CVE-2018-20343

This is a PoC for CVE-2018-20343, a vulnerability in Ken Silverman's Build Engine. The generated .map file triggers a buffer overflow and overwrites the stack as shown below:

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Open Watcom under DosBox 0.74-2: the analyst controls the stack

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Under FreeDos 1.2: the analyst controls EIP

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The vulnerable code is in ENGINE.C:

1935   kread(fil,&numsectors,2);
1936   kread(fil,&sector[0],sizeof(sectortype)*numsectors);

Variable sector has a declared size of MAXSECTORS * sizeof(struct sectortype) = 1024 * 40 = 40960 bytes. The analyst controls numsectors (because it is read from the map file) and can thus create a map file to initialize numsectors with a value greater than 40960 to trigger the overflow.

Ken Silverman pointed out that code relying on the kread function, such as the following code snippets, might also be vulnerable:

In ENGINE.C:

1938  kread(fil,&numwalls,2);
1939  kread(fil,&wall[0],sizeof(walltype)*numwalls);
1940
1941  kread(fil,&numsprites,2);
1942  kread(fil,&sprite[0],sizeof(spritetype)*numsprites);

2033   kread(fil,palette,768);
2034   kread(fil,&numpalookups,2);
[...]
2046   kread(fil,palookup[globalpal],numpalookups<<8);
2047   kread(fil,transluc,65536);

2453       kread(fil,&artversion,4);
2454       if (artversion != 1) return(-1);
2455       kread(fil,&numtiles,4);
2456       kread(fil,&localtilestart,4);
2457       kread(fil,&localtileend,4);
2458       kread(fil,&tilesizx[localtilestart],(localtileend-localtilestart+1)<<1);
2459       kread(fil,&tilesizy[localtilestart],(localtileend-localtilestart+1)<<1);
2460       kread(fil,&picanm[localtilestart],(localtileend-localtilestart+1)<<2);

2520     kread(fil,&dasiz,4);
2521       //Must store filenames to use cacheing system :(
2522     voxlock[voxindex][i] = 200;
2523     allocache(&voxoff[voxindex][i],dasiz,&voxlock[voxindex][i]);
2524     ptr = (char *)voxoff[voxindex][i];
2525     kread(fil,ptr,dasiz);

In KDMENG.C:

1155     kread(fil,&dawaversionum,4);
1156     if (dawaversionum != 0) { kclose(fil); return; }
1157  
1158     kread(fil,&numwaves,4);
1159     for(i=0;i<numwaves;i++)
1160     {
1161       kread(fil,&instname[i][0],16);
1162       kread(fil,&wavleng[i],4);
1163       kread(fil,&repstart[i],4);
1164       kread(fil,&repleng[i],4);
1165       kread(fil,&finetune[i],4);

1194     kread(fil,snd,totsndbytes);

1214   kread(fil,&kdmversionum,4);
1215   if (kdmversionum != 0) return(-2);
1216   kread(fil,&numnotes,4);
1217   kread(fil,&numtracks,4);
1218   kread(fil,trinst,numtracks);
1219   kread(fil,trquant,numtracks);
1220   kread(fil,trvol1,numtracks);
1221   kread(fil,trvol2,numtracks);
1222   kread(fil,nttime,numnotes<<2);
1223   kread(fil,nttrack,numnotes);
1224   kread(fil,ntfreq,numnotes);
1225   kread(fil,ntvol1,numnotes);
1226   kread(fil,ntvol2,numnotes);
1227   kread(fil,ntfrqeff,numnotes);
1228   kread(fil,ntvoleff,numnotes);
1229   kread(fil,ntpaneff,numnotes);

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The PoC for this vulnerability has been tested using the following environments:

• Ken Silverman's Build engine KENBUILD.ZIP (sha256sum 704a5eee1b722ed5f452d3fb1aff197f0d201ff9737839846268c6b1e07f8e1f)
• target binary GAME.EXE (sha256sum 120acdf872af56ac65982a0c21e3aa2fdd06b70bd6203869ba777ce3e6daa5e4) compiled with the Open Watcom compiler 1.9
• DOSBox 0.74-2 || FreeDOS 1.2 under QEMU.

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The Build engine has been used in many games. Below is a list of games based on the Build engine which have been tested against the PoC (which overflows the sector variable):

Game	Release Date	Version	Status
Duke Nukem 3D	1996	1.5	vulnerable
Shadow Warrior	1997	1.2	vulnerable
Redneck Rampage	1997	1.01	vulnerable
Blood	1997	1.0	not vulnerable