AWS-LC X.509 Name Constraints Bypass via Wildcard/Unicode CN

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### Summary AWS-LC is an open-source, general-purpose cryptographic library. ### Impact A logic error in CN (Common Name) validation allows certificates with wildcard or raw UTF-8 Unicode CN values to bypass name constraints enforcement. The `cn2dnsid` function does not recognize these CN patterns as valid DNS identifiers, causing `NAME_CONSTRAINTS_check_CN` to skip validation. However, `X509_check_host` accepts these CN values when no dNSName SAN is present, allowing certificates to bypass name constraints while still being used for hostname verification. Customers of AWS services do not need to take action. Applications using aws-lc-sys should upgrade to the most recent release of aws-lc-sys. ### Impacted versions: * aws-lc-sys >= v0.32.0, < v0.39.0 ### Patches The patch is included in aws-lc-sys v0.39.0. ### Workarounds Applications that set `X509_CHECK_FLAG_NEVER_CHECK_SUBJECT` to disable CN fallback are not affected. Applications that only encounter certificates with dNSName SANs (standard for public WebPKI) are also not affected. Otherwise, there is no workaround and applications using aws-lc-sys should upgrade to the most recent releases of aws-lc-sys. ### References If you have any questions or comments about this advisory, we ask that you contact AWS Security via our [vulnerability reporting page](https://aws.amazon.com/security/vulnerability-reporting/) or directly via email to [[email protected]](mailto:[email protected]). Please do not create a public GitHub issue. ### Credits Oleh Konko from 1seal (https://1seal.org/)

Source published

20 Mar 2026 20:34:59 UTC

Source modified

10 Sep 2026 03:50:36 UTC

Saved here

24 Oct 2026 13:19:54 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

Source: osv · GitHub Advisory Database · CC-BY-4.0. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.

CVSS_V4 · CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

Publisher rating · HIGH

A logic error in CN (Common Name) validation allows certificates with wildcard or raw UTF-8 Unicode CN values to bypass name constraints enforcement. The `cn2dnsid` function does not recognize these CN patterns as valid DNS identifiers, causing `NAME_CONSTRAINTS_check_CN` to skip validation. However, `X509_check_host` accepts these CN values when no dNSName SAN is present, allowing certificates to bypass name constraints while still being used for hostname verification. Customers of AWS services do not need to take action. Applications using `aws-lc-sys` should upgrade to the most recent release of `aws-lc-sys`. ## Workarounds Applications that set `X509_CHECK_FLAG_NEVER_CHECK_SUBJECT` to disable CN fallback are not affected. Applications that only encounter certificates with dNSName SANs (standard for public WebPKI) are also not affected. Otherwise, there is no workaround and applications using `aws-lc-sys` should upgrade to the most recent releases of `aws-lc-sys`.

Source published

19 Mar 2026 12:00:00 UTC

Source modified

9 Nov 2026 11:00:03 UTC

Saved here

9 Nov 2026 12:02:17 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

Source: osv · OSV · upstream source license applies. The publisher's wording is preserved. ChangeIntel groups the information and labels changes. The JSON record includes additional source fields.