Date: Sat, 15 Sep 2012 19:22:06 -0500 From: Raphael Geissert <geissert@...ian.org> To: oss-security@...ts.openwall.com Subject: Re: CVE request - mcrypt buffer overflow flaw On Tuesday 11 September 2012 10:19:38 Eygene Ryabinkin wrote: > Unfortunately, mcrypt's check_file_head() in combination with > decrypt_general() is a bit worse: it allows to overwrite up to 50 > bytes of stack buffers from decrypt_general(), namely local_algorithm, > local_mode, local_keymode. And in some curcumstances to overwrite > even 2-3 extra bytes (not more, since buf will contain '\0'), though > it is not very much controllable path. > > The problem is that no length checks are done in combos > read_until_null/strcpy. Function read_until_null() allows for up to > 100 bytes to be read and it won't NUL-terminate the buffer, so strcpy > can do perform access even further (read from tmp_buf and writes to > the said buffers; but this is the uncontrolled way I was talking > about). > > The modified PoC is at > http://codelabs.ru/security/mcrypt/poc-cve-2012-4409.py > With it I was able to overwrite the salt_size@...rypt_general() > and to trigger the call to malloc() for the chunk of 0x42424242 bytes > via _mcrypt_malloc() that lead to bus error because of subsequent > memmove(): [...] > I wasn't yet able to smash the stack of decrypt_general(), because > BUFFER_SIZE is 1024 and tmp_buf prevents me to reach the top of the > stack frame (provided that compiler won't rearrange local variables), > so I was not able to go past it. Thus it looks like a temporary > memory consumption/DoS. Another week, another couple of patches. One makes it use strncpy and forces a NUL on the last byte of local_algorithm, local_mode, and local_keymode. Their values are checked later on, so it seems safe to pass unvalidated data. The size of the buffers is hard-coded to avoid making many changes to the code. Once those issues were fixed I noticed that salt_size is not initialized if the salt flag is not set. The result is an inconditional call to malloc, with an uninitialized int as argument. This can lead to a non-attacker-controlled memory consumption DoS in most cases. It makes me think nobody actually ever used it without a salt. Once again, I've not taken a look at other bits of mcrypt, so there might be more issues around. Cheers, -- Raphael Geissert - Debian Developer www.debian.org - get.debian.net View attachment "uninitialized_salt_size.patch" of type "text/x-patch" (363 bytes) View attachment "buffer_overflows.patch" of type "text/x-patch" (784 bytes)
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