RUSTSEC-2026-0044: AWS-LC X.509 Name Constraints Bypass via Wildcard/Unicode CN

What to do

1. See which version you have: cargo tree -i aws-lc-sys (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p aws-lc-sys. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example aws-lc-sys = "0.39.0") or update the dependency that pulls it in.
3. Check again with cargo audit, or paste your Cargo.lock into our Cargo.toml checker.

Details from the advisory

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.

References

• Package: crates.io/crates/aws-lc-sys
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0044.html
• Advisory: github.com/aws/aws-lc-rs/security/advisories/GHSA-394x-vwmw-crm3

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.

Other advisories for aws-lc-sys

• Vulnerability MediumRUSTSEC-2026-0045 · 2026-03-02Timing Side-Channel in AES-CCM Tag Verification in AWS-LC
• Vulnerability HighRUSTSEC-2026-0046 · 2026-03-02PKCS7_verify Certificate Chain Validation Bypass in AWS-LC
• Vulnerability HighRUSTSEC-2026-0047 · 2026-03-02PKCS7_verify Signature Validation Bypass in AWS-LC
• Vulnerability HighRUSTSEC-2026-0048 · 2026-03-19CRL Distribution Point Scope Check Logic Error in AWS-LC

Full security history of aws-lc-sys