Worst perl5 bugs

A small list of the worst perl5 bugs, all fixed in cperl


It’s trivial to DoS a perl5 system.


Examples for a 64bit system, but also trivial on 32bit. It creates a huge array or hash, which runs out of memory in the VMM subsystem which eventually kills the process. cperl dies with “Too many elements”, here even at compile-time.

No Hash Security

Nothing is done against the root-cause of a hash flood denial of service attack with colliding keys, only some security theatre by using slower hash functions and slower collision resolution KEY_PERTURB_RANDOM. If the seed is exposed, trivially on perl5 as it is at a fixed known address offset readable via unpack, or exposed via the command line, there is no prevention. Only cperl is secure, and also much faster. See e.g. cperl t/op/hashflood.t Or perl5241cdelta/“Protect and warn on hash flood DoS”.


Pointing that out on p5p led to the developer simply ignoring it. Instead they are working on making it even slower, but not improving the horrible implementation and security.

Language Maintainance

Silent integer overwraps

@a=(0,1); print $a[~1] => 0

~1 is essentially (UV)-2 or 0xffff_fffe.

@a=(1);print $a[18446744073709551615]' => 1

Silent overwrap of 18446744073709551615 to -1.

The same happens with overlong hash keys, they are not converted to SVs which can hold overlong strings. Everything in the buffer after I32 s ignored. Or with overlong hashes, where you can create huge hashes >I32 but can only iterate over the first I32 entries.

With cperl the “Too many elements” error is now triggered when accessing or extending an out of bounds array index or trying to insert too many hash keys. This is to prevent from silent hash or array overflows. Previously extending a hash beyond it’s capable size was silently ignored, leading to performance degradation with overly high fill factors and extending an array failed only on memory exhaustion, but the signed index led to an index overflow between I32 and U32, resp. I64 and U64.

Even worse, accessing overflown unsigned array indices would silently access the signed counterpart, indices at the end.

chop/chomp only works on half of overlarge arrays.

Or ~“a”x2G complement of overlarge strings, silently processing only the half - as with overlong hash keys.

There was also a smartmatch Array - CodeRef rule, which passed only over half the array elements. The Hash part was also wrong, but the wrong number was not used.

regex match group of >2GB string len.

repeat count >2GB and don’t silently cap it at IV_MAX. Which was at least better then silent wrap around.


Binary names

Only cperl is binary safe against \0 in names, which is esp. unsafe with package names, being mapped 1:1 to filenames.

Insecure unicode names

Unicode allows to be identifiers not identifiable, i.e. confusable evading visual inspection of 3rd party code. Bidi spoofs can contain right-to-left overwriting L-T-R characters, combining marks, mixed scripts (e.g. Cyrillic and Greek), …

There’s a TR39 security guideline for identifiers which cperl implements. perl5 has no idea about that and is not willing to fix it, even if perlcc prominently warns about that since 5.16.

No Unicode confusables +UFFA0, +U3164. In deviation from Unicode 1.1 we treat the two HANGUL FILLER characters +UFFA0 and +U3164 not as valid ID_Start and ID_Continue characters for perl identifiers.

overlong names

The “panic: hash key too long” error is now thrown with overlarge hash keys in every hv_common access and in Cpanel::JSON::XS. perl5 still silently ignores those failures, and truncates the keys.

Many more similar “panic: (file|keyword|mro|stash)? name too long” errors were added to the parser and compiler to protect from overlong names (> I32_MAX, 2147483647).

Insecure taint mode

perl5 has several known taint loopholes, see perlsec. cperl has them all fixed.

Of course it is much faster to use tainted variables, as you don’t have to check and sanitize every single variable, only external, tainted ones.

Minor issues from perl540cdelta

DynaLoader format string hardening

Replace all % characters in user-controlled library filenames, passed via the system dl_error call verbatim to printf, without any arguments on the stack, which could lead to execution of arbitrary stack code. No CVE. This affects all systems with dynamic loading where the attacker can cause a dynamic loading error.

CVSSv2 Severity: 7.2 (AV:L/AC:L/Au:N/C:C/I:C/A:C/E:U/RL:OF/RC:C/CDP:MH/TD:H/CR:H/IR:H/AR:ND)

XSLoader relative paths with eval or #line

Upstream XSLoader 0.22 (perl 5.26) fixed a minor security problem with XSLoader within eval or with a #line directive, which can load a local relative shared library, which is not in @INC. See https://rt.cpan.org/Ticket/Display.html?id=115808

cperl XSLoader was already protected against the eval case since 5.22, when being rewritten in C. cperl-5.24.0 fixed now also ignoring a relative filename in a #line directive, when the relative path is not in @INC.

handle method calls on protected stashes


Known bug upstream, not fixed there. This problem appears more often with cperl with its protected coretypes than upstream.

fedora: Do not crash when inserting a non-stash into a stash

Fedora Patch 37 RT#128238

fedora: Do not treat %: as a stash

Fedora Patch36 RT#128238

fedora: Fix precedence in hv_ename_delete

Fedora Patch35 RT#128086

fedora: Do not use unitialized memory in $h{\const} warnings

Fedora Patch34 RT#128189

fedora: Do not mangle errno from failed socket calls

Fedora Patch32 RT#128316

fedora: Backport memory leak when compiling a regular expression with a POSIX class

E.g. when C is used.

Fedora Patch31 RT#128313

suse: perl5.24.0.dif

Many Configure and linux hints enhancements, esp for lib64, probe fixes, gdbm and ODBM fixes, gnu readline integration with the debugger. See https://build.opensuse.org/package/show/devel:languages:perl/perl

suse: fix regexp backref overflows

With many backref groups (>I32)

suse: perl-saverecontext.diff RT#76538

Handle get magic with globs in the regex compiler. Correctly restore context, esp. when loading unicode swashes. Reported at 5.12, patched for suse 5.14, still ignored with 5.24.

Minor issues from perl541cdelta

Warn on metasploit CVE-2015-1592

There are known and exploitable attack vectors published for years, and there’s no effort to even detect or warn about it.

cperl detects of the destructive attack against Movable-Type, the third vector only, which tries to delete mt-config.cgi was added to was added to cperl Storable 3.01c.

Warns with “SECURITY: Movable-Type CVE-2015-1592 Storable metasploit attack” but does not protect against it.

Warn on metasploit reverse shells

cperl detects the metasploit payload unix/reverse_perl and some existing variants. This is just a dumb match at startup against existing exploits in the wild, but not future variants. Warns with “SECURITY: metasploit reverse/bind shell payload”, but do not protect against it. This warning is thrown even without C<-w>.

Also detects the CVE-2012-1823 reverse/bind shell payload, which is widely exploited too. The security warning is called “SECURITY: CVE-2012-1823 reverse/bind shell payload”.

Fixed overwriting the HVhek_UNSHARED bit in the hash loop

Broken with v5.9

This fixed -DNODEFAULT_SHAREKEYS. In the default configuration without NODEFAULT_SHAREKEYS since 5.003_001 all hash keys are stored twice, once in the hash and once again in PL_strtab, the global string table, with the benefit of faster hash loops and copies. Almost all hashtables get the SHAREKEYS bit.

With -Accflags=-DNODEFAULT_SHAREKEYS simple scripts are 20-30% faster. https://github.com/perl11/cperl/issues/201 but practical usage is dominated by copying hashes, which is faster with shared keys.


A security problem. Was in the very first cperl release 5.22.1, because we detected it and developed the fixes. With full toolchain support, in all modules. perl5 caught up 2 years later, 5.26. But of course they changed the established name to their own -Ddefault_inc_excludes_dot

perl4 ‘ package seperator

cperl deleted that, and fixed all issues.

unicode bugs

e.g range is broken in perl5, fixed in cperl 5.24.1c. Apparently fixed with 5.26 finally.

my $r = chr 255; utf8::upgrade $r; my $num = ("a" .. $r);

utf8 padnames

In perl5 all padnames are utf8 encoded by default. In cperl only those who are utf8 encoded. https://github.com/perl11/cperl/issues/208

compiler toolchain support

perl5 links with CC and ignores the linker LD, which disables advanced llvm thin, lto and cfe support. e.g. clang-4 is produces 20% faster code, and with cfe much safer code.

perl5 is inable to produce reproducible builds. cperl does it by default.

lexical $_ support

perl5 was not able to find and fix the trivial bugs. Their core features and modules rely on that, but they removed it. esp. given/when, smartmatch, List::Utils. cperl supports it.

use encoding

perl5 was not able to find and fix the trivial bugs. Many foreign devs rely on that, but they removed it. cperl supports it.


perl5 removed it, while it is necessary to track custom ops. cperl supports it.

for qw(…)

perl5 removed support for qw() with bogus justification. You need to write for (qw(..)) {} The promised parser improvements never arrived.

cperl allows for qw(...) and supporting it is trivial.

.pmc loading and reflection

perl5 removed timestamp checks for pugs with 5.8, a module doesn’t know if it’s loaded from a .pmc, and force loading a .pm is not possible.

cperl fixed that for the upcoming JitCache support, which adds expensively optimized subs for a package to a .pmc. But only some, not all subs. So a .pmc can never replace a full .pm. So reflection and loading .pm needs to be enabled.

Core modules


The CPAN version was never updated. The core version suffers from several severe core bugs, similar to the inability in core to support huge >2GB data.

cperl Storable fixes JD’s stack-overflow write (totally a CVE), detects the known MetaSploit attack vector and supports large objects, strings, hashed and arrays.

There are also more stack-overflow attacks fixed in my CPAN version.


YAML is slow, incompatible with itself and insecure by default.

e.g. Parse-CPAN-Meta security: cperl is 10x faster, can read its own files and sets $YAML::XS::DisableCode, $YAML::XS::DisableBlessed while parsing META.yml or CPAN .yml files.

Very similar to Storable. At least with YAML the upstream maintainer is listening, but he needs >1 year to merge my fixes, which is not acceptable. Nothing published yet upstream. Needs to be forked eventually.


Look at the relevant pod section in Cpanel::JSON::XS


For the security bugs see on cperl:

grep -A20 '=head1 Security' pod/perl*cdelta.pod

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