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Integer underflow in pointer validation.

Discovered by

Kenton Varda <>






  • 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


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. 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.

This problem is also caught by capnp/fuzz-test.c++, which has been merged into master but likely doesn't have as broad coverage.