In the Linux kernel, the following vulnerability has been resolved:
tcp: fix corruption of urgent data on multi-segment retransmit
On the normal xmit path, while in urgent mode we refuse to build a
multi-segment TSO packet, so every segment gets its own urg_ptr:
<pre>
/* tcp_write_xmit() */
limit = mss_now;
if (tso_segs > 1 && !tcp_urg_mode(tp))
limit = tcp_mss_split_point(...);
</pre>
The retransmit path has no such guard. __tcp_retransmit_skb() builds a
segs > 1 skb and hands it to the GSO layer, which only advances th->seq
per segment and copies urg_ptr verbatim:
<pre>
/* __tcp_retransmit_skb() */
len = cur_mss * segs; /* segs > 1, no urg_mode check */
...
/* tcp_gso_segment(): bumps seq only, urg_ptr is copied */
</pre>
urg_ptr is an offset from the segment's own seq, so a copied value points
at a different place on each segment. The receiver rebuilds the absolute
urgent seq as seg.seq + urg_ptr, so it walks a moving urgent point instead
of the one OOB byte:
<pre>
seg1 seq 1 urg_ptr 5001 -> urgent @ 5001 (ok)
seg2 seq 1001 urg_ptr 5001 -> urgent @ 6001 (wrong, +MSS)
seg3 seq 2001 urg_ptr 5001 -> urgent @ 7001 (wrong, +2*MSS)
</pre>
The real OOB byte is never pointed at, so the receiver stops splicing it
out and delivers it as normal in-band data, corrupting the stream.
Guard the retransmit length like the xmit path: keep segs = 1 while in
urgent mode.