Go · github.com/edgelesssys/contrast
Constrata's coordinator transit engine `ciphertextContainer.UnmarshalJSON` panics on attacker-controlled short ciphertexts
ciphertextContainer.UnmarshalJSON decodes the third :-separated component of a vault:vX:base64... ciphertext and then unconditionally takes a 12-byte prefix slice for the AES-GCM nonce: c.nonce = fullCiphertext[:aesGCMNonceSize]. If the decoded blob is shorter than 12 bytes, the slice expression panics. The panic happens before any cryptographic operation, while the JSON body of the request is still being parsed inside the request handler. Because the handler is invoked from net/http's standard handler goroutine, the panic is recovered to a 500 response, but the request handler aborts mid-execution and the recovered panic appears in the Coordinator's logs. An authenticated workload that holds a valid mesh certificate for any WorkloadSecretID can trigger the panic at will, producing log spam, request-failure metrics, and a slow but cheap denial of service against the transit-engine endpoint.
coordinator/internal/transitengineapi/crypto.go:64-88:
// UnmarshalJSON umarshalls a json string to a ciphertextContainer holding the version prefix,
// decoded base64 nonce and ciphertext.
func (c *ciphertextContainer) UnmarshalJSON(data []byte) error {
var encoded string
if err := json.Unmarshal(data, &encoded); err != nil {
return err
}
// Split "vault:vX:base64" format
parts := strings.SplitN(encoded, ":", 3)
if len(parts) < 3 {
return fmt.Errorf("invalid ciphertext format")
}
version, err := extractVersion(parts[1])
if err != nil {
return fmt.Errorf("ciphertext version: %w", err)
}
c.keyVersion = version
fullCiphertext, err := base64.StdEncoding.DecodeString(parts[2])
if err != nil {
return fmt.Errorf("decoding ciphertext: %w", err)
}
c.nonce = fullCiphertext[:aesGCMNonceSize] // PANIC when len(fullCiphertext) < 12
c.ciphertext = fullCiphertext[aesGCMNonceSize:]
return nil
}
aesGCMNonceSize = 12 (defined at line 33). There is no length check on fullCiphertext. If parts[2] decodes to fewer than 12 bytes (which happens for any base64 string shorter than ~16 characters), the slice expression fullCiphertext[:aesGCMNonceSize] triggers Go's runtime panic runtime error: slice bounds out of range [:12] with length N.
UnmarshalJSON is reached from parseRequest:
// coordinator/internal/transitengineapi/transitengineapi.go:292-302
func parseRequest(r *http.Request, into any) error {
defer r.Body.Close()
if err := validateContentType(r); err != nil {
return err
}
if err := json.NewDecoder(r.Body).Decode(into); err != nil {
return err
}
return nil
}
which is called inside getDecryptHandler (line 178-237) before any other processing.
The transit-engine HTTP server (transitengineapi.go:74-100) configures tls.RequireAndVerifyClientCert with the Coordinator's mesh CA pool. The handler is wrapped by authorizationMiddleware (line 348-357) which calls authorizeWorkloadSecret (line 241-254). That function reads the WorkloadSecretOID extension from the peer cert and requires it to match the URL path's {name} segment.
Any workload that has gone through the normal initializer / meshapi flow (coordinator/internal/meshapi/meshapi.go:71-119) and has a non-empty WorkloadSecretID in its PolicyEntry is issued a mesh cert with the matching extension, so the path-name authorisation is automatically satisfied for whichever workloadSecretID the manifest assigned to that workload. There is no rate limiting, no proof-of-work, and no audit log on triggering the panic.
net/http wraps each handler in a recovered goroutine, so the panic does not crash the Coordinator process. Instead:
http: panic serving <peer>: runtime error: slice bounds out of range to stderr together with a goroutine stack trace.http.Server.serve calls c.close() in the recovery path).promRegistry) record the request as failed.A workload that wants to amplify the impact can:
/version endpoint.The panic also avoids returning a JSON error body to the caller, so callers that depend on a structured error are forced into a less informative failure mode (HTTP-level connection close).
The bug is deterministic. Drop the following test into coordinator/internal/transitengineapi/crypto_test.go:
func TestCiphertextContainer_UnmarshalJSON_ShortBlobPanics(t *testing.T) {
// "AAAA" base64-decodes to 3 bytes, well under aesGCMNonceSize=12.
body := []byte(`"vault:v1:AAAA"`)
defer func() {
if r := recover(); r == nil {
t.Fatalf("expected panic, got nil")
}
}()
var c ciphertextContainer
_ = c.UnmarshalJSON(body) // panics: slice bounds out of range [:12] with length 3
}
End-to-end against a running Coordinator (omitted for static review; would require a Contrast cluster and a mesh-certificate-holding workload):
$ curl -k --cert workload.crt --key workload.key \
-H 'Content-Type: application/json' \
-d '{"ciphertext":"vault:v1:AAAA","associated_data":""}' \
https://coordinator:8200/v1/transit/decrypt/<my-workload-secret-id>
# Connection: closed without HTTP response body.
# Coordinator log:
# http: panic serving 10.0.0.5:54321: runtime error: slice bounds out of range [:12] with length 3
# goroutine 4711 [running]:
# net/http.(*conn).serve.func1(...)
# net/http/server.go:1883 +0xb0
# panic({0x...?, 0x...?})
# runtime/panic.go:770 +0x132
# github.com/edgelesssys/contrast/coordinator/internal/transitengineapi.(*ciphertextContainer).UnmarshalJSON(...)
# coordinator/internal/transitengineapi/crypto.go:85 +0x...
net/http's panic recovery, but each panicked request consumes CPU for the recovery / stack dump and floods the operator's logs.transitengineapi package, and exact line numbers of internal source. This is a low-grade fingerprint, but it is leaked even to operators who would normally only see the binary version through controlled means.CVSS rationale: AV:N, AC:L, PR:L (any workload with a transit-engine permission can do this), UI:N, S:U, C:N / I:N / A:L (low availability impact: log noise + per-request CPU cost; no full DoS because Go's HTTP panic recovery keeps the process up). Score 3.1.
Validate the decoded length before slicing. The minimal change at coordinator/internal/transitengineapi/crypto.go:81-87:
fullCiphertext, err := base64.StdEncoding.DecodeString(parts[2])
if err != nil {
return fmt.Errorf("decoding ciphertext: %w", err)
}
if len(fullCiphertext) < aesGCMNonceSize {
return fmt.Errorf("ciphertext is too short: got %d bytes, expected at least %d for the nonce", len(fullCiphertext), aesGCMNonceSize)
}
c.nonce = fullCiphertext[:aesGCMNonceSize]
c.ciphertext = fu
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Check my repoSources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.