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POSecurity vulnerabilities in fairly simple c code
 

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    <p>I'm studying for my last ever exam (yey!), and have run into a problem I have a hard time figuring out. It's an old exam question where you are supposed to find at least two vulnerabilities that can be exploited in a function which reads a ppm image file. The only issue I can identify is if cols and/or rows are given unexpected values, either too large (causing integer overflow) or negative, which leads to img->raster having an incorrect size, opening up the possibility of a heap-based buffer-overflow attack.</p> <p>As far as I can reason, the unchecked malloc should not be exploitable.</p> <pre><code>struct image *read_ppm(FILE *fp) { int version; int rows, cols, maxval; int pixBytes=0, rowBytes=0, rasterBytes; uint8_t *p; struct image *img; /* Read the magic number from the file */ if ((fscanf(fp, " P%d ", &amp;version) &lt; 1) || (version != 6)) { return NULL; } /* Read the image dimensions and color depth from the file */ if (fscanf(fp, " %d %d %d ", &amp;cols, &amp;rows, &amp;maxval) &lt; 3) { return NULL; } /* Calculate some sizes */ pixBytes = (maxval &gt; 255) ? 6 : 3; // Bytes per pixel rowBytes = pixBytes * cols; // Bytes per row rasterBytes = rowBytes * rows; // Bytes for the whole image /* Allocate the image structure and initialize its fields */ img = malloc(sizeof(*img)); if (img == NULL) return NULL; img-&gt;rows = rows; img-&gt;cols = cols; img-&gt;depth = (maxval &gt; 255) ? 2 : 1; img-&gt;raster = (void*)malloc(rasterBytes); /* Get a pointer to the first pixel in the raster data. */ /* It is to this pointer that all image data will be written. */ p = img-&gt;raster; /* Iterate over the rows in the file */ while (rows--) { /* Iterate over the columns in the file */ cols = img-&gt;cols; while (cols--) { /* Try to read a single pixel from the file */ if (fread(p, pixBytes, 1, fp) &lt; 1) { /* If the read fails, free memory and return */ free(img-&gt;raster); free(img); return NULL; } /* Advance the pointer to the next location to which we should read a single pixel. */ p += pixBytes; } } /* Return the image */ return img; } </code></pre> <p>Original (the last question): <a href="http://www.ida.liu.se/~TDDC90/exam/old/TDDC90%20TEN1%202009-12-22.pdf" rel="noreferrer">http://www.ida.liu.se/~TDDC90/exam/old/TDDC90%20TEN1%202009-12-22.pdf</a></p> <p>Thanks for any help.</p>
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    1. COThis puts me in mind of the (2008 winning) example from [The Underhanded C Contest](http://underhanded.xcott.com/), could be some clues there.
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