_nx_icmpv6_validate_options() scans the option area with while (length > 2) (common/src/nx_icmpv6_validate_options.c:79). An area whose size leaves a one- or two-byte residue exits the loop with that tail unexamined; the residue is not negative, so the function returns NX_SUCCESS. Its zero-length rejection never sees those bytes.
Every consumer then re-walks the same area, reading a two-byte option header at the residue and subtracting nx_icmpv6_option_length << 3 with no zero check and no remaining-length check. Three outcomes follow, selected by bytes the attacker controls.
Zero length byte. The walker subtracts zero and advances zero. All four handlers loop forever — _nx_icmpv6_process_ra (nx_icmpv6_process_ra.c:245, :528), _nx_icmpv6_process_ns (:251, :329), _nx_icmpv6_process_na (:147, :156) and _nx_icmpv6_process_redirect (:247, :350). The walk runs in the IP thread, which is the highest-priority thread and does not yield inside the loop, so the system stops until a watchdog reset and the frame can be replayed after each one.
Non-zero length byte on a short residue. The three unsigned counters underflow — 2 - 8 becomes 0xFFFFFFFA — and the walk continues past the packet buffer, reading until it faults or meets a zero length byte and freezes. The Router Advertisement counter is signed and exits cleanly in this case.
One-byte residue. The walker reads a two-byte option header, over-reading one byte.
During a runaway walk, stray bytes parsing as a link-layer address option are copied into the neighbor cache (nx_icmpv6_process_ns.c:280, :293) and subsequently used as the destination MAC for frames to that neighbour, placing off-packet memory on the link. Confirmed by inspection, not reproduced.
Weakness
The product reads data past the end, or before the beginning, of the intended buffer.
Potential Mitigations
- Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
- When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
- Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.