Impact
Anyone using the tendermint-light-client and related packages to perform light client verification (e.g. IBC-rs, Hermes).
At present, the light client does not check that the chain IDs of the trusted and untrusted headers match, resulting in a possible attack vector where someone who finds a header from an untrusted chain that satisfies all other verification conditions (e.g. enough overlapping validator signatures) could fool a light client.
The attack vector is currently theoretical, and no proof-of-concept exists yet to exploit it on live networks.
Patches
Users of the light client-related crates can currently upgrade to v0.28.0.
Workarounds
None