zlib 1.2.11 through 1.3.2 contains a heap buffer overflow: after an underlying write() fails, gz write() returns without resetting strm.next in, leaving it pointed at the caller's buffer. A later gz* write call then derives a position from the stale pointer and writes past a heap allocation; any write() failure reaches it, including EPIPE on a blocking descriptor, and in versions before 1.3.1.2 the failed write must be followed by a gzclearerr() call.