RUSTSEC-2025-0004: ssl::select_next_proto use after free

What to do

1. See which version you have: cargo tree -i openssl (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p openssl. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example openssl = "0.10.70") 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

In openssl versions before 0.10.70, ssl::select_next_proto can return a slice pointing into the server argument's buffer but with a lifetime bound to the client argument. In situations where the server buffer's lifetime is shorter than the client buffer's, this can cause a use after free. This could cause the server to crash or to return arbitrary memory contents to the client.

openssl 0.10.70 fixes the signature of ssl::select_next_proto to properly constrain the output buffer's lifetime to that of both input buffers.

In standard usage of ssl::select_next_proto in the callback passed to SslContextBuilder::set_alpn_select_callback, code is only affected if the server buffer is constructed *within* the callback. For example:

Not vulnerable - the server buffer has a 'static lifetime:

builder.set_alpn_select_callback(|_, client_protos| {
    ssl::select_next_proto(b"\x02h2", client_protos).ok_or_else(AlpnError::NOACK)
});

Not vulnerable - the server buffer outlives the handshake:

let server_protos = b"\x02h2".to_vec();
builder.set_alpn_select_callback(|_, client_protos| {
    ssl::select_next_proto(&server_protos, client_protos).ok_or_else(AlpnError::NOACK)
});

Vulnerable - the server buffer is freed when the callback returns:

builder.set_alpn_select_callback(|_, client_protos| {
    let server_protos = b"\x02h2".to_vec();
    ssl::select_next_proto(&server_protos, client_protos).ok_or_else(AlpnError::NOACK)
});

References

• Package: crates.io/crates/openssl
• Advisory: rustsec.org/advisories/RUSTSEC-2025-0004.html
• Web: github.com/sfackler/rust-openssl/pull/2360
• Web: github.com/sfackler/rust-openssl/pull/2360

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

• Vulnerability HighRUSTSEC-2016-0001 · 2016-11-05SSL/TLS MitM vulnerability due to insecure defaults
• Vulnerability CriticalRUSTSEC-2018-0010 · 2018-06-01Use after free in CMS Signing
• VulnerabilityRUSTSEC-2023-0022 · 2023-03-24`openssl` `X509NameBuilder::build` returned object is not thread safe
• VulnerabilityRUSTSEC-2023-0023 · 2023-03-24`openssl` `SubjectAlternativeName` and `ExtendedKeyUsage::other` allow arbitrary file read
• VulnerabilityRUSTSEC-2023-0024 · 2023-03-24`openssl` `X509Extension::new` and `X509Extension::new_nid` null pointer dereference
• VulnerabilityRUSTSEC-2023-0044 · 2023-06-20`openssl` `X509VerifyParamRef::set_host` buffer over-read
• UnsoundRUSTSEC-2023-0072 · 2023-11-23`openssl` `X509StoreRef::objects` is unsound
• VulnerabilityRUSTSEC-2024-0357 · 2024-07-21`MemBio::get_buf` has undefined behavior with empty buffers

Full security history of openssl