Created attachment 132097 [details]
Proof of concept

There is a NULL pointer dereference in cairo-bentley-ottmann-rectangular.c:558 in the function sweep_line_delete_edge:

543	sweep_line_delete_edge (sweep_line_t *sweep, edge_t *edge)
544	{
545	    if (edge->right != NULL) {
546		edge_t *next = edge->next;
547		if (next->x == edge->x) {
548		    next->top = edge->top;
549		    next->right = edge->right;
550		} else
551		    edge_end_box (sweep, edge, sweep->current_y);
552	    }
553	
554	    if (sweep->cursor == edge)
555		sweep->cursor = edge->prev;
556	
557	    edge->prev->next = edge->next;
558	    edge->next->prev = edge->prev;

edge->next is 0 at line 558 so 0->prev is a NULL pointer dereference of 0x8 because ->prev is at +8 in edge struct, the segfault occurs at address 0x8.

I don't know exactly why the ->next field is NULL, I guess a solution could be to check for this condition at the beginning of the function.

This bug was found when using a poppler util, pdftocairo. A PoC is attached. To reproduce the bug use:
pdftocairo -svg PoC.pdf

This vulnerability has been found by Offensive Research at Salesforce.com:
Alberto Garcia (@algillera), Francisco Oca (@francisco_oca) & Suleman Ali (@Salbei_)