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Date: Mon, 21 Sep 2015 15:52:07 +0200
From: Stefan Cornelius <>
Subject: Re: Heap overflow and DoS in unzip 6.0

On Tue, 15 Sep 2015 13:10:17 -0300
Gustavo Grieco <> wrote:
> AFAIK, upstream is still working on the heap overflow issue (the DoS
> is fixed in the last unzip beta). In concrete, they said:
> "My initial (quick) analysis suggested that the basic problem is
> corrupt (unrealistic) Extra-Field sizes, which UnZip does not check
> well enough. I need to verify that that is the cause, and then figure
> out what to do about it. Currently, if the program sees an Extra-Field
> length of, say, 0x1007, it tends to believe it, even if that's bigger
> than the actual archive member's length (or perhaps even bigger than
> the whole archive). If it actually tries to read data which haven't
> been read, then valgrind gets upset (with good reason). At least
> that's what I believe so far."

I've tried to contact upstream via the contact form to share my
analysis. So far, I've had no response, so I'll share it here:

I believe the root cause for this issue is in extract.c around line
        if (G.lrec.compression_method == STORED) {
            zusz_t csiz_decrypted = G.lrec.csize;

            if (G.pInfo->encrypted)
                csiz_decrypted -= 12; ##### BOOOOM 
            if (G.lrec.ucsize != csiz_decrypted) {
                Info(slide, 0x401, ((char *)slide,
                  FmZofft(G.lrec.ucsize, NULL, "u"),

So there can be an integer overflow if csiz_decrypted is smaller than
12: if (G.pInfo->encrypted)
                csiz_decrypted -= 12;

Did anyone come to a similar conclusion? Also, the subsequent unzip
processing after this underflow exposed a lot of other places where
additional hardening may be a very good idea. Is somebody working on

Stefan Cornelius / Red Hat Product Security

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