Skip to content

Out-of-bounds read due to logic error handling list-of-list.

Moderate
kentonv published GHSA-qqff-4vw4-f6hx Nov 30, 2022

Package

cargo capnp (Rust)

Affected versions

<0.15.2

Patched versions

0.15.2, 0.14.11, 0.13.7
capnproto (C++)
<0.10.3
0.5.4,0.7.1,0.8.1,0.9.2,0.10.3

Description

Problem

Out-of-bounds read due to logic error handling list-of-list.

Discovered by

David Renshaw <dwrenshaw@gmail.com>, the maintainer of Cap'n Proto's Rust
implementation, which is affected by the same bug. David discovered this bug
while running his own fuzzer.

Announced

2022-11-30

CVE

CVE-2022-46149

Impact

  • Remotely segfault a peer by sending it a malicious message, if the victim
    performs certain actions on a list-of-pointer type.
  • Possible exfiltration of memory, if the victim performs additional certain
    actions on a list-of-pointer type.
  • To be vulnerable, an application must perform a specific sequence of actions,
    described below. At present, we are not aware of any vulnerable
    application
    , but we advise updating regardless.

Fixed in

Unfortunately, the bug is present in inlined code, therefore the fix will
require rebuilding dependent applications.

C++ fix:

Rust fix:

  • capnp crate version 0.15.2, 0.14.11, or 0.13.7

Details

A specially-crafted pointer could escape bounds checking by exploiting
inconsistent handling of pointers when a list-of-structs is downgraded to a
list-of-pointers.

For an in-depth explanation of how this bug works, see David Renshaw's
blog post
. This details below focus only on determining whether an
application is vulnerable.

In order to be vulnerable, an application must have certain properties.

First, the application must accept messages with a schema in which a field has
list-of-pointer type. This includes List(Text), List(Data),
List(List(T)), or List(C) where C is an interface type. In the following
discussion, we will assume this field is named foo.

Second, the application must accept a message of this schema from a malicious
source, where the attacker can maliciously encode the pointer representing the
field foo.

Third, the application must call getFoo() to obtain a List<T>::Reader for
the field, and then use it in one of the following two ways:

  1. Pass it as the parameter to another message's setFoo(), thus copying the
    field into a new message. Note that copying the parent struct as a whole
    will not trigger the bug; the bug only occurs if the specific field foo
    is get/set on its own.

  2. Convert it into AnyList::Reader, and then attempt to access it through
    that. This is much less likely; very few apps use the AnyList API.

The dynamic API equivalents of these actions (capnp/dynamic.h) are also
affected.

If the application does these steps, the attacker may be able to cause the
Cap'n Proto implementation to read beyond the end of the message. This could
induce a segmentation fault. Or, worse, data that happened to be in memory
immediately after the message might be returned as if it were part of the
message. In the latter case, if the application then forwards that data back
to the attacker or sends it to another third party, this could result in
exfiltration of secrets.

Any exfiltration of data would have the following limitations:

  • The attacker could exfiltrate no more than 512 KiB of memory immediately
    following the message buffer.
    • The attacker chooses in advance how far past the end of the message to
      read.
    • The attacker's message itself must be larger than the exfiltrated data.
      Note that a sufficiently large message buffer will likely be allocated
      using mmap() in which case the attack will likely segfault.
  • The attack can only work if the 8 bytes immediately following the
    exfiltrated data contains a valid in-bounds Cap'n Proto pointer. The
    easiest way to achieve this is if the pointer is null, i.e. 8 bytes of zero.
    • The attacker must specify exactly how much data to exfiltrate, so must
      guess exactly where such a valid pointer will exist.
    • If the exfiltrated data is not followed by a valid pointer, the attack
      will throw an exception. If an application has chosen to ignore exceptions
      (e.g. by compiling with -fno-exceptions and not registering an
      alternative exception callback) then the attack may be able to proceed
      anyway.

Severity

Moderate
5.4
/ 10

CVSS base metrics

Attack vector
Network
Attack complexity
High
Privileges required
None
User interaction
None
Scope
Changed
Confidentiality
Low
Integrity
None
Availability
Low
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:N/A:L

CVE ID

CVE-2022-46149

Weaknesses

No CWEs