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Fix heap buffer overflow in AES key unwrap #133

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Fix heap buffer overflow in AES key unwrap #133

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9 changes: 9 additions & 0 deletions src/jwe.c
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Original file line number Diff line number Diff line change
Expand Up @@ -591,6 +591,15 @@ _cjose_jwe_decrypt_ek_aes_kw(_jwe_int_recipient_t *recipient, cjose_jwe_t *jwe,
return false;
}

// the wrapped key (RFC 3394) is always the plaintext CEK length plus 8 bytes;
// enforce this before calling AES_unwrap_key, which would otherwise copy the
// attacker-controlled encrypted_key into the fixed-size jwe->cek buffer
if (recipient->enc_key.raw_len != jwe->cek_len + 8)
{
CJOSE_ERROR(err, CJOSE_ERR_INVALID_ARG);
return false;
}

// AES unwrap the CEK in to jwe->cek
int len = AES_unwrap_key(&akey, (const unsigned char *)NULL, jwe->cek, (const unsigned char *)recipient->enc_key.raw,
recipient->enc_key.raw_len);
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78 changes: 78 additions & 0 deletions test/check_jwe.c
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Expand Up @@ -1197,6 +1197,83 @@ START_TEST(test_cjose_jwe_multiple_recipients)
}
END_TEST

// build a compact JWE from a valid AES-KW JWE but with the encrypted_key segment
// replaced by an oversized (attacker-controlled) base64url blob, then confirm that
// importing parses fine but decryption fails gracefully instead of overflowing the
// fixed-size CEK buffer in AES_unwrap_key (RFC 3394 wrapped key is always cek_len + 8)
static void _decrypt_oversized_aes_kw_ek(const char *alg, const char *enc, const char *key)
{
cjose_err err;

cjose_jwk_t *jwk = cjose_jwk_import(key, strlen(key), &err);
ck_assert_msg(NULL != jwk, "cjose_jwk_import failed: %s", err.message);

cjose_header_t *hdr = cjose_header_new(&err);
ck_assert(cjose_header_set(hdr, CJOSE_HDR_ALG, alg, &err));
ck_assert(cjose_header_set(hdr, CJOSE_HDR_ENC, enc, &err));

const char *plain = "Setec Astronomy";
cjose_jwe_t *jwe = cjose_jwe_encrypt(jwk, hdr, (const uint8_t *)plain, strlen(plain), &err);
ck_assert_msg(NULL != jwe, "cjose_jwe_encrypt failed: %s", err.message);

char *compact = cjose_jwe_export(jwe, &err);
ck_assert_msg(NULL != compact, "cjose_jwe_export failed: %s", err.message);

// locate the boundaries of the encrypted_key (second) segment
char *first_dot = strchr(compact, '.');
ck_assert(NULL != first_dot);
char *second_dot = strchr(first_dot + 1, '.');
ck_assert(NULL != second_dot);

// craft an oversized encrypted_key: 1024 raw bytes base64url-encoded; this decodes
// to far more than any CEK length (16/24/32 ... 64) + 8 bytes
uint8_t oversized[1024];
memset(oversized, 0x41, sizeof(oversized));
char *oversized_b64u = NULL;
size_t oversized_b64u_len = 0;
ck_assert(cjose_base64url_encode(oversized, sizeof(oversized), &oversized_b64u, &oversized_b64u_len, &err));

// reassemble: <header>.<oversized_ek>.<iv>.<ciphertext>.<tag>
size_t header_len = first_dot - compact;
const char *tail = second_dot; // includes the leading '.'
size_t tail_len = strlen(tail);
size_t tampered_len = header_len + 1 + oversized_b64u_len + tail_len;
char *tampered = (char *)malloc(tampered_len + 1);
ck_assert(NULL != tampered);
memcpy(tampered, compact, header_len);
tampered[header_len] = '.';
memcpy(tampered + header_len + 1, oversized_b64u, oversized_b64u_len);
memcpy(tampered + header_len + 1 + oversized_b64u_len, tail, tail_len);
tampered[tampered_len] = '\0';

// import must still succeed (parsing the oversized segment is legal)
cjose_jwe_t *jwe_bad = cjose_jwe_import(tampered, tampered_len, &err);
ck_assert_msg(NULL != jwe_bad, "cjose_jwe_import of oversized encrypted_key failed for alg %s: %s", alg, err.message);

// decryption must fail cleanly (no heap overflow) with CJOSE_ERR_INVALID_ARG
size_t plain_len = 0;
uint8_t *decrypted = cjose_jwe_decrypt(jwe_bad, jwk, &plain_len, &err);
ck_assert_msg(NULL == decrypted, "cjose_jwe_decrypt succeeded on oversized encrypted_key for alg %s", alg);
ck_assert_msg(CJOSE_ERR_INVALID_ARG == err.code, "cjose_jwe_decrypt returned wrong err.code %d for alg %s", err.code, alg);

free(tampered);
cjose_get_dealloc()(oversized_b64u);
cjose_get_dealloc()(compact);
cjose_jwe_release(jwe_bad);
cjose_jwe_release(jwe);
cjose_header_release(hdr);
cjose_jwk_release(jwk);
}

START_TEST(test_cjose_jwe_decrypt_aes_kw_oversized_ek)
{
_decrypt_oversized_aes_kw_ek(CJOSE_HDR_ALG_A128KW, CJOSE_HDR_ENC_A128CBC_HS256, JWK_OCT_16);
_decrypt_oversized_aes_kw_ek(CJOSE_HDR_ALG_A192KW, CJOSE_HDR_ENC_A192CBC_HS384, JWK_OCT_24);
_decrypt_oversized_aes_kw_ek(CJOSE_HDR_ALG_A256KW, CJOSE_HDR_ENC_A256CBC_HS512, JWK_OCT_32);
_decrypt_oversized_aes_kw_ek(CJOSE_HDR_ALG_A256KW, CJOSE_HDR_ENC_A256GCM, JWK_OCT_32);
}
END_TEST

Suite *cjose_jwe_suite()
{
Suite *suite = suite_create("jwe");
Expand All @@ -1218,6 +1295,7 @@ Suite *cjose_jwe_suite()
tcase_add_test(tc_jwe, test_cjose_jwe_import_invalid_serialization);
tcase_add_test(tc_jwe, test_cjose_jwe_decrypt_bad_params);
tcase_add_test(tc_jwe, test_cjose_jwe_multiple_recipients);
tcase_add_test(tc_jwe, test_cjose_jwe_decrypt_aes_kw_oversized_ek);
suite_add_tcase(suite, tc_jwe);

return suite;
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