diff --git a/perl-5.33.0-perl-17844-don-t-update-SvCUR-until-after-we-ve-done.patch b/perl-5.33.0-perl-17844-don-t-update-SvCUR-until-after-we-ve-done.patch
new file mode 100644
index 0000000..ce31651
--- /dev/null
+++ b/perl-5.33.0-perl-17844-don-t-update-SvCUR-until-after-we-ve-done.patch
@@ -0,0 +1,72 @@
+From 45f235c116d4deab95c576aff77fe46d609f8553 Mon Sep 17 00:00:00 2001
+From: Tony Cook <tony@develop-help.com>
+Date: Mon, 15 Apr 2019 15:23:32 +1000
+Subject: [PATCH] (perl #17844) don't update SvCUR until after we've done
+ moving
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+SvCUR() before the SvGROW() calls could result in reading beyond the
+end of a buffer.
+
+It wasn't a problem in the normal case, since sv_grow() just calls
+realloc() which has its own notion of how big the memory block is, but
+if the SV is SvOOK() sv_backoff() tries to move SvCUR()+1 bytes, which
+might be larger than the currently allocated size of the PV.
+
+Signed-off-by: Petr Písař <ppisar@redhat.com>
+---
+ doop.c     |  2 +-
+ t/op/bop.t | 11 ++++++++++-
+ 2 files changed, 11 insertions(+), 2 deletions(-)
+
+diff --git a/doop.c b/doop.c
+index 88220092c3..c9c953212e 100644
+--- a/doop.c
++++ b/doop.c
+@@ -1087,7 +1087,6 @@ Perl_do_vop(pTHX_ I32 optype, SV *sv, SV *left, SV *right)
+     lsave = lc;
+     rsave = rc;
+ 
+-    SvCUR_set(sv, len);
+     (void)SvPOK_only(sv);
+     if (SvOK(sv) || SvTYPE(sv) > SVt_PVMG) {
+ 	dc = SvPV_force_nomg_nolen(sv);
+@@ -1103,6 +1102,7 @@ Perl_do_vop(pTHX_ I32 optype, SV *sv, SV *left, SV *right)
+ 	sv_usepvn_flags(sv, dc, needlen, SV_HAS_TRAILING_NUL);
+ 	dc = SvPVX(sv);		/* sv_usepvn() calls Renew() */
+     }
++    SvCUR_set(sv, len);
+ 
+     if (len >= sizeof(long)*4 &&
+ 	!(PTR2nat(dc) % sizeof(long)) &&
+diff --git a/t/op/bop.t b/t/op/bop.t
+index eecd90387f..07f057d0a9 100644
+--- a/t/op/bop.t
++++ b/t/op/bop.t
+@@ -18,7 +18,7 @@ BEGIN {
+ # If you find tests are failing, please try adding names to tests to track
+ # down where the failure is, and supply your new names as a patch.
+ # (Just-in-time test naming)
+-plan tests => 501;
++plan tests => 502;
+ 
+ # numerics
+ ok ((0xdead & 0xbeef) == 0x9ead);
+@@ -669,3 +669,12 @@ foreach my $op_info ([and => "&"], [or => "|"], [xor => "^"]) {
+         like $@, $expected, $description;
+     }
+ }
++
++{
++    # perl #17844 - only visible with valgrind/ASAN
++    fresh_perl_is(<<'EOS',
++formline X000n^\\0,\\0^\\0for\0,0..10
++EOS
++                  '',
++                  {}, "[perl #17844] access beyond end of block");
++}
+-- 
+2.25.4
+
diff --git a/perl.spec b/perl.spec
index 32abd41..d332b84 100644
--- a/perl.spec
+++ b/perl.spec
@@ -201,6 +201,10 @@ Patch25:        perl-5.33.0-reentr.c-Buffer-sizes-for-asctime_r-ctime_r-are-smal
 # in upstream after 5.33.0
 Patch26:        perl-5.33.0-reentr.c-Prevent-infinite-looping.patch
 
+# Fix a buffer overread in when reallocating formats, GH#17844,
+# in upstream after 5.33.0
+Patch27:        perl-5.33.0-perl-17844-don-t-update-SvCUR-until-after-we-ve-done.patch
+
 # Link XS modules to libperl.so with EU::CBuilder on Linux, bug #960048
 Patch200:       perl-5.16.3-Link-XS-modules-to-libperl.so-with-EU-CBuilder-on-Li.patch
 
@@ -4213,6 +4217,7 @@ you're not running VMS, this module does nothing.
 %patch24 -p1
 %patch25 -p1
 %patch26 -p1
+%patch27 -p1
 %patch200 -p1
 %patch201 -p1
 
@@ -4246,6 +4251,7 @@ perl -x patchlevel.h \
     'Fedora Patch24: Fix running actions after stepping in a debugger (GH#17901)' \
     'Fedora Patch25: Fix a buffer size for asctime_r() and ctime_r() functions' \
     'Fedora Patch26: Prevent from an integer overflow in RenewDouble() macro' \
+    'Fedora Patch27: Fix a buffer overread in when reallocating formats (GH#17844)' \
     'Fedora Patch200: Link XS modules to libperl.so with EU::CBuilder on Linux' \
     'Fedora Patch201: Link XS modules to libperl.so with EU::MM on Linux' \
     %{nil}
@@ -6974,6 +6980,7 @@ popd
 - Fix running actions after stepping in a debugger (GH#17901)
 - Fix a buffer size for asctime_r() and ctime_r() functions
 - Prevent from an integer overflow in RenewDouble() macro
+- Fix a buffer overread in when reallocating formats (GH#17844)
 
 * Tue Jul 28 2020 Fedora Release Engineering <releng@fedoraproject.org> - 4:5.32.0-458
 - Rebuilt for https://fedoraproject.org/wiki/Fedora_33_Mass_Rebuild