> From: openssl-users On Behalf Of Dr. Stephen Henson > Sent: Friday, February 20, 2015 17:24 > On Fri, Feb 20, 2015, Nathaniel McCallum wrote: > > > I'd like to use ASN1_item_d2i_bio() (or something similar) to parse an > > incoming message. However, given that types like ASN1_OCTET_STRING > > have (essentially) unbounded length, how do I prevent an attacker from > > DOS'ing via OOM? > > > > Is there some way to set a max packet size? > > > > No there isn't but if the input is in DER form you can peek the first few > bytes and get the tag+length fields to determine the size of the structure. If > the input uses indefinite length encoding that isn't possible however. > Some other possibilities: If the bio is memBIO or fileBIO its input size is known before you start, at least if it contains only one root item. More generally you could layer a simple filter BIO that limits total reads to a chosen amount like 1M, probably measured from a CTRL operation -- or a more complex one that looks dynamically at your memory-used and/or memory-available and chooses whether/when to force EOF, but that would be dependent on your particular platform and not portable. Alternatively or in addition, OpenSSL allows you to provide your own malloc/realloc/free implementations used instead of the standard ones. But these are used for *all* OpenSSL heap allocations, so you might need some care to count the space used "for" or at least during a d2i as opposed to other purposes and times.
