RUSTSEC-2019-0029: ChaCha20 counter overflow can expose repetitions in the keystream

What to do

1. See which version you have: cargo tree -i chacha20 (it also shows which of your dependencies pulls it in).
2. Update within your version requirement: cargo update -p chacha20. If the fix is a newer major or minor than your requirement allows, raise it in Cargo.toml (for example chacha20 = "0.2.3") 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 ChaCha20 stream cipher can produce a maximum of 2^32 blocks (~256GB) before the 32-bit counter overflows. Releases of the chacha20 crate prior to v0.2.3 allow generating keystreams larger than this, including seeking past the limit. When this occurs, the keystream is duplicated, with failure modes similar to nonce reuse (i.e. exposure of the XOR of two plaintexts).

The v0.2.3 release now panics in this event, rather than exposing the duplicated keystream. Note this is a "hot fix" solution to the problem and future releases will pursue returning an error in this case.

Users of the chacha20poly1305 crate are unaffected by this as this crate properly asserts the length of the plaintext is less than the maximum allowed (P_MAX as described in RFC 8439 Section 2.8).

References

• Package: crates.io/crates/chacha20
• Advisory: rustsec.org/advisories/RUSTSEC-2019-0029.html
• Web: github.com/RustCrypto/stream-ciphers/pull/64

From the RustSec Advisory Database (CC0-1.0) via the OSV export, downloaded Oct 24, 2026. Severity score computed from the published CVSS vector.