Linux Kernel SCTP ASCONF-ACK Parameter Walk Loops Indefinitely on an Unpadded Short Parameter Due to SCTP_PAD4 Rounding, Causing a Soft Lockup

Status: NEW  |  Advisory ID: CVE-2026-98123

Key Details

What to Know

In the Linux kernel, the following vulnerability has been resolved:

sctp: fix soft lockup from unpadded ASCONF-ACK parameter iteration

sctp_verify_asconf() walks ASCONF-ACK parameters with

sctp_walk_params(), which advances by SCTP_PAD4(length), while the

consumer sctp_get_asconf_response() 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

asconf_ack_len 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 SCTP_PARAM_ERR_CAUSE case of sctp_verify_asconf() 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 SCTP_PARAM_ERR_CAUSE parameters shorter than

sizeof(struct sctp_addip_param) + sizeof(struct sctp_errhdr) 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 ERR_CAUSE

parameter the consumer can see, so the consumer's asconf_ack_len

check is dropped and it returns err_param->cause directly. The

consumer padding fix is still required because odd lengths remain

valid for SCTP_PARAM_ERR_CAUSE per RFC 5061.

The issue was found by ZeroHive, a vulnerability hunting agent at

Tencent Yunding Lab. (NVD)

References