1. The number has been stolen and someone is changing the password. ;

2. The phone is bound to another account and the account is changing the password. ;

3. The account is logged in from a different place and there is an abnormality. Alibaba sends a verification code for security reasons. ;

4. Sometimes there are system errors. ;

5. A hacker is using the current mobile phone number to crack. Since the bound mobile phone number requires a verification code to be sent every time the password is changed, a text message will be received. ; Extended information: ; 1. The verification code is generally used to prevent batch registration. It is difficult for human eyes to read it, let alone machines. Second, Baidu Tieba requires a verification code to post before becoming a member of the bar, probably to prevent large-scale anonymous replies. At present, many websites have adopted verification code technology to prevent users from using robots to automatically register, log in, and flood. The so-called verification code is to generate an image with a string of randomly generated numbers or symbols, and add some interference to the image, such as randomly drawing a few straight lines and some dots (to prevent OCR). The user recognizes the verification code information in the form and enters it into the website for verification. Only after successful verification can a certain function be used. 2. Generally, verification codes are required in places where user IDs are registered and in major forums. 3. Common verification codes: (1) Four digits and letters, which may be all letters or all numbers, a random 4-digit string, the most primitive verification code, has almost zero verification effect. The CSDN website user login uses GIF format, a commonly used random digital picture verification code. The characters on the picture are relatively standard, and the verification effect is better than the previous one. (2) Chinese characters are the latest verification codes currently registered, randomly generated, and difficult to type, such as the QQ appeal page. (3) When applying for MS hotmail, it is in BMP format, random numbers + random uppercase English letters + random interference pixels + random positions. ; (4) Korean or Japanese. Now you need to type Korean when registering for MS on Paopao HF, which increases the difficulty. ; (5) When registering for Google Gmail, the format is JPG, with random English letters + random colors + random positions + random lengths. ; (6) Other major forums use XBM format, with random content. ; (7) Ad verification code: Just enter part of the content in the ad. The feature is that it can bring extra income to the website and also give users a refreshing feeling. ; (8) Question verification code: Question verification code is mainly filled in the form of questions and answers. ; Reference: Baidu Encyclopedia: Verification Code