CVE-2026-45409 Internationalized Domain Names in Applications (IDNA): Specially crafted inputs to idna.encode() can bypass CVE-2024-3651 fix
Attackers can cause denial of service in applications using the Python IDNA library by sending specially crafted inputs that consume excessive resources.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-45409?
CVE-2026-45409 is a medium-severity denial of service vulnerability in kjd idna, published on 5 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-1333
Which versions are affected by CVE-2026-45409?
3 open source packages ship an affected version, including idna. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
idna PyPI | All versions | 3.15 |
idna PyPI | All versions | 3.15 |
idna PyPI | All versions | 3.15 |
Linux distribution packages (27)
python3-idna AlmaLinux:10 | All versions | 3.7-6.el10_2 |
python3-idna AlmaLinux:8 | All versions | 2.5-8.el8_10 |
python3-idna AlmaLinux:9 | All versions | 2.10-8.el9_8 |
python-idna Red Hat:enterprise_linux:10.2 | All versions | 0:3.7-6.el10_2 |
python3-idna Red Hat:enterprise_linux:10.2 | All versions | 0:3.7-6.el10_2 |
python-idna Red Hat:enterprise_linux:8::baseos | All versions | 0:2.5-8.el8_10 |
python3-idna Red Hat:enterprise_linux:8::baseos | All versions | 0:2.5-8.el8_10 |
python-idna Red Hat:enterprise_linux:9::baseos | All versions | 0:2.10-8.el9_8 |
python3-idna Red Hat:enterprise_linux:9::baseos | All versions | 0:2.10-8.el9_8 |
python-idna Red Hat:hummingbird:1 | All versions | 0:3.18-1.hum1 |
python3-idna Red Hat:hummingbird:1 | All versions | 0:3.18-1.hum1 |
python-idna Rocky Linux:10 | All versions | 0:3.7-6.el10_2 |
python-idna Rocky Linux:8 | All versions | 0:2.5-8.el8_10 |
python-idna Rocky Linux:9 | All versions | 0:2.10-8.el9_8 |
python-idna SUSE:Linux Enterprise Micro 5.3 | All versions | 2.6-150000.3.9.1 |
python-idna SUSE:Linux Enterprise Micro 5.4 | All versions | 2.6-150000.3.9.1 |
python-idna SUSE:Linux Enterprise Micro 5.5 | All versions | 2.6-150000.3.9.1 |
python-idna SUSE:Linux Enterprise Module for Basesystem 15 SP7 | All versions | 2.6-150000.3.9.1 |
python-idna SUSE:Linux Enterprise Module for Public Cloud 12 | All versions | 2.5-3.16.1 |
python-idna SUSE:Linux Enterprise Module for Public Cloud 15 SP4 | All versions | 3.4-150400.11.13.1 |
python-idna SUSE:Linux Enterprise Module for Python 3 15 SP7 | All versions | 3.4-150400.11.13.1 |
python-idna SUSE:Linux Enterprise Server 16.0 | All versions | 3.10-160000.3.1 |
python-idna SUSE:Linux Enterprise Server LTSS Extended Security 12 SP5 | All versions | 2.5-3.16.1 |
python-idna SUSE:Linux Enterprise Server for SAP applications 16.0 | All versions | 3.10-160000.3.1 |
python-idna SUSE:Linux Micro 6.2 | All versions | 3.10-160000.3.1 |
python-idna openSUSE:Leap 16.0 | All versions | 3.10-160000.3.1 |
python-idna openSUSE:Tumbleweed | All versions | 3.15-1.1 |
Products include Kjd Internationalized Domain Names in Applications.
How do I fix CVE-2026-45409?
Upgrade idna to 3.15 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-45409 being exploited?
There is no known exploitation of CVE-2026-45409 in the wild and no public exploit code.
Other vulnerabilities in kjd idna
Sources
Also known as ALSA-2026:54290, ALSA-2026:54481, ALSA-2026:54484, BELL-CVE-2026-45409, CLEANSTART-2026-CS22541, CLEANSTART-2026-WY69230 and 55 more.