CVE-2026-80587: mptcp: avoid combining some incoming suboptions
In the Linux kernel, the following vulnerability has been resolved:
mptcp: avoid combining some incoming suboptions
Some MPTCP suboptions are mutually exclusive according to the RFC8684, but also because in different places, the code doesn't expect some combinations to be present. That's specially true for suboptions that would be present twice, but with different attributes.
The new restrictions are the same as the ones applied on the output side, with mptcpwriteoptions. The same rules can be reused with a small fix: an MPFASTCLOSE can be used with a DSS when the sender picks this option [1], which is not the case on Linux. Here are the rules:
Which options can be used together?
X: mutually exclusive O: often used together C: can be used together in some cases P: could be used together but we prefer not to (optimisations)
| Opt: | MPC | MPJ | DSS | ADD | RM | PRIO | FAIL | FC | |------|------|------|------|------|------|------|------|------| | MPC |------|------|------|------|------|------|------|------| | MPJ | X |------|------|------|------|------|------|------| | DSS | X | X |------|------|------|------|------|------| | ADD | X | X | P |------|------|------|------|------| | RM | C | C | C | P |------|------|------|------| | PRIO | X | C | C | C | C |------|------|------| | FAIL | X | X | C | X | X | X |------|------| | FC | X | X | P | X | X | X | X |------| | RST | X | X | X | X | X | X | O | O | |------|------|------|------|------|------|------|------|------|
The only difference is with the 'P': another stack could send and ADDADDR with other suboptions (DSS, RMADDR), and this should be allowed.
A few points of attention:
- In theory, an MPCAPABLE could be used with a RMADDR, but there is no reason to add it with a SYN. Note that even with a 4th ACK, it doesn't seem to be useful, except when IDs are known in advance via another channel. Better not to break that.
- Now, combining both an MPCAPABLE and an MPJOIN will no longer result to a reject of the two options, but only the second suboption is ignored. That seems OK to do that for this unexpected error. At least now all inconsistent combinations are handled the same way. This could change later in next. This also means the explicit checks for having both MPC + MPJ in subflow.c will now be unreachable. That's fine, they will be removed in a follow-up patch.
- In case of conflicting combinations, the extra suboption(s) is/are ignored: having such combinations either means the remote peer is buggy, or is evil. The simplest action is then taken in this case: stop processing the current suboption.
- In mpopt->suboptions, there is also a bit reserved to the checksum, which can be used in an MPCAPABLE and a DSS. Each time a DSS option can be used in parallel with another option, the checksum can be set, so the verification is combined into a new OPTIONSMPTCPDSS macro.
- An MPCAPABLE ACK can carry a Data-Level Length, and an optional Checksum: they are the same as the ones found in a DSS, because a DSS cannot be used in parallel to an MPCAPABLE. Similarly, even if there is room, a DSS cannot be used with an MPJOIN.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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Fixed in 6.6.157.1-1 - Upgrade
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Event History
Frequently Asked Questions
What traffic must a system process for this issue to be relevant?
The issue concerns incoming MPTCP suboptions in the Linux kernel. Systems that do not use or process MPTCP connections are not described as exposed by the provided information.
What condition triggers the vulnerable behavior?
An incoming MPTCP packet must contain suboption combinations that are mutually exclusive under RFC8684 or are not expected by different parts of the code, including duplicate suboptions with different attributes.
What does the available fix change?
The fix restricts combinations of incoming MPTCP suboptions, reusing rules already applied when Linux writes MPTCP options. It also accounts for the fact that Linux does not send MP_FASTCLOSE together with DSS in the referenced case.