RUSTSEC-2024-0421: `idna` accepts Punycode labels that do not produce any non-ASCII when decoded

What to do

1. See which version you have: cargo tree -i idna (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p idna. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example idna = "1.0.0") 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

idna 0.5.0 and earlier accepts Punycode labels that do not produce any non-ASCII output, which means that either ASCII labels or the empty root label can be masked such that they appear unequal without IDNA processing or when processed with a different implementation and equal when processed with idna 0.5.0 or earlier.

Concretely, example.org and xn--example-.org become equal after processing by idna 0.5.0 or earlier. Also, example.org.xn-- and example.org. become equal after processing by idna 0.5.0 or earlier.

In applications using idna (but not in idna itself) this may be able to lead to privilege escalation when host name comparison is part of a privilege check and the behavior is combined with a client that resolves domains with such labels instead of treating them as errors that preclude DNS resolution / URL fetching and with the attacker managing to introduce a DNS entry (and TLS certificate) for an xn---masked name that turns into the name of the target when processed by idna 0.5.0 or earlier.

Remedy

Upgrade to idna 1.0.3 or later, if depending on idna directly, or to url 2.5.4 or later, if depending on idna via url. (This issue was fixed in idna 1.0.0, but versions earlier than 1.0.3 are not recommended for other reasons.)

When upgrading, please take a moment to read about alternative Unicode back ends for idna.

If you are using Rust earlier than 1.81 in combination with SQLx 0.8.2 or earlier, please also read an issue about combining them with url 2.5.4 and idna 1.0.3.

Additional information

This issue resulted from idna 0.5.0 and earlier implementing the UTS 46 specification literally on this point and the specification having this bug. The specification bug has been fixed in revision 33 of UTS 46.

Acknowledgements

Thanks to kageshiron for recognizing the security implications of this behavior.

References

• Package: crates.io/crates/idna
• Advisory: rustsec.org/advisories/RUSTSEC-2024-0421.html
• Web: bugzilla.mozilla.org/show_bug.cgi?id=1887898

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