From 69d1b3fc29677af8ade8dc15dba83f0589cb63d6 Mon Sep 17 00:00:00 2001

From: Lasse Collin <lasse.collin@tukaani.org>

Date: Tue, 29 Mar 2022 19:19:12 +0300

Subject: [PATCH] xzgrep: Fix escaping of malicious filenames (ZDI-CAN-16587).

Malicious filenames can make xzgrep to write to arbitrary files

or (with a GNU sed extension) lead to arbitrary code execution.

xzgrep from XZ Utils versions up to and including 5.2.5 are

affected. 5.3.1alpha and 5.3.2alpha are affected as well.

This patch works for all of them.

This bug was inherited from gzip's zgrep. gzip 1.12 includes

a fix for zgrep.

The issue with the old sed script is that with multiple newlines,

the N-command will read the second line of input, then the

s-commands will be skipped because it's not the end of the

file yet, then a new sed cycle starts and the pattern space

is printed and emptied. So only the last line or two get escaped.

One way to fix this would be to read all lines into the pattern

space first. However, the included fix is even simpler: All lines

except the last line get a backslash appended at the end. To ensure

that shell command substitution doesn't eat a possible trailing

newline, a colon is appended to the filename before escaping.

The colon is later used to separate the filename from the grep

output so it is fine to add it here instead of a few lines later.

The old code also wasn't POSIX compliant as it used \n in the

replacement section of the s-command. Using \<newline> is the

POSIX compatible method.

LC_ALL=C was added to the two critical sed commands. POSIX sed

manual recommends it when using sed to manipulate pathnames

because in other locales invalid multibyte sequences might

cause issues with some sed implementations. In case of GNU sed,

these particular sed scripts wouldn't have such problems but some

other scripts could have, see:

    info '(sed)Locale Considerations'

This vulnerability was discovered by:

cleemy desu wayo working with Trend Micro Zero Day Initiative

Thanks to Jim Meyering and Paul Eggert discussing the different

ways to fix this and for coordinating the patch release schedule

with gzip.

---

 src/scripts/xzgrep.in | 20 ++++++++++++--------

 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/src/scripts/xzgrep.in b/src/scripts/xzgrep.in

index b180936..e5186ba 100644

--- a/src/scripts/xzgrep.in

+++ b/src/scripts/xzgrep.in

@@ -180,22 +180,26 @@ for i; do

          { test $# -eq 1 || test $no_filename -eq 1; }; then

       eval "$grep"

     else

+      # Append a colon so that the last character will never be a newline

+      # which would otherwise get lost in shell command substitution.

+      i="$i:"

+

+      # Escape & \ | and newlines only if such characters are present

+      # (speed optimization).

       case $i in

       (*'

 '* | *'&'* | *'\'* | *'|'*)

-        i=$(printf '%s\n' "$i" |

-            sed '

-              $!N

-              $s/[&\|]/\\&/g

-              $s/\n/\\n/g

-            ');;

+        i=$(printf '%s\n' "$i" | LC_ALL=C sed 's/[&\|]/\\&/;; $!s/$/\\/');;

       esac

-      sed_script="s|^|$i:|"

+

+      # $i already ends with a colon so don't add it here.

+      sed_script="s|^|$i|"

       # Fail if grep or sed fails.

       r=$(

         exec 4>&1

-        (eval "$grep" 4>&-; echo $? >&4) 3>&- | sed "$sed_script" >&3 4>&-

+        (eval "$grep" 4>&-; echo $? >&4) 3>&- |

+            LC_ALL=C sed "$sed_script" >&3 4>&-

       ) || r=2

       exit $r

     fi >&3 5>&-

-- 

2.35.1