oss-security - CVE-2023-49284: fish command substitution output can trigger shell expansion

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Date: Fri, 8 Dec 2023 10:54:02 -0800
From: Alan Coopersmith <alan.coopersmith@...cle.com>
To: oss-security@...ts.openwall.com
Subject: CVE-2023-49284: fish command substitution output can trigger shell
 expansion

https://github.com/fish-shell/fish-shell/security/advisories/GHSA-2j9r-pm96-wp4f
reports:

Package: fish-shell
Affected versions: < 3.6.2
Patched versions: 3.6.2
CVSS: 3.9 (CVSS:3.1/AV:L/AC:L/PR:L/UI:R/S:U/C:L/I:N/A:L)
CVE ID: CVE-2023-49284

Impact:

fish shell uses certain Unicode non-characters internally for marking wildcards 
and expansions. It will incorrectly allow these markers to be read on command 
substitution output, rather than transforming them into a safe internal 
representation.

While this may cause unexpected behavior with direct input (for example, echo 
\UFDD2HOME has the same output as echo $HOME), this may become a minor security 
problem if the output is being fed from an external program into a command 
substitution where this output may not be expected.

Consider the following:

In foo.py:

print("\ufdd2HOME") # Perhaps this value is retrieved from a database or 
external source

At the shell:

 > echo $(python3 foo.py)
/home/fishuser

This design flaw was introduced in very early versions of fish, predating the 
version control system, and is thought to be present in every version of fish 
released in the last 15 years or more, although with different characters.

Code execution does not appear to be possible, but denial of service (through 
large brace expansion) or information disclosure (such as variable expansion) is 
potentially possible under certain circumstances.

Patches:

fish shell 3.6.2 has been released to correct this issue.

-- 
         -Alan Coopersmith-                 alan.coopersmith@...cle.com
          Oracle Solaris Engineering - https://blogs.oracle.com/solaris

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