A bug in slapd's UTF8StringNormalize() function can cause a one-byte buffer overflow when it is passed a zero-length string. The code then writes a '\0' past the one-byte long buffer allocated on the heap, which could possibly allow a remote authenticated user to crash slapd. As per the upstream report , this bug has been present since 2003-04-07  so should affect all versions of openldap we currently ship.
A patch to correct the flaw has been committed  (depends on the previous patch ).
Doug Lea's Malloc stores chunks whose size is smaller than 512 bytes in one of the small bins, which holds identically sized chunks. The size of a chunk is always a multiple of 8 bytes, and the first small bin holds 16 bytes chunks. Since the minimum allocated size is 16 bytes, it seems no data that can result in application crash can be overwritten as a result of this.
Resolved in Fedora Rawhide (openldap-2.4.26-6.fc17).
The Red Hat Security Response Team does not consider this to be a security issue. For additional information, refer to: https://bugzilla.redhat.com/show_bug.cgi?id=749324#c1.
For additional information, OpenLDAP has a Slab Allocator implementation atop of glibc malloc with the size of its allocated objects rounded up to double word boundaries, which is also its minimum size. Thus, this can not even be called an off-by-one when not falling back to glibc malloc.