UBUNTU-CVE-2026-98123: Medium severity ubuntu/linux vulnerability

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.