comparison src/capnproto-git-20161025/security-advisories/2015-03-02-1-c++-integer-underflow.md @ 133:1ac99bfc383d

Add Cap'n Proto source

+Problem
+=======
+
+Integer underflow in pointer validation.
+
+Discovered by
+=============
+
+Kenton Varda <[e-mail]>
+
+Announced
+=========
+
[phone]
+
+CVE
+===
+
+CVE-2015-2311
+
+Impact
+======
+
+- Remotely segfault a peer by sending it a malicious message.
+- Possible exfiltration of memory, depending on application behavior.
+- If the application performs a sequence of operations that "probably" no
+  application does (see below), possible memory corruption / code execution.
+
+Fixed in
+========
+
+- git commit [26bcceda72372211063d62aab7e45665faa83633][0]
+- release 0.5.1.1:
+  - Unix: https://capnproto.org/capnproto-c++-0.5.1.1.tar.gz
+  - Windows: https://capnproto.org/capnproto-c++-win32-0.5.1.1.zip
+- release 0.4.1.1:
+  - Unix: https://capnproto.org/capnproto-c++-0.4.1.1.tar.gz
+- release 0.6 (future)
+
+[0]: https://github.com/sandstorm-io/capnproto/commit/26bcceda72372211063d62aab7e45665faa83633
+
+Details
+=======
+
+*The following text contains speculation about the exploitability of this
+bug. This is provided for informational purposes, but as such speculation is
+often shown to be wrong, you should not rely on the accuracy of this
+section for the safety of your service. Please update your library.*
+
+A `Text` pointer, when non-null, must point to a NUL-terminated string, meaning
+it must have a size of at least 1. Under most circumstances, Cap'n Proto will
+reject zero-size text objects. However, if an application performs the
+following sequence, they may hit a code path that was missing a check:
+
+1. Receive a message containing a `Text` value, but do not actually look at
+   that value.
+2. Copy the message into a `MessageBuilder`.
+3. Call the `get()` method for the `Text` value within the `MessageBuilder`,
+   obtaining a `Text::Builder` for the *copy*.
+
+In this case, the `Text::Builder` will appear to point at a string with size
+2^32-1, starting at a location within the Cap'n Proto message.
+
+The `Text::Builder` is writable. If the application decided to overwrite the
+text in-place, it could overwrite arbitrary memory in the next 4GB of virtual
+address space. However, there are several reasons to believe this is unusual:
+
+- Usually, when an application `get()`s a text field, it only intends to
+  read it. Overwriting the text in-place is unusual.
+- Calling `set()` on the field -- the usual way to overwrite text -- will
+  create an all-new text object and harmlessly discard the old, invalid
+  pointer.
+
+Note that even if an application does overwrite the text, it would still be
+hard for the attacker to exploit this for code execution unless the attacker
+also controls the data that the application writes into the field.
+
+This vulnerability is somewhat more likely to allow exfiltration of memory.
+However, this requires the app to additionally echo the text back to the
+attacker. To do this without segfaulting, the app would either need to attempt
+to read only a subset of the text, or would need to have 2^32 contiguous bytes
+of virtual memory mapped into its address space.
+
+A related problem, also fixed in this change, occurs when a `Text` value
+has non-zero size but lacks a NUL terminator. Again, if an application
+performs the series of operations described above, the NUL terminator check
+may be bypassed. If the app then passes the string to an API that assumes
+NUL-terminated strings, the contents of memory after the text blob may be
+interpreted as being part of the string, up to the next zero-valued byte.
+This again could lead to exfiltration of data, this time without the high
+chance of segfault, although only up to the next zero-valued byte, which
+are typically quite common.
+
+Preventative measures
+=====================
+
+This problem was discovered through preventative measures implemented after
+the security problem discussed in the [previous advisory][1]. Specifically, this
+problem was found by using template metaprogramming to implement integer
+bounds analysis in order to effectively "prove" that there are no integer
+overflows in the core pointer validation code (capnp/layout.c++).
+Tentatively, I believe that this analysis exhaustively covers this file.
+The instrumentation has not been merged into master yet as it requires some
+cleanup, but [check the Cap'n Proto blog for an in-depth discussion][2].
+
+This problem is also caught by capnp/fuzz-test.c++, which *has* been
+merged into master but likely doesn't have as broad coverage.
+
+[1]: https://github.com/sandstorm-io/capnproto/tree/master/security-advisories/2015-03-02-0-c++-integer-overflow.md
+[2]: https://capnproto.org/news/2015-03-02-security-advisory-and-integer-overflow-protection.html