CVE-2026-98123: sctp: fix soft lockup from unpadded ASCONF-ACK parameter iteration
In the Linux kernel, the following vulnerability has been resolved:
sctp: fix soft lockup from unpadded ASCONF-ACK parameter iteration
sctpverifyasconf() walks ASCONF-ACK parameters with sctpwalkparams(), which advances by SCTPPAD4(length), while the consumer sctpgetasconfresponse() iterates the same parameters advancing by the raw length, without padding. A single odd-length parameter desynchronises the two walks and makes the consumer interpret attacker-controlled bytes at a misaligned offset.
When those bytes yield a length of zero, the while loop over asconfacklen makes no progress, spinning forever in softirq context, and the watchdog reports a soft lockup. All reads stay within the received skb, so the lockup is a pure remote denial of service. A remote peer can trigger it with a crafted ASCONF-ACK on an ADD-IP enabled association with an outstanding ASCONF (RFC 5061 section 4.1.2 requires the chunk to be authenticated, but the predefined empty key id 0 allows the peer to compute the same association HMAC from publicly exchanged parameters, so the gate does not help).
The SCTPPARAMERRCAUSE case of sctpverifyasconf() also performs no length check, letting a parameter without a complete error header reach the consumer, which reads errhdr.cause past the end of the parameter, an out-of-bounds read.
Reject SCTPPARAMERRCAUSE parameters shorter than sizeof(struct sctpaddipparam) + sizeof(struct sctperrhdr) at the verifier, and advance the consumer iterator with the same padding rule as the verifier to keep the two walks in lockstep. The verifier change guarantees a complete error header in every ERRCAUSE parameter the consumer can see, so the consumer's asconfacklen check is dropped and it returns errparam->cause directly. The consumer padding fix is still required because odd lengths remain valid for SCTPPARAMERRCAUSE per RFC 5061.
The issue was found by ZeroHive, a vulnerability hunting agent at Tencent Yunding Lab.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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Fixed in 6.6.158.1-1 - Upgrade
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ubuntu/linux-riscv-7.0to a version that resolves this vulnerability.Fixed in 7.0.0-42.42.1~24.04.1 - Upgrade
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ubuntu/linux-awsto a version that resolves this vulnerability.Fixed in 7.0.0-1016.17 - Upgrade
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 7.0.0-42.42.1
Event History
Frequently Asked Questions
Which systems are exposed to this denial of service?
Systems are exposed when they have an SCTP association with ADD-IP enabled and an outstanding ASCONF request. A remote peer on that association can send the crafted ASCONF-ACK needed to trigger the soft lockup.
Does ASCONF-ACK authentication prevent exploitation?
No. Although RFC 5061 requires the chunk to be authenticated, the predefined empty key ID 0 lets the peer compute the same association HMAC from publicly exchanged parameters, so this requirement does not block the attack.
What is the impact of a successful attack?
The attacker can cause an infinite loop in softirq context, leading the watchdog to report a soft lockup. The described impact is a remote denial of service; reads remain within the received skb.
What configuration change can reduce exposure before applying a fix?
Avoid enabling or using SCTP ADD-IP associations where possible, particularly associations that can have outstanding ASCONF requests. The attack requires both ADD-IP and an outstanding ASCONF on the targeted association.