RUSTSEC-2021-0129: Invalid handling of `X509_verify_cert()` internal errors in libssl

What to do

1. See which version you have: cargo tree -i openssl-src (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p openssl-src. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example openssl-src = "300.0.4") 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

Internally libssl in OpenSSL calls X509_verify_cert() on the client side to verify a certificate supplied by a server. That function may return a negative return value to indicate an internal error (for example out of memory). Such a negative return value is mishandled by OpenSSL and will cause an IO function (such as SSL_connect() or SSL_do_handshake()) to not indicate success and a subsequent call to SSL_get_error() to return the value SSL_ERROR_WANT_RETRY_VERIFY. This return value is only supposed to be returned by OpenSSL if the application has previously called SSL_CTX_set_cert_verify_callback(). Since most applications do not do this the SSL_ERROR_WANT_RETRY_VERIFY return value from SSL_get_error() will be totally unexpected and applications may not behave correctly as a result. The exact behaviour will depend on the application but it could result in crashes, infinite loops or other similar incorrect responses.

This issue is made more serious in combination with a separate bug in OpenSSL 3.0 that will cause X509_verify_cert() to indicate an internal error when processing a certificate chain. This will occur where a certificate does not include the Subject Alternative Name extension but where a Certificate Authority has enforced name constraints. This issue can occur even with valid chains.

References

• Package: crates.io/crates/openssl-src
• Advisory: rustsec.org/advisories/RUSTSEC-2021-0129.html
• Web: www.openssl.org/news/secadv/20211214.txt

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 openssl-src

• Vulnerability HighRUSTSEC-2020-0015 · 2020-04-25Crash causing Denial of Service attack
• Vulnerability MediumRUSTSEC-2021-0055 · 2021-05-01NULL pointer deref in signature_algorithms processing
• Vulnerability HighRUSTSEC-2021-0056 · 2021-05-01CA certificate check bypass with X509_V_FLAG_X509_STRICT
• Vulnerability HighRUSTSEC-2021-0057 · 2021-05-01Integer overflow in CipherUpdate
• Vulnerability MediumRUSTSEC-2021-0058 · 2021-05-01Null pointer deref in `X509_issuer_and_serial_hash()`
• Vulnerability CriticalRUSTSEC-2021-0097 · 2021-08-24SM2 Decryption Buffer Overflow
• Vulnerability HighRUSTSEC-2021-0098 · 2021-08-24Read buffer overruns processing ASN.1 strings
• VulnerabilityRUSTSEC-2022-0014 · 2022-03-15Infinite loop in `BN_mod_sqrt()` reachable when parsing certificates

Full security history of openssl-src