The root cause of the CWE-401 (Missing Release of Memory after Effective Lifetime) vulnerability in the given code is that the memory allocated by `strdup` at line 76 is not properly released or freed before the function exits or in case of an error condition.

The code allocates memory for `pool->nonce1` using `strdup`, which duplicates the string returned by `json_string_value(json_array_get(res_val, 1))`. However, there is no corresponding `free` or memory release operation for `pool->nonce1` in case the function exits due to an error condition (e.g., at lines 62, 67, 73, 79, or 85) or when the function returns successfully.

This can lead to a memory leak, as the allocated memory for `pool->nonce1` is not released after its effective lifetime, which is a violation of CWE-401. The memory leak can potentially cause resource exhaustion and other related issues if the function is called repeatedly without properly freeing the allocated memory.

To fix this vulnerability, the code should include a proper memory release operation (e.g., `free(pool->nonce1)`) in the error handling cases and before the function returns successfully.