GHSA-ggw3-5987-rx77 Pomerium Pre-Auth Memory Exhaustion via Unbounded zstd Decompression in HPKE Callback
SummaryThe 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 53–54CWE**CWE-400**: Uncontrolled Resource Consumption**CWE-1284**: Improper Validation of Specified Quantity in InputDescriptionUnbounded 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`):`WithDecoderLowmem(true)` reduces the decoder's own memory footprint but applies no cap on the output. A 19 KB input can produce 128 MiB of output; a 38 KB input can produce 256 MiB.By contrast, the codebase applies `LimitReader` when decompressing in `internal/zero/api/download.go:75`:The protection is available but not applied to `decodeQueryStringV2`, confirming this is an inconsistent defense.HPKE Does Not Block the Attack — Sender Validation Is Too Late`DecryptURLValues` for the V2 format (`url.go:107–126`):`Open` uses `SetupAuth` (HPKE authenticated mode). It only verifies that `sealed` was created with a key pair whose public half is `senderPublicKey`. Because the attacker supplies both `k` (sender public key) and `q` (sealed payload), they choose a consistent key pair themselves. The `Open` call succeeds with their own freshly-generated keys.Sender identity is validated **after** `DecryptURLValues` returns (`stateless.go:391–397`):The decompression memory spike occurs unconditionally before rejection.Pre-Auth Execution Chain on the Proxy CallbackThe proxy registers the callback route without any session or signature middleware (`proxy/handlers.go:53–54`):For Stateless-flow deployments, `p.Callback` → `authenticateflow.Stateless.Callback` → `hpke.DecryptURLValues` (unbounded decompress) → `validateSenderPublicKey` (rejects). This is by design: the callback endpoint must be pre-auth because it is the landing page after an IdP OAuth redirect.Pomerium's HPKE receiver public key is served publicly and without authentication (`internal/controlplane/http.go:82`):The full attack requires no credentials of any kind.**Self-hosted (Stateful) deployments are NOT affected.** The stateful `Callback` calls `s.VerifySignature(r)` as its very first operation, verifying an HMAC-SHA256 signature over the URL before touching the body. If the signature is missing or invalid, the function returns immediately without decrypting or decompressing anything.Proof of ConceptRepeated calls amplify the effect proportionally. The server-side rejection from `validateSenderPublicKey` does not prevent the allocation.Impact**Pre-auth denial of service** against any Pomerium proxy using the hosted/stateless authenticate flow (Pomerium Zero / `authenticate.pomerium.app`).An attacker who can reach the proxy can allocate hundreds of megabytes of server memory per HTTP request by sending a ~20–40 KB payload.Sustained attack with concurrent requests can exhaust available memory and crash the proxy process, blocking all user access to every application protected by that Pomerium deployment.No credentials, session cookies, or insider access required — only network reachability to the proxy's HTTPS port.Recommended RemediationOption 1: Cap decompressed output size in `decodeQueryStringV2` (preferred)Apply a reasonable upper bound on the decompressed query string. Legitimate HPKE-encrypted query strings contain URL parameters (redirect URIs, scopes, timestamps) and are never more than a few hundred kilobytes:This fixes the root cause at the lowest layer and protects all callers unconditionally.Option 2: Validate sender public key before decompressingRestructure `DecryptURLValues` so the sender's public key is compared against the known authenticate service key before the decompression step is reached. This requires passing the expected public key into `DecryptURLValues` or splitting the decrypt and decompress steps:This eliminates the DoS attack path entirely for the callback endpoint but does not fix the underlying missing bound in `decodeQueryStringV2`, leaving other current or future callers at risk.CreditThis vulnerability was discovered and reported by [bugbunny.ai](https://bugbunny.ai).
CVSS v3.1 score: 7.5