GHSA-ggw3-5987-rx77: Pomerium Pre-Auth Memory Exhaustion via Unbounded zstd Decompression in HPKE Callback

Summary 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. Severity High (CVSS 3.1: 7.5) CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H - Attack Vector: Network — the /.pomerium/callback route on the proxy service is externally reachable. - Attack Complexity: Low — the receiver public key is publicly available at /.well-known/pomerium/hpke-public-key; no special conditions apply. - Privileges Required: None — the callback endpoint is intentionally pre-authentication (it is the OAuth landing page). - User Interaction: None - Scope: Unchanged — the DoS is confined to the Pomerium proxy process itself. - Confidentiality Impact: None - Integrity Impact: None - Availability Impact: High — repeated attacks can exhaust process memory and crash the proxy. Affected Component - pkg/hpke/url.go — decodeQueryStringV2 (line 171) - internal/authenticateflow/stateless.go — Callback (line 385–393) - proxy/handlers.go — Callback (line 105–107), route registered at line 53–54 CWE - CWE-400: Uncontrolled Resource Consumption - CWE-1284: Improper Validation of Specified Quantity in Input Description Unbounded zstd Decompression in decodeQueryStringV2 pkg/hpke/url.go defines two decoders. The V1 path is plaintext. The V2 path zstd-compresses the query string before encryption. Decoding reverses this with no output size cap (url.go:166–176): var zstdDecoder, _ = zstd.NewReader(nil, zstd.WithDecoderLowmem(true), ) func de