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Internationalized Domain Names in Applications (IDNA): Specially crafted inputs to idna.encode() can bypass CVE-2024-3651 fix

Published Jun 5, 2026

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 products

Remediation

Red Hat statement

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 fix in version 3.14 rejects long inputs early before further processing, and version 3.15 extends this approach to alternate per-label conversion and codec functions.

Red Hat mitigation

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.

Weaknesses (2)

CWE-1333

CWE-770

Allocation of Resources Without Limits

References (7)

Change history (0)

No recorded changes yet.

Sources
CVE.org / MITRE
Status PUBLISHED
Assigner GitHub_M
Published Jun 5, 2026
Updated Jun 8, 2026
Reserved May 12, 2026
Link CVE-2026-45409
CISA Vulnrichment
Updated Jun 8, 2026
NVD
Status Analyzed
Modified Jul 23, 2026
Link CVE-2026-45409
Red Hat
Severity Moderate
Public date Jun 5, 2026
Links CVE-2026-45409 - Bugzilla
GHSA-65PC-FJ4G-8RJX