Created attachment 132101 [details]
Proof of concept

There is a bug attempting free on address which was not malloc() in cairo-clip.c:137:
127	void
128	_cairo_clip_destroy (cairo_clip_t *clip)
129	{
130	    if (clip == NULL || _cairo_clip_is_all_clipped (clip))
131		return;
132	
133	    if (clip->path != NULL)
134		_cairo_clip_path_destroy (clip->path);
135	
136	    if (clip->boxes != &clip->embedded_box)
137		free (clip->boxes);

In some cases clip->boxes could be allocated but in the PoC is not allocated is assigned to clip->embedded_box a member of the same class at cairo-clip-boxes.c:165
149	static cairo_clip_t *
150	_cairo_clip_intersect_rectangle_box (cairo_clip_t *clip,
151					     const cairo_rectangle_int_t *r,
152					     const cairo_box_t *box)
153	{
154	    cairo_box_t extents_box;
155	    cairo_bool_t changed = FALSE;
156	    int i, j;
157	
158	    if (clip == NULL) {
159		clip = _cairo_clip_create ();
160		if (clip == NULL)
161		    return _cairo_clip_set_all_clipped (clip);
162	    }
163	
164	    if (clip->num_boxes == 0) {
165		clip->boxes = &clip->embedded_box;

There is already a check in _cairo_clip_destroy to avoid similar cases:
136	    if (clip->boxes != &clip->embedded_box)

But with the PoC the clip where the embedded_box is comming is different the the clip being destroyed.

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_)