diff --git a/wolfcrypt/src/dh.c b/wolfcrypt/src/dh.c
index 849ff81025..26166c7310 100644
--- a/wolfcrypt/src/dh.c
+++ b/wolfcrypt/src/dh.c
@@ -1122,13 +1122,24 @@ static int GeneratePrivateDh186(DhKey* key, WC_RNG* rng, byte* priv,
     byte cBuf[DH_MAX_SIZE + 64 / WOLFSSL_BIT_SIZE];
 #endif
 
-    /* Parameters validated in calling functions. */
+    /* Pointer parameters validated by the public entry wc_DhGenerateKeyPair. */
 
     if (mp_iszero(&key->q) == MP_YES) {
         WOLFSSL_MSG("DH q parameter needed for FIPS 186-4 key generation");
         return BAD_FUNC_ARG;
     }
 
+    /* Bound *privSz so cSz (= *privSz + 8) cannot exceed the cBuf capacity.
+     * Note: DH_MAX_SIZE is documented as a bit count, but the cBuf declaration
+     * above uses it directly as a byte count (cBuf is DH_MAX_SIZE + 8 bytes).
+     * This check matches that convention so *privSz (in bytes) is bounded by
+     * the actual byte capacity of cBuf. The same bound is applied to the
+     * WOLFSSL_SMALL_STACK path to avoid unbounded heap allocation. */
+    if (*privSz > DH_MAX_SIZE) {
+        WOLFSSL_MSG("DH private key size exceeds DH_MAX_SIZE");
+        return BAD_FUNC_ARG;
+    }
+
     qSz = (word32)mp_unsigned_bin_size(&key->q);
     pSz = (word32)mp_unsigned_bin_size(&key->p);
 
diff --git a/wolfcrypt/test/test.c b/wolfcrypt/test/test.c
index 6725de6aca..e225347f56 100644
--- a/wolfcrypt/test/test.c
+++ b/wolfcrypt/test/test.c
@@ -29605,6 +29605,27 @@ static wc_test_ret_t dh_fips_generate_test(WC_RNG *rng)
     if (ret != 0)
         ERROR_OUT(WC_TEST_RET_ENC_EC(ret), exit_gen_test);
 
+#if !defined(WOLFSSL_NO_DH186) && !defined(HAVE_SELFTEST) && \
+    !defined(HAVE_FIPS)
+    /* Regression: an oversized *privSz must be rejected before
+     * GeneratePrivateDh186 writes (*privSz + 8) bytes of RNG output into the
+     * stack-allocated cBuf (sized DH_MAX_SIZE + 8 in non-WOLFSSL_SMALL_STACK
+     * builds). The key still has q set here, so the call dispatches through
+     * GeneratePrivateDh186. Only exercised when the local src/dh.c bound
+     * check is in play (not HAVE_SELFTEST / HAVE_FIPS builds, which use
+     * separate validated modules). */
+    {
+        word32 hugePrivSz = (word32)DH_MAX_SIZE + 1;
+        word32 outPubSz   = sizeof(pub);
+        ret = wc_DhGenerateKeyPair(key, rng, priv, &hugePrivSz, pub, &outPubSz);
+    #if defined(WOLFSSL_ASYNC_CRYPT)
+        ret = wc_AsyncWait(ret, &key->asyncDev, WC_ASYNC_FLAG_NONE);
+    #endif
+        if (ret != WC_NO_ERR_TRACE(BAD_FUNC_ARG))
+            ERROR_OUT(WC_TEST_RET_ENC_EC(ret), exit_gen_test);
+    }
+#endif /* !WOLFSSL_NO_DH186 && !HAVE_SELFTEST && !HAVE_FIPS */
+
     wc_FreeDhKey(key);
     ret = wc_InitDhKey_ex(key, HEAP_HINT, devId);
     if (ret != 0)