```toml
[advisory]
id = "RUSTSEC-2021-0129"
package = "openssl-src"
aliases = ["CVE-2021-4044", "GHSA-mmjf-f5jw-w72q"]
categories = ["denial-of-service"]
date = "2021-12-14"
url = "https://www.openssl.org/news/secadv/20211214.txt"

[versions]
patched = [">= 300.0.4"]
unaffected = ["< 300.0.0"]
```

# Invalid handling of `X509_verify_cert()` internal errors in libssl

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.