerability caused by quadratic algorithmic complexity in the storeAtts() function in xmlparse.c, where processing N specified attributes with non-normalized values triggers an O(N^2) linear scan of elementType->defaultAtts to determine CDATA status. A remote unauthenticated attacker can supply a single well-formed XML document of a few megabytes to an application parsing untrusted XML to cause excessive CPU consumption, resulting in denial of service without requiring authentication, external entity resolution, or non-default parser options.

Weakness: CWE-407 (Inefficient Algorithmic Complexity)

## Which versions are affected by CVE-2026-66046?

It affects 1 product.


Products: Libexpat Project Libexpat.

## How do I fix CVE-2026-66046?

Upgrade to a fixed version.

## Is CVE-2026-66046 being exploited?

There is no known exploitation of CVE-2026-66046 in the wild and no public exploit code.

## Other vulnerabilities in Libexpat Project Libexpat

- [CVE-2024-45490](https://insights.sbomify.com/CVE-2024-45490/): Integer overflow in n/a (critical 9.8)
- [CVE-2021-46143](https://insights.sbomify.com/CVE-2021-46143/): Integer overflow in n/a (high 8.1)
- [CVE-2017-11742](https://insights.sbomify.com/CVE-2017-11742/): Privilege escalation in n/a (high 7.8)
- [CVE-2025-59375](https://insights.sbomify.com/CVE-2025-59375/): Denial of service in libexpat (high 7.5)
- [CVE-2023-52425](https://insights.sbomify.com/CVE-2023-52425/): Denial of service in n/a (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-66046 and fix it if it is affected.

What it is: CVE-2026-66046 is a high-severity denial of service vulnerability in Libexpat Project Libexpat, published on 18 Aug 2026. 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-2026-66046 in the wild and no public exploit code.

Affected product: Libexpat Project Libexpat. 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-2026-66046/ (Markdown: https://insights.sbomify.com/CVE-2026-66046/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66046
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66046
- OSV: https://osv.dev/vulnerability/CVE-2026-66046

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66046/
Summaries are generated and may contain mistakes; the official description above is the record.