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