diff --git a/0001-generator-Fix-assertion-in-ext-mode-BLOCK_STATUS-CVE.patch b/0001-generator-Fix-assertion-in-ext-mode-BLOCK_STATUS-CVE.patch
new file mode 100644
index 0000000..d660188
--- /dev/null
+++ b/0001-generator-Fix-assertion-in-ext-mode-BLOCK_STATUS-CVE.patch
@@ -0,0 +1,88 @@
+From 4451e5b61ca07771ceef3e012223779e7a0c7701 Mon Sep 17 00:00:00 2001
+From: Eric Blake <eblake@redhat.com>
+Date: Mon, 30 Oct 2023 12:50:53 -0500
+Subject: [PATCH] generator: Fix assertion in ext-mode BLOCK_STATUS,
+ CVE-2023-5871
+
+Another round of fuzz testing revealed that when a server negotiates
+extended headers and replies with a 64-bit flag value where the client
+used the 32-bit API command, we were correctly flagging the server's
+response as being an EOVERFLOW condition, but then immediately failing
+in an assertion failure instead of reporting it to the application.
+
+The following one-byte change to qemu.git at commit fd9a38fd43 allows
+the creation of an intentionally malicious server:
+
+| diff --git i/nbd/server.c w/nbd/server.c
+| index 859c163d19f..32e1e771a95 100644
+| --- i/nbd/server.c
+| +++ w/nbd/server.c
+| @@ -2178,7 +2178,7 @@ static void nbd_extent_array_convert_to_be(NBDExtentArray *ea)
+|
+|      for (i = 0; i < ea->count; i++) {
+|          ea->extents[i].length = cpu_to_be64(ea->extents[i].length);
+| -        ea->extents[i].flags = cpu_to_be64(ea->extents[i].flags);
+| +        ea->extents[i].flags = ~cpu_to_be64(ea->extents[i].flags);
+|      }
+|  }
+
+and can then be detected with the following command line:
+
+$ nbdsh -c - <<\EOF
+> def f(a,b,c,d):
+>   pass
+>
+> h.connect_systemd_socket_activation(["/path/to/bad/qemu-nbd",
+>   "-r", "-f", "raw", "TODO"])
+> h.block_staus(h.get_size(), 0, f)
+> EOF
+nbdsh: generator/states-reply-chunk.c:626: enter_STATE_REPLY_CHUNK_REPLY_RECV_BS_ENTRIES: Assertion `(len | flags) <= UINT32_MAX' failed.
+Aborted (core dumped)
+
+whereas a fixed libnbd will give:
+
+nbdsh: command line script failed: nbd_block_status: block-status: command failed: Value too large for defined data type
+
+We can either relax the assertion (by changing to 'assert ((len |
+flags) <= UINT32_MAX || cmd->error)'), or intentionally truncate flags
+to make the existing assertion reliable.  This patch goes with the
+latter approach.
+
+Sadly, this crash is possible in all existing 1.18.x stable releases,
+if they were built with assertions enabled (most distros do this by
+default), meaning a malicious server has an easy way to cause a Denial
+of Service attack by triggering the assertion failure in vulnerable
+clients, so we have assigned this CVE-2023-5871.  Mitigating factors:
+the crash only happens for a server that sends a 64-bit status block
+reply (no known production servers do so; qemu 8.2 will be the first
+known server to support extended headers, but it is not yet released);
+and as usual, a client can use TLS to guarantee it is connecting only
+to a known-safe server.  If libnbd is compiled without assertions,
+there is no crash or other mistaken behavior; and when assertions are
+enabled, the attacker cannot accomplish anything more than a denial of
+service.
+
+Reported-by: Richard W.M. Jones <rjones@redhat.com>
+Fixes: 20dadb0e10 ("generator: Prepare for extent64 callback", v1.17.4)
+Signed-off-by: Eric Blake <eblake@redhat.com>
+(cherry picked from commit 177308adb17e81fce7c0f2b2fcf655c5c0b6a4d6)
+Signed-off-by: Eric Blake <eblake@redhat.com>
+---
+ generator/states-reply-chunk.c | 1 +
+ 1 file changed, 1 insertion(+)
+
+diff --git a/generator/states-reply-chunk.c b/generator/states-reply-chunk.c
+index 5a31c19..8ab7e8b 100644
+--- a/generator/states-reply-chunk.c
++++ b/generator/states-reply-chunk.c
+@@ -600,6 +600,7 @@ STATE_MACHINE {
+             break; /* Skip this and later extents; we already made progress */
+           /* Expose this extent as an error; we made no progress */
+           cmd->error = cmd->error ? : EOVERFLOW;
++          flags = (uint32_t)flags;
+         }
+       }
+ 
+-- 
+2.39.3
+
diff --git a/libnbd.spec b/libnbd.spec
index 5072976..fe4f142 100644
--- a/libnbd.spec
+++ b/libnbd.spec
@@ -9,7 +9,7 @@
 
 Name:           libnbd
 Version:        1.18.1
-Release:        1%{?dist}
+Release:        2%{?dist}
 Summary:        NBD client library in userspace
 
 License:        LGPL-2.0-or-later AND BSD-3-Clause
@@ -28,7 +28,8 @@ Source3:        copy-patches.sh
 # Patches are stored in the upstream repository:
 # https://gitlab.com/nbdkit/libnbd/-/commits/rhel-9.4/
 
-# (no patches)
+# Patches.
+Patch0001:     0001-generator-Fix-assertion-in-ext-mode-BLOCK_STATUS-CVE.patch
 
 %if 0%{patches_touch_autotools}
 BuildRequires: autoconf, automake, libtool
@@ -380,6 +381,10 @@ make %{?_smp_mflags} check || {
 
 
 %changelog
+* Wed Nov 01 2023 Richard W.M. Jones <rjones@redhat.com> - 1.18.1-2
+- Fix assertion in ext-mode BLOCK_STATUS (CVE-2023-5871)
+  resolves: RHEL-15143
+
 * Tue Oct 24 2023 Richard W.M. Jones <rjones@redhat.com> - 1.18.1-1
 - Rebase to 1.18.1
   resolves: RHEL-14476