RUSTSEC-2026-0348: Brotli certificate decompression can allocate up to 1 GiB before authentication

What to do

1. See which version you have: cargo tree -i rustls (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p rustls. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example rustls = "0.23.46") 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

The built-in Brotli certificate decompressor (enabled by the non-default brotli crate feature) bounded the size of the decompressed output, but accepted a peer-selected history window. Brotli's large-window extension was enabled, so a five-byte header could cause an allocation of 1 GiB. A peer can abort a process whose allocator cannot satisfy that request.

No trusted certificate is required by the attacker. Affected configurations are:

• clients that enable the brotli feature and connect to a malicious server, and
• servers that enable the brotli feature and request client certificates.

Applications that do not enable the brotli feature are not affected.

References

• Package: crates.io/crates/rustls
• Advisory: rustsec.org/advisories/RUSTSEC-2026-0348.html
• Web: github.com/rustls/rustls/pull/3297

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 rustls

• Vulnerability HighRUSTSEC-2024-0336 · 2024-04-19`rustls::ConnectionCommon::complete_io` could fall into an infinite loop based on network input
• VulnerabilityRUSTSEC-2024-0399 · 2024-11-22rustls network-reachable panic in `Acceptor::accept`
• Vulnerability MediumRUSTSEC-2026-0285 · 2026-09-14TLS 1.3 handshake messages incorrectly accepted across encryption level boundaries
• Vulnerability MediumRUSTSEC-2026-0346 · 2026-10-14`Debug` impls for client session values expose TLS secrets
• Vulnerability LowRUSTSEC-2026-0347 · 2026-10-14Buffered application data sent to outer server after ECH rejection
• Vulnerability LowRUSTSEC-2026-0349 · 2026-10-14Unauthenticated ServerHello can downgrade later connections to classical key exchange

Full security history of rustls