landave

8 jaar geledendiscuss

Wow. It appears that movaxbx.ru copied multiple of my blog posts as well[1][2][3][4].

I wonder how hard it would be to take legal action (in particular to stop them from doing this altogether, as opposed to just taking down a single article). The site appears to be hosted in Russia[5]. From a quick glance over the Wikipedia page[6], it seems to me that Russian copyright law is quite similar to copyright law in most developed countries.

[1]: https://movaxbx.**/2018/06/05/f-secure-anti-virus-remote-cod...

[2]: https://movaxbx.**/2018/05/04/7-zip-from-uninitialized-memor...

[3]: https://movaxbx.**/2018/05/04/7-zip-multiple-memory-corrupti...

[4]: https://movaxbx.**/2018/05/04/bitdefender-heap-buffer-overfl...

[5]: http://ip-api.com/#movaxbx.ru

[6]: https://en.wikipedia.org/wiki/Copyright_law_of_Russia

landave

8 jaar geledendiscuss

LGPL or not, if I remember correctly, F-Secure does not enforce a valid signature of the 7-Zip library, so you can replace it yourself. Don't quote me on this though.

However, F-Secure applies several patches to harden 7-Zip and to fix bugs that are not yet fixed in the public 7-Zip version. So it is not clear whether it is always such a good idea to do this.

landave

8 jaar geledendiscuss

Why do you think so? It seems they have updated to 7-Zip 18.05 on May 11, 2018: https://forums.malwarebytes.com/topic/228610-vulnerability-i...

landave

8 jaar geledendiscuss

I think this is correct. Since _solidAllowed is set to false at the beginning of Code(), it will remain false if an exception occurs in the middle of decoding (CVE-2018-5996). This will enforce PpmError being set to true for the next item, which in turn will enforce the (possibly broken) PPMD state to be reinitialized. In some sense, this means that the new bug fix is a generalization of the first one, fixing both CVE-2018-5996 and CVE-2018-10115.

landave

8 jaar geledendiscuss

> does say VS2017 produce a smaller executable file, or a faster executable?

If I recall correctly, Igor once said that he tested the new VS compiler and it produced neither smaller nor faster executables. I believe there was almost no difference.

landave

8 jaar geledendiscuss

Okay, so these packages come with more mitigations than 7-Zip on Windows. However, looking at the source code, I am pretty sure they are affected by the same bug.

landave

8 jaar geledendiscuss

That's right, they patched CVE-2017-17969, which affected ZIP decompression. Interestingly, I believe they didn't patch CVE-2018-5996 (affecting RAR), which I published [0] on January 23 together with CVE-2017-17969.

[0]: https://landave.io/2018/01/7-zip-multiple-memory-corruptions...

landave

8 jaar geledendiscuss

You missed my point. It is trivial to find out what the magic number is. What is more important though: How exactly is the magic number matched? From what you have written, one might be tempted to simply check whether a file begins with this magic number. And this would be wrong. If you take a look at the matching in CPP/7zip/Archive/Rar/RarHandler.cpp:

    Byte marker[NHeader::kMarkerSize];
    RINOK(ReadStream_FALSE(stream, marker, NHeader::kMarkerSize));
    if (memcmp(marker, kMarker, NHeader::kMarkerSize) == 0)
      m_Position += NHeader::kMarkerSize;
    else
    {
      if (searchHeaderSizeLimit && *searchHeaderSizeLimit == 0)
        return S_FALSE;
      RINOK(stream->Seek(m_StreamStartPosition, STREAM_SEEK_SET, NULL));
      RINOK(FindSignatureInStream(stream, kMarker, NHeader::kMarkerSize,
          searchHeaderSizeLimit, arcStartPos));
      m_Position = arcStartPos + NHeader::kMarkerSize;
      RINOK(stream->Seek(m_Position, STREAM_SEEK_SET, NULL));
    }

7-Zip finds the magic number if it appears within some searchHeaderSizeLimit, i.e., the file does not need to start (at offset 0) with the magic number. For example, 7-Zip will extract a RAR file which begins with [00 52 61 72 21 1A 07 00] (instead of [52 61 72 21 1A 07 00]) just fine.

landave

8 jaar geledendiscuss

Just looked at both the packages source, and it looks like they are affected. At least all the vulnerable code is in the source package.

landave

8 jaar geledendiscuss

Of course, but I would still strongly advise against this.

If you really cannot avoid implementing something like this, you should inspect the 7-Zip code in order to be 100% sure that the magic number detection in your filter is identical (or matches a superset) to the one from 7-Zip.

landave

8 jaar geledendiscuss

HE-ASLR I am discussing with him right now, and I think we will get this.

But honestly, I don't think we will ever see a 7-Zip with /GS or CFG. Not only would this cost about 1% in binary size, it would cost an additional 1% in runtime performance loss. Additionally, it would require compiling 7-Zip with a modern compiler like VS2017. You're just asking for too much.

landave

8 jaar geledendiscuss

Note that the standard 'p7zip' package from Debian/Ubuntu doesn't support RAR. However, they have an additional package 'p7zip-full' or 'p7zip-rar' for RAR support. I didn't check explicitly, but I assume these are affected.

landave

8 jaar geledendiscuss

DEP was previously disabled because Igor used to compile 7-Zip with VC6, which doesn't support the /NXCOMPAT flag. I convinced him back in January to enable it for 7-Zip 18.01. Note, however, that 64-bit versions of Windows enforce DEP even if the /NXCOMPAT flag is missing. Since Windows 10, the 32-bit version does this as well.

ASLR was primarily disabled because Igor wanted to strip the relocation from the binaries in order to save about 0.5-1% in file size. I have discussed this with him, and convinced him to enabled full ASLR for 7-Zip 18.05.

So now we have 7-Zip 18.05 with full ASLR and DEP. Stack canaries (/GS) are still disabled though.

landave

8 jaar geledendiscuss

There were some misunderstandings that I want to clear up (maybe I will add them in an update to the blog post):

1. Some people mentioned that this would "only affect RAR files" and it would be safe to extract 7z files with 7-Zip prior to version 18.05. This is wrong, because 7-Zip detects the file type from the magic numbers at the beginning of the file. So the exploit can be renamed to 'exploit.7z' and it works just as well.

On /r/sysadmin, someone even mentioned that a temporary solution might be to block RAR files. By the same argument, this is unlikely to be effective.

2. Almost all versions prior to 18.05 are affected. I manually checked version 15.05 and 17.01, and they are definitely affected.

3. Not only 7-Zip itself is affected, but essentially all software that uses 7z.dll as library to extract files. This includes various anti-virus software. However, exploitation may be more difficult (though not impossible) if ASLR&DEP is properly enabled (on all modules).

landave

8 jaar geledendiscuss

I assume you mean a performance comparison? The runtime performance cost of ASLR on Windows is zero once a binary has been loaded, since the code is relocated at load time.

Stack canaries might cause a slight performance hit, but it is usually below one percent, since it creates only a small cost per function call for a fraction of all functions.

landave

8 jaar geledendiscuss

So I just tried to compile 7-Zip with VS2017 and /DYNAMICBASE. The main binary 7z.dll is 1,569,792 bytes in total, 9344 bytes (0.595%) of which are used by the relocation table. Enabling stack canaries (/GS) gives me a 1,578,496 byte binary (including the relocation table), so another 8704 bytes more.

landave

8 jaar geledendiscuss

What do you mean exactly by "these things"?

It may be that the blog post is difficult to understand simply because I have written it poorly...

landave

8 jaar geledendiscuss

Thanks for pointing this out. I just fixed it.

landave

8 jaar geledendiscuss

Yes, 18.00 beta is the patched version. The current release (non-beta) version is not patched yet. Moreover, the POSIX port of p7zip is not patched yet at all.

landave

8 jaar geledendiscuss

The RAR PPMd bug can only be triggered if many conditions are satisfied. For example, the RAR archive needs to be mostly correctly structured, and needs to have at least two items that are compressed with the right flags (e.g., RAR version 3, PPMd). Furthermore, the compressed streams need to be constructed such that the bugs are triggered. Hence, I believe the bug is difficult to hit with straightforward coverage-guided fuzzing.

Submissions

Bitdefender: UPX Unpacking Featuring Ten Memory Corruptions

Bitdefender: UPX Unpacking Featuring Ten Memory Corruptions

F-Secure Anti-Virus: Remote Code Execution via Solid RAR Unpacking

7-Zip: From Uninitialized Memory to Remote Code Execution

7-Zip: From Uninitialized Memory to Remote Code Execution

7-Zip: Multiple Memory Corruptions via RAR and ZIP

7-Zip: Multiple Memory Corruptions via RAR and ZIP

Palo Alto Networks White Paper: Protect Yourself from Antivirus [pdf]

Bitdefender Anti-Virus: Heap Buffer Overflow via 7z LZMA

Bitdefender Remote Stack Buffer Overflow via 7z PPMD

Avast Antivirus Remote Stack Buffer Overflow with Magic Numbers

Announcing a new blog series on anti-virus software

Ask HN: Legal situation of disclosing anti-virus bugs

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