Rustls: TLS 1.3 handshake messages incorrectly accepted across encryption level boundaries
- Published
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- The rating the maker of the software gives it. Shown when it differs from the CVSS level.What do these mean? →
- Medium (ghsa)
- Fix
- Fixed in 0.23.45
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
Rustls, the TLS library written in Rust, is affected by this flaw in its TLS 1.3 handshake processing. Because it can accept handshake messages sent in plaintext where they should have been encrypted, a network-position attacker could inject such messages, though the handshake transcript remains authenticated so the attacker cannot alter or complete the handshake.
Summarized by anthropic/claude-haiku-5.5. Verify details against the original sources.
Read the original description
Rustls accepted TLS 1.3 handshake messages sent at the wrong encryption level when they followed a key-changing message in the same record. The handshake transcript is still authenticated, so a network-position attacker cannot use this to alter or complete a handshake; the practical effect is that a peer could send handshake messages that should be encrypted in plaintext without rustls rejecting the connection. This issue affects rustls versions 0.23.13 through 0.23.44 inclusive. Original report Summary Rustls (tested version 0.23.44, and it appears still an issue on latest master though I have not tested this) accepts a TLS 1.3 server flight where a plaintext EncryptedExtensions is packed into the same record as the ServerHello. This is very similar to Go's CVE-2025-61730 fixed in https://github.com/golang/go/commit/5046bdf8a612b35a2c1a9e168054c1d5c65e7dd7 Details It appears `Deframer::aligned` considers itself aligned as long as all messages remaining in the buffer are complete. Rustls processes the ServerHello, installs the handshake keys, then pulls the complete (plaintext) EncryptedExtensions out of the buffer. This is insufficient per RFC 8446 section 5.1's > Handshake messages MUST NOT span key changes. Implementations MUST verify that all messages immediately preceding a key change align with a record boundary; if not, then they MUST terminate the connection with an "unexpected_message" alert. Because the ClientHello, EndOfEarlyData, ServerHello, Finished, and KeyUpdate messages can immediately precede a key change, implementations MUST send these messages in alignment with a record boundary. Impact An on path attacker can inject plaintext messages that are accepted. Tool Use Disclosure This was discovered by a new TLS/DTLS test suite currently under construction. The specific testcase was inspired by the Go CVE. Other tested implementations (Botan, OpenSSL, BoringSSL, Go, wolfSSL) reject this protocol flow. Daybreak Blue reviewed the rustls code to identify probable root cause.
Timeline
- Advisory published
- Record last updated
Affected versions
- rustlscrates.io
From 0.23.13 · fixed in 0.23.45