# RHSA-2026:63134: osbuild-composer security update

RHSA-2026:63134, released June 26, 2026, fixes 5 CVEs in osbuild-composer (version 134.1-10.el10_0). None of them is reported as exploited. Patch Radar priority: Urgent.

- Web page: https://patchblox.com/security/fix/linux/rhsa-2026-63134
- Platform: Linux
- Released: Jun 26, 2026
- Priority: Urgent (Closes CVE-2026-39821, rated Critical)
- CVEs: 5, exploited: 0, on CISA KEV: 0
- Vendor advisory: https://access.redhat.com/errata/RHSA-2026:63134

## Applies to

| Product | Release | Fixed in |
|---|---|---|
| [osbuild-composer](https://patchblox.com/security/data/product/linux/osbuild-composer.md) | RHEL 10 | 134.1-10.el10_0 |

## CVEs this fix closes

| CVE | Priority | Title | CVSS | EPSS | Exploitation |
|---|---|---|---:|---:|---|
| [CVE-2026-39821](https://patchblox.com/security/data/cve/CVE-2026-39821.md) | Urgent | Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna | 10.0 | 0.0183 |  |
| [CVE-2026-55677](https://patchblox.com/security/data/cve/CVE-2026-55677.md) | High | osbuild-composer: A service for building customized OS artifacts, such as VM images and OSTree commits, that uses osbuild under the hood | 8.2 | 0.0052 |  |
| [CVE-2026-25681](https://patchblox.com/security/data/cve/CVE-2026-25681.md) | High | Invoking  incorrect handling of character references in DOCTYPE nodes in golang.org/x/net/html | 8.7 | 0.0022 |  |
| [CVE-2026-27136](https://patchblox.com/security/data/cve/CVE-2026-27136.md) | High | Invoking  duplicate attributes can cause XSS in golang.org/x/net/html | 8.7 | 0.0022 |  |
| [CVE-2026-42502](https://patchblox.com/security/data/cve/CVE-2026-42502.md) | High | Invoking  incorrect handling of HTML elements in foreign content in golang.org/x/net/html | 8.2 | 0.0022 |  |

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Data as of 2026-11-07T12:40:37Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo