CVE-2026-45409 — idna
MEDIUMCVE-2026-45409 is a medium-severity (CVSS 5.3) CWE-1333 vulnerability in idna. A fix is available for idna — see the affected versions and patch details below.
Internationalized Domain Names in Applications (IDNA): Specially crafted inputs to idna.encode() can bypass CVE-2024-3651 fix
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-45409.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-45409 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 380,526 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
idnaReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Internationalized Domain Names in Applications (IDNA) for Python provides support for Internationalized Domain Names in Applications (IDNA) and Unicode IDNA Compatibility Processing. In versions prior to 3.15, payloads such as "\u0660" * N or "\u30fb" * N + "\u6f22" utilize the valid_contexto function prior to length rejection, and for high values of N will take a long time to process. This is the same issue as CVE-2024-3651, however the original remediation in 2024 was not a complete fix. A specially crafted argument to the idna.encode() function could consume significant resources. This may lead to a denial-of-service. Starting in version 3.14, the function rejects long inputs as soon as practicable prior to any further processing to minimize resource consumption. In version 3.15, this approach was extended to lesser used alternate functions (i.e. per-label conversions and codec support). A workaround is available. Domain names cannot exceed 253 characters in length. If this length limit is enforced prior to passing the domain to the idna.encode() function, it should no longer consume significant resources. This is triggered by arbitrarily large inputs that would not occur in normal usage, but may be passed to the library assuming there is no preliminary input validation by the higher-level application.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | idna | all versions | 3.15pip install --upgrade 'idna==3.15' |
Affected Products
internationalized domain names in applicationskjdDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for idna, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update idna to 3.15 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-45409 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
A flaw was found in the python-idna library. This is an incomplete fix for CVE-2024-3651. The idna.encode() function performs contextual validation checks (valid_contexto) before rejecting oversized inputs, allowing specially crafted long payloads to cause excessive CPU consumption and lead to a denial of service. The…
To mitigate this denial-of-service vulnerability, applications utilizing the `idna` Python library should implement input validation to ensure that domain names do not exceed the standard 253-character length limit before being passed to the `idna.encode()` function. This operational control prevents the processing of excessively long inputs that could lead to resource exhaustion and service unavailability. Applications that pass user-controlled data directly to `idna.encode()` without validation checks (e.g.: DNS length < 254 chars) are affected.Source: Red Hat security advisory for CVE-2026-45409 (CC BY 4.0)
| Product | Fixed in | Advisory |
|---|---|---|
| Red Hat Enterprise Linux 10 | python-idna-0:3.7-6.el10_2 | RHSA-2026:54481 |
| Red Hat Enterprise Linux 8 | python-idna-0:2.5-8.el8_10 | RHSA-2026:54290 |
| Red Hat Enterprise Linux 9 | python-idna-0:2.10-8.el9_8 | RHSA-2026:54484 |
| Red Hat Discovery 2 | discovery/discovery-ui-rhel9:1788206196 | RHSA-2026:61783 |
| Red Hat Hardened Images | jaeger-main-2.19.0-1.hum1 | RHSA-2026:25039 |
| Red Hat Hardened Images | python-idna-main-3.18-1.hum1 | RHSA-2026:25503 |
| Red Hat Hardened Images | python-rpds-py-main-2026.6.3-1.hum1 | RHSA-2026:34119 |
| Red Hat Openshift Data Foundation 4.18 | odf4/cephcsi-rhel9:1786705347 | RHSA-2026:56431 |
Frequently Asked Questions
Is CVE-2026-45409 in your dependencies?
Find it across PyPI, including transitive dependencies.