Certain malformed configuration file lines can lead to unpredictable crashes. This bug is not exploitable by an IRC user, but has resulted in a lot of email to the Undernet Coder Committee.
The server configuration file consists of several lines, each starting with a single letter, followed by a colon (':'), followed by several fields, each separated by another colon. Except for connection classes (designated as "Y-lines"), the first three fields of all configuration file lines are strings, represented in the C with pointers. Empty fields are represented with NULL pointers.
Certain lines, particularly C, H, and U-lines, are searched via particular fields. If a given field is empty, a valid configuration entry is stored in the in-memory representation, but which contains a NULL pointer for this field. This results in unpredictable assertion failures during certain operations involving linking servers or searching for U-lines. (If the assertions did not fail, string comparisons immediately after the assertions would result in crashes.)
The next version of Undernet's ircu will contain a brand new configuration file format. Much of the code leading to these crashes will be rewritten, so this will not be a problem for future versions of ircu. In the meantime, please check your server configuration. None of the first three fields of a C, H, L, or U-line should be empty. Examples of proper C, H, and U-lines follow. (If you are using L-lines, please send email to firstname.lastname@example.org stating what you are using them for: The Undernet Coder Committee is strongly considering removing support for them in future versions of ircu.)
Also, please be aware that ircd.conf is sensitive to whitespace. Editing your ircd.conf on a Windows machine may result in carriage return/newline pairs as line endings, as opposed to the UNIX standard newline. The carriage return may be interpreted as a part of the string and render H-lines and U-lines inoperable.