int n_ssl3_mac(SSL *ssl, SSL3_RECORD *rec, unsigned char *md, int send)
{
    unsigned char *mac_sec, *seq;
    const EVP_MD_CTX *hash;
    unsigned char *p, rec_char;
    size_t md_size;
    int npad;
    int t;

    if (send) {
        mac_sec = &(ssl->s3->write_mac_secret[0]);
        seq = RECORD_LAYER_get_write_sequence(&ssl->rlayer);
        hash = ssl->write_hash;
    } else {
        mac_sec = &(ssl->s3->read_mac_secret[0]);
        seq = RECORD_LAYER_get_read_sequence(&ssl->rlayer);
        hash = ssl->read_hash;
    }

    t = EVP_MD_CTX_size(hash);
    if (t < 0)
        return -1;
    md_size = t;
    npad = (48 / md_size) * md_size;

    if (!send &&
        EVP_CIPHER_CTX_mode(ssl->enc_read_ctx) == EVP_CIPH_CBC_MODE &&
        ssl3_cbc_record_digest_supported(hash)) {
        /*
         * This is a CBC-encrypted record. We must avoid leaking any
         * timing-side channel information about how many blocks of data we
         * are hashing because that gives an attacker a timing-oracle.
         */

        /*-
         * npad is, at most, 48 bytes and that's with MD5:
         *   16 + 48 + 8 (sequence bytes) + 1 + 2 = 75.
         *
         * With SHA-1 (the largest hash speced for SSLv3) the hash size
         * goes up 4, but npad goes down by 8, resulting in a smaller
         * total size.
         */
        unsigned char header[75];
        unsigned j = 0;
        memcpy(header + j, mac_sec, md_size);
        j += md_size;
        memcpy(header + j, ssl3_pad_1, npad);
        j += npad;
        memcpy(header + j, seq, 8);
        j += 8;
        header[j++] = rec->type;
        header[j++] = rec->length >> 8;
        header[j++] = rec->length & 0xff;

        /* Final param == is SSLv3 */
        if (ssl3_cbc_digest_record(hash,
                                   md, &md_size,
                                   header, rec->input,
                                   rec->length + md_size, rec->orig_len,
                                   mac_sec, md_size, 1) <= 0)
            return -1;
    } else {
        unsigned int md_size_u;
        /* Chop the digest off the end :-) */
        EVP_MD_CTX *md_ctx = EVP_MD_CTX_new();

        if (md_ctx == NULL)
            return -1;

        rec_char = rec->type;
        p = md;
        s2n(rec->length, p);
        if (EVP_MD_CTX_copy_ex(md_ctx, hash) <= 0
            || EVP_DigestUpdate(md_ctx, mac_sec, md_size) <= 0
            || EVP_DigestUpdate(md_ctx, ssl3_pad_1, npad) <= 0
            || EVP_DigestUpdate(md_ctx, seq, 8) <= 0
            || EVP_DigestUpdate(md_ctx, &rec_char, 1) <= 0
            || EVP_DigestUpdate(md_ctx, md, 2) <= 0
            || EVP_DigestUpdate(md_ctx, rec->input, rec->length) <= 0
            || EVP_DigestFinal_ex(md_ctx, md, NULL) <= 0
            || EVP_MD_CTX_copy_ex(md_ctx, hash) <= 0
            || EVP_DigestUpdate(md_ctx, mac_sec, md_size) <= 0
            || EVP_DigestUpdate(md_ctx, ssl3_pad_2, npad) <= 0
            || EVP_DigestUpdate(md_ctx, md, md_size) <= 0
            || EVP_DigestFinal_ex(md_ctx, md, &md_size_u) <= 0) {
            EVP_MD_CTX_reset(md_ctx);
            return -1;
        }
        md_size = md_size_u;

        EVP_MD_CTX_free(md_ctx);
    }

    ssl3_record_sequence_update(seq);
    return (md_size);
}

This weakness involves numeric computation errors, such as integer overflows, underflows, or precision losses, which can lead to miscalculations and exploitable behaviors in software.