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PatchSiren cyber security CVE debrief

CVE-2026-45409 kjd CVE debrief

CVE-2026-45409 is a denial-of-service vulnerability affecting Internationalized Domain Names in Applications (IDNA) for Python, specifically versions prior to 3.15. The vulnerability arises from the handling of specially crafted arguments to the `idna.encode()` function, which could consume significant resources. This issue is related to CVE-2024-3651 but was not fully addressed in the previous remediation. The vulnerability is triggered by arbitrarily large inputs that would not occur in normal usage but may be passed to the library without preliminary input validation by the higher-level application.

Vendor
kjd
Product
idna
CVSS
MEDIUM 6.9
CISA KEV
Not listed in stored evidence
Original CVE published
2026-06-05
Original CVE updated
2026-07-23
Advisory published
2026-06-05
Advisory updated
2026-07-23

Who should care

Developers and administrators using Internationalized Domain Names in Applications (IDNA) for Python, especially those who handle domain names that may be passed to the `idna.encode()` function without proper validation.

Technical summary

The `idna.encode()` function in versions of IDNA for Python prior to 3.15 does not efficiently handle long inputs. Payloads such as a large number of Unicode characters can cause the function to consume significant resources, leading to a denial-of-service. This is due to the function's processing of inputs before rejecting them based on length. Starting with version 3.14, the function was modified to reject long inputs sooner to minimize resource consumption. Version 3.15 extended this approach to other functions.

Defensive priority

MEDIUM

Recommended defensive actions

  • Update to version 3.15 or later of Internationalized Domain Names in Applications (IDNA) for Python.
  • Enforce a domain name length limit of 253 characters before passing the domain to the `idna.encode()` function.

Evidence notes

The vulnerability is documented in the official CVE record [cve-org] and detailed in the NVD [nvd]. A security advisory is available [ref-4].

Sources and references

Verified primary and authoritative sources

  • CVE-2026-45409 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-45409

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-45409 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-45409

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.