CVE-2026-39821 Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna
Critical 9.6
Published
Updated
Check this codebase for CVE-2026-39821 and fix it if it is affected.
What it is: CVE-2026-39821 is a critical vulnerability in golang.org/x/net/idna, published on 22 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-39821 in the wild and no public exploit code.
Affected packages and fixed versions:
- golang.org/x/net (Go): affected all versions; fixed in 0.55.0
- stdlib (Go): affected all versions; fixed in 1.25.13, 1.26.6, 1.27.0-rc.3
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-2026-39821/ (Markdown: https://insights.sbomify.com/CVE-2026-39821/index.md)
The Go idna library incorrectly accepts ASCII-only Punycode labels, allowing attackers to bypass hostname-based privilege checks and escalate privileges.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-39821?
CVE-2026-39821 is a critical vulnerability in golang.org/x/net/idna, published on 22 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Official description
The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an ASCII-only label. For example, ToUnicode("xn--example-.com") incorrectly returns the name "example.com" rather than an error. This behavior can lead to privilege escalation in programs using the idna package. For example, a program which performs privilege checks on the ASCII hostname may reject "example.com" but permit "xn--example-.com". If that program subsequently converts the ASCII hostname to Unicode, it will inadvertently permits access to the Unicode name "example.com".
Weakness: CWE-1289
Which versions are affected by CVE-2026-39821?
2 open source packages ship an affected version, including golang.org/x/net and stdlib. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
golang.org/x/net Go | All versions | 0.55.0 |
stdlib Go | All versions | 1.25.13, 1.26.6, 1.27.0-rc.3 |
Linux distribution packages (38)
git-lfs AlmaLinux:10 | All versions | 3.7.1-5.el10_2.5 |
grafana-pcp AlmaLinux:10 | All versions | 5.3.0-7.el10_2 |
git-lfs AlmaLinux:8 | All versions | 3.4.1-11.el8_10 |
grafana AlmaLinux:8 | All versions | 9.2.10-31.el8_10 |
grafana-pcp AlmaLinux:8 | All versions | 5.1.1-16.el8_10 |
grafana-selinux AlmaLinux:8 | All versions | 9.2.10-31.el8_10 |
git-lfs AlmaLinux:9 | All versions | 3.7.1-4.el9_8.1 |
grafana-pcp AlmaLinux:9 | All versions | 5.1.1-17.el9_8 |
git-lfs Red Hat:enterprise_linux:10.2 | All versions | 0:3.7.1-5.el10_2.5 |
git-lfs-debuginfo Red Hat:enterprise_linux:10.2 | All versions | 0:3.7.1-5.el10_2.5 |
git-lfs-debugsource Red Hat:enterprise_linux:10.2 | All versions | 0:3.7.1-5.el10_2.5 |
grafana-pcp Red Hat:enterprise_linux:10.2 | All versions | 0:5.3.0-7.el10_2 |
grafana-pcp-debuginfo Red Hat:enterprise_linux:10.2 | All versions | 0:5.3.0-7.el10_2 |
grafana-pcp-debugsource Red Hat:enterprise_linux:10.2 | All versions | 0:5.3.0-7.el10_2 |
git-lfs Red Hat:enterprise_linux:8::appstream | All versions | 0:3.4.1-11.el8_10 |
git-lfs-debuginfo Red Hat:enterprise_linux:8::appstream | All versions | 0:3.4.1-11.el8_10 |
git-lfs-debugsource Red Hat:enterprise_linux:8::appstream | All versions | 0:3.4.1-11.el8_10 |
grafana Red Hat:enterprise_linux:8::appstream | All versions | 0:9.2.10-31.el8_10 |
grafana-debuginfo Red Hat:enterprise_linux:8::appstream | All versions | 0:9.2.10-31.el8_10 |
grafana-debugsource Red Hat:enterprise_linux:8::appstream | All versions | 0:9.2.10-31.el8_10 |
grafana-pcp Red Hat:enterprise_linux:8::appstream | All versions | 0:5.1.1-16.el8_10 |
grafana-pcp-debuginfo Red Hat:enterprise_linux:8::appstream | All versions | 0:5.1.1-16.el8_10 |
grafana-pcp-debugsource Red Hat:enterprise_linux:8::appstream | All versions | 0:5.1.1-16.el8_10 |
grafana-selinux Red Hat:enterprise_linux:8::appstream | All versions | 0:9.2.10-31.el8_10 |
git-lfs Red Hat:enterprise_linux:9::appstream | All versions | 0:3.7.1-4.el9_8.1 |
git-lfs-debuginfo Red Hat:enterprise_linux:9::appstream | All versions | 0:3.7.1-4.el9_8.1 |
git-lfs-debugsource Red Hat:enterprise_linux:9::appstream | All versions | 0:3.7.1-4.el9_8.1 |
grafana-pcp Red Hat:enterprise_linux:9::appstream | All versions | 0:5.1.1-17.el9_8 |
grafana-pcp-debuginfo Red Hat:enterprise_linux:9::appstream | All versions | 0:5.1.1-17.el9_8 |
grafana-pcp-debugsource Red Hat:enterprise_linux:9::appstream | All versions | 0:5.1.1-17.el9_8 |
ose-aws-ecr-image-credential-provider Red Hat:openshift:4.16::el8 | All versions | 0:4.16.0-202610062120.p2.gac983ec.assembly.stream.el8 |
ose-aws-ecr-image-credential-provider Red Hat:openshift:4.16::el9 | All versions | 0:4.16.0-202610062120.p2.gac983ec.assembly.stream.el9 |
openshift-clients Red Hat:openshift:4.20::el8 | All versions | 0:4.20.0-202608101541.p2.g1367d50.assembly.stream.el8 |
openshift-clients-redistributable Red Hat:openshift:4.20::el8 | All versions | 0:4.20.0-202608101541.p2.g1367d50.assembly.stream.el8 |
openshift-clients Red Hat:openshift:4.20::el9 | All versions | 0:4.20.0-202608101541.p2.g1367d50.assembly.stream.el9 |
openshift-clients-redistributable Red Hat:openshift:4.20::el9 | All versions | 0:4.20.0-202608101541.p2.g1367d50.assembly.stream.el9 |
ose-azure-acr-image-credential-provider Red Hat:openshift:4.22::el8 | All versions | 0:4.22.0-202606291123.p2.ga17a3e3.assembly.stream.el8 |
ose-azure-acr-image-credential-provider Red Hat:openshift:4.22::el9 | All versions | 0:4.22.0-202606291123.p2.ga17a3e3.assembly.stream.el9 |
Products include Golang Net.
How do I fix CVE-2026-39821?
Upgrade golang.org/x/net to 0.55.0 or later.
Is CVE-2026-39821 being exploited?
There is no known exploitation of CVE-2026-39821 in the wild and no public exploit code.
Other vulnerabilities in golang.org/x/net/idna
- CVE-2026-25680 Invoking denial of service when parsing arbitrary HTML in golang.org/x/net/html
- CVE-2026-25681 Invoking incorrect handling of character references in DOCTYPE nodes in golang.org/x/net/html
- CVE-2026-27136 Invoking duplicate attributes can cause XSS in golang.org/x/net/html
- CVE-2026-42502 Invoking incorrect handling of HTML elements in foreign content in golang.org/x/net/html
- CVE-2026-42506 Invoking incorrect handling of namespaced elements in foreign content in golang.org/x/net/html
Sources
Also known as ALSA-2026:30853, ALSA-2026:30854, ALSA-2026:30855, ALSA-2026:35826, ALSA-2026:35829, ALSA-2026:35830 and 2299 more.