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

CVE-2026-66046 is fixed in 11 products on 1 platform, first by expat 2.2.5-3ubuntu0.9+esm5, released August 20, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-66046
- Priority: High
- CVSS: 7.5 (Important)
- EPSS: 0.0074
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Aug 18, 2026
- Weakness: Inefficient Algorithmic Complexity
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-66046

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | MozillaFirefox opensuse-leap 16.0 | 153.5.0-160000.1.1 | [OPENSUSE-SU-2026:22139-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-22139-1.md) | Nov 7, 2026 |
| Linux | MozillaFirefox sles 15.6 | 153.5.0-150400.157.16.1 | [SUSE-SU-2026:4834-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4834-1.md) | Nov 7, 2026 |
| Linux | MozillaFirefox sles 15.7 | 153.5.0-150400.157.16.1 | [SUSE-SU-2026:4834-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4834-1.md) | Nov 7, 2026 |
| Linux | MozillaFirefox sles 16.0 | 153.5.0-160000.1.1 | [SUSE-SU-2026:24260-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-24260-1.md) | Nov 7, 2026 |
| Linux | MozillaThunderbird sles 15.7 | 153.5.0-150400.13.9.1 | [SUSE-SU-2026:4736-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4736-1.md) | Nov 7, 2026 |
| Linux | expat Debian 12.15 (bookworm) | 2.5.0-1+deb12u4 | [DEBIAN-CVE-2026-66046](https://patchblox.com/security/data/fix/linux/debian-cve-2026-66046.md) | Sep 5, 2026 |
| Linux | expat Debian 13.5 (trixie) | 2.9.0-1~deb13u1 | [DEBIAN-CVE-2026-66046](https://patchblox.com/security/data/fix/linux/debian-cve-2026-66046.md) | Sep 5, 2026 |
| Linux | expat Debian 14 | 2.8.4-1 | [DEBIAN-CVE-2026-66046](https://patchblox.com/security/data/fix/linux/debian-cve-2026-66046.md) | Sep 5, 2026 |
| Linux | expat RHEL 10 | 2.7.3-1.el10_2.5 | [RHSA-2026:74001](https://patchblox.com/security/data/fix/linux/rhsa-2026-74001.md) | Oct 1, 2026 |
| Linux | expat RHEL 8 | 0:2.5.0-4.el8_10 | [RHSA-2026:72448](https://patchblox.com/security/data/fix/linux/rhsa-2026-72448.md) | Sep 29, 2026 |
| Linux | expat RHEL 9 | 0:2.5.0-6.el9_8.5 | [RHSA-2026:72663](https://patchblox.com/security/data/fix/linux/rhsa-2026-72663.md) | Sep 29, 2026 |
| Linux | expat Ubuntu 18.04.6 LTS (bionic) | 2.2.5-3ubuntu0.9+esm5 | [expat 2.2.5-3ubuntu0.9+esm5](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-expat-2.2.5-3ubuntu0.9-esm5.md) | Aug 20, 2026 |
| Linux | expat Ubuntu 20.04 LTS (focal) | 2.2.9-1ubuntu0.8+esm3 | [expat 2.2.9-1ubuntu0.8+esm3](https://patchblox.com/security/data/fix/linux/ubuntu-focal-expat-2.2.9-1ubuntu0.8-esm3.md) | Aug 20, 2026 |
| Linux | expat Ubuntu 22.04.1 LTS (jammy) | 2.4.7-1ubuntu0.9 | [expat 2.4.7-1ubuntu0.9](https://patchblox.com/security/data/fix/linux/ubuntu-jammy-expat-2.4.7-1ubuntu0.9.md) | Aug 20, 2026 |
| Linux | expat Ubuntu 24.04.5 LTS (noble) | 2.6.1-2ubuntu0.6 | [expat 2.6.1-2ubuntu0.6](https://patchblox.com/security/data/fix/linux/ubuntu-noble-expat-2.6.1-2ubuntu0.6.md) | Aug 20, 2026 |
| Linux | expat Ubuntu 26.04.1 LTS (resolute) | 2.7.4-1ubuntu0.2 | [expat 2.7.4-1ubuntu0.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-expat-2.7.4-1ubuntu0.2.md) | Aug 20, 2026 |
| Linux | expat almalinux 10 | 2.7.3-1.el10_2.5 | [ALSA-2026:74001](https://patchblox.com/security/data/fix/linux/alsa-2026-74001.md) | Sep 30, 2026 |
| Linux | expat almalinux 8 | 2.5.0-4.el8_10 | [ALSA-2026:72448](https://patchblox.com/security/data/fix/linux/alsa-2026-72448.md) | Sep 28, 2026 |
| Linux | expat almalinux 9 | 2.5.0-6.el9_8.5 | [ALSA-2026:72663](https://patchblox.com/security/data/fix/linux/alsa-2026-72663.md) | Sep 28, 2026 |
| Linux | expat amzn 2023 | 2.8.3-1.amzn2023.0.1 | [ALAS2023-2026-2157](https://patchblox.com/security/data/fix/linux/alas2023-2026-2157.md) | Sep 14, 2026 |
| Linux | expat ol 10 | 0:2.7.3-1.el10_2.5 | [ELSA-2026-74001](https://patchblox.com/security/data/fix/linux/elsa-2026-74001.md) | Oct 1, 2026 |
| Linux | expat ol 8 | 0:2.5.0-4.el8_10 | [ELSA-2026-72448](https://patchblox.com/security/data/fix/linux/elsa-2026-72448.md) | Sep 29, 2026 |
| Linux | expat ol 9 | 0:2.5.0-6.el9_8.5 | [ELSA-2026-72663](https://patchblox.com/security/data/fix/linux/elsa-2026-72663.md) | Sep 28, 2026 |
| Linux | expat opensuse-leap 16.0 | 2.8.4-160000.1.1 | [OPENSUSE-SU-2026:21945-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-21945-1.md) | Sep 24, 2026 |
| Linux | expat rocky 10 | 0:2.7.3-1.el10_2.5 | [RLSA-2026:74001](https://patchblox.com/security/data/fix/linux/rlsa-2026-74001.md) | Oct 1, 2026 |
| Linux | expat rocky 8 | 0:2.5.0-4.el8_10 | [RLSA-2026:72448](https://patchblox.com/security/data/fix/linux/rlsa-2026-72448.md) | Sep 29, 2026 |
| Linux | expat rocky 9 | 0:2.5.0-6.el9_8.5 | [RLSA-2026:72663](https://patchblox.com/security/data/fix/linux/rlsa-2026-72663.md) | Sep 30, 2026 |
| Linux | expat sles 15.6 | 2.8.5-150400.3.40.1 | [SUSE-SU-2026:4623-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4623-1.md) | Nov 7, 2026 |
| Linux | expat sles 15.7 | 2.8.5-150700.3.15.1 | [SUSE-SU-2026:4619-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4619-1.md) | Nov 7, 2026 |
| Linux | expat sles 16.0 | 2.8.4-160000.1.1 | [SUSE-SU-2026:23912-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-23912-1.md) | Sep 24, 2026 |
| Linux | expat-debuginfo amzn 2023 | 2.8.3-1.amzn2023.0.1 | [ALAS2023-2026-2157](https://patchblox.com/security/data/fix/linux/alas2023-2026-2157.md) | Sep 14, 2026 |
| Linux | expat-debugsource amzn 2023 | 2.8.3-1.amzn2023.0.1 | [ALAS2023-2026-2157](https://patchblox.com/security/data/fix/linux/alas2023-2026-2157.md) | Sep 14, 2026 |
| Linux | expat-devel RHEL 8 | 0:2.5.0-4.el8_10 | [RHSA-2026:72448](https://patchblox.com/security/data/fix/linux/rhsa-2026-72448.md) | Sep 29, 2026 |
| Linux | expat-devel RHEL 9 | 0:2.5.0-6.el9_8.5 | [RHSA-2026:72663](https://patchblox.com/security/data/fix/linux/rhsa-2026-72663.md) | Sep 29, 2026 |
| Linux | expat-devel almalinux 10 | 2.7.3-1.el10_2.5 | [ALSA-2026:74001](https://patchblox.com/security/data/fix/linux/alsa-2026-74001.md) | Sep 30, 2026 |
| Linux | expat-devel almalinux 8 | 2.5.0-4.el8_10 | [ALSA-2026:72448](https://patchblox.com/security/data/fix/linux/alsa-2026-72448.md) | Sep 28, 2026 |
| Linux | expat-devel almalinux 9 | 2.5.0-6.el9_8.5 | [ALSA-2026:72663](https://patchblox.com/security/data/fix/linux/alsa-2026-72663.md) | Sep 28, 2026 |
| Linux | expat-devel amzn 2023 | 2.8.3-1.amzn2023.0.1 | [ALAS2023-2026-2157](https://patchblox.com/security/data/fix/linux/alas2023-2026-2157.md) | Sep 14, 2026 |
| Linux | expat-devel ol 10 | 0:2.7.3-1.el10_2.5 | [ELSA-2026-74001](https://patchblox.com/security/data/fix/linux/elsa-2026-74001.md) | Oct 1, 2026 |
| Linux | expat-devel ol 8 | 0:2.5.0-4.el8_10 | [ELSA-2026-72448](https://patchblox.com/security/data/fix/linux/elsa-2026-72448.md) | Sep 29, 2026 |
| Linux | expat-devel ol 9 | 0:2.5.0-6.el9_8.5 | [ELSA-2026-72663](https://patchblox.com/security/data/fix/linux/elsa-2026-72663.md) | Sep 28, 2026 |
| Linux | expat-static amzn 2023 | 2.8.3-1.amzn2023.0.1 | [ALAS2023-2026-2157](https://patchblox.com/security/data/fix/linux/alas2023-2026-2157.md) | Sep 14, 2026 |
| Linux | firefox amzn 2023 | 140.14.0-1.amzn2023.0.1 | [ALAS2023-2026-2105](https://patchblox.com/security/data/fix/linux/alas2023-2026-2105.md) | Aug 31, 2026 |
| Linux | firefox-debuginfo amzn 2023 | 140.14.0-1.amzn2023.0.1 | [ALAS2023-2026-2105](https://patchblox.com/security/data/fix/linux/alas2023-2026-2105.md) | Aug 31, 2026 |
| Linux | firefox-debugsource amzn 2023 | 140.14.0-1.amzn2023.0.1 | [ALAS2023-2026-2105](https://patchblox.com/security/data/fix/linux/alas2023-2026-2105.md) | Aug 31, 2026 |
| Linux | libexpat1 Ubuntu 18.04.6 LTS (bionic) | 2.2.5-3ubuntu0.9+esm5 | [libexpat1 2.2.5-3ubuntu0.9+esm5](https://patchblox.com/security/data/fix/linux/ubuntu-bionic-libexpat1-2.2.5-3ubuntu0.9-esm5.md) | Aug 20, 2026 |
| Linux | libexpat1 Ubuntu 20.04 LTS (focal) | 2.2.9-1ubuntu0.8+esm3 | [libexpat1 2.2.9-1ubuntu0.8+esm3](https://patchblox.com/security/data/fix/linux/ubuntu-focal-libexpat1-2.2.9-1ubuntu0.8-esm3.md) | Aug 20, 2026 |
| Linux | libexpat1 Ubuntu 22.04.1 LTS (jammy) | 2.4.7-1ubuntu0.9 | [libexpat1 2.4.7-1ubuntu0.9](https://patchblox.com/security/data/fix/linux/ubuntu-jammy-libexpat1-2.4.7-1ubuntu0.9.md) | Aug 20, 2026 |
| Linux | libexpat1 Ubuntu 24.04.5 LTS (noble) | 2.6.1-2ubuntu0.6 | [libexpat1 2.6.1-2ubuntu0.6](https://patchblox.com/security/data/fix/linux/ubuntu-noble-libexpat1-2.6.1-2ubuntu0.6.md) | Aug 20, 2026 |
| Linux | libexpat1 Ubuntu 26.04.1 LTS (resolute) | 2.7.4-1ubuntu0.2 | [libexpat1 2.7.4-1ubuntu0.2](https://patchblox.com/security/data/fix/linux/ubuntu-resolute-libexpat1-2.7.4-1ubuntu0.2.md) | Aug 20, 2026 |

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