The HPKE V2 URL decode path in "pkg/hpke/url.go" decompresses attacker-controlled zstd data without any size limit. On Pomerium deployments using the stateless authentication flow (Pomerium Zero / hosted authenticate), the proxy's "/.pomerium/callback" endpoint is reachable without credentials and processes attacker-crafted HPKE-encrypted payloads before the sender's identity is validated. Because Pomerium's HPKE receiver public key is publicly served, an attacker can encrypt a decompression bomb, deliver it to the callback endpoint, and cause unbounded memory allocation — crashing or degrading the proxy process.