Integer underflow in the Lists_MakeMask() function in lists.c in ngIRCd before 0.8.2 allows remote attackers to cause a denial of service (application crash) and possibly execute arbitrary code via a long MODE line that causes an incorrect length calculation, which leads to a buffer overflow.

The (1) Conn_StartLogin and (2) cb_Read_Resolver_Result functions in conn.c in ngIRCd 18 through 20.2, when the configuration option NoticeAuth is enabled, does not properly handle the return code for the Handle_Write function, which allows remote attackers to cause a denial of service (assertion failure and server crash) via unspecified vectors, related to a "notice auth" message not being sent to a new client.

The Server-Server protocol implementation in ngIRCd before 26~rc2 allows an out-of-bounds access, as demonstrated by the IRC_NJOIN() function.