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Medium, CVSS 6.9amended

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

CVE-2026-45409GHSA-65pc-fj4g-8rjx

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (idna 3.15), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • PyPI idna

    Affected
    < 3.15
    Fixed in
    3.15

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
6.9 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2026-45409: 0.41% estimated exploitation activity in the next 30 days; percentile 35.13 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

This is the same issue as CVE-2024-3651, however the original remediation in 2024 was not a complete fix. Payloads such as "\u0660" N or "\u30fb" N + "\u6f22" utilize the validcontexto function prior to length rejection, and for high values of N will take a long time to process. Impact A specially crafted argument to the idna.encode() function could consume significant resources. This may lead to a denial-of-service. Patches 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). Workarounds 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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from ghsa record GHSA-65pc-fj4g-8rjx

  2. Revision 2 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 1 · created ·

    Published from osv record GHSA-65pc-fj4g-8rjx

Agent brief

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# Patch Next advisory brief: CVE-2026-45409

Advisory: Internationalized Domain Names in Applications (IDNA): Specially crafted inputs to idna.encode() can bypass CVE-2024-3651 fix
IDs: CVE-2026-45409, GHSA-65pc-fj4g-8rjx
Record: https://patchnext.com/advisories/cve-2026-45409 (revision 3, source checked 26 Oct 2026, 13:46 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: idna 3.15. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI idna: affected < 3.15; fixed in 3.15

## Suggested inspection (read-only)
- `python -m pip show idna`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on idna.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-65pc-fj4g-8rjx (OSV record GHSA-65pc-fj4g-8rjx lists affected ranges: PyPI idna >=0 <3.15 (fixed))
- Verified: https://github.com/advisories/GHSA-65pc-fj4g-8rjx (GitHub-reviewed advisory GHSA-65pc-fj4g-8rjx lists affected ranges: PyPI idna >=0 <3.15)

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