virtualenv bash activate runs commands from an environment name with a backslash before $

Summary

The bash activate script escapes the environment name for only one of the two passes bash makes over PS1, so a backslash in the name undoes the escaping and a name containing \$(...) runs as commands in the user's shell each time bash draws the prompt.

Affected area

src/virtualenv/activation/bash/activate.sh, the block that prepends the environment name to PS1.

Attacker-controlled input

The environment name: the environment directory's name, or the --prompt or VIRTUALENV_PROMPT value. An attacker needs to influence it, for example through a CI job or wrapper tool that passes a branch, project or job name through, or a directory in a cloned repository or unpacked archive. --prompt . takes the current directory's name.

Details

bash handles PS1 in two passes each time it draws the prompt: it first decodes backslash prompt escapes, turning \\ into \ and \$ into $ (or # for root), and then runs parameter expansion, command substitution and arithmetic expansion on the result. Since 21.13.0 the script escapes backslashes, dollar signs and backticks in the name, but only for the second pass, so the decode pass strips the escaping that a backslash in the name precedes. Releases before 21.13.0 do not escape the name at all. A bash running as root decodes \$ to #, so the payload shows the wrong text there but runs nothing. zsh does no decode pass and is not affected.

Proof of concept

$ virtualenv --prompt 'a\$(touch PWNED)' env
$ bash --norc --noprofile -c 'source env/bin/activate; printf "%s" "${PS1@P}"'
$ ls PWNED
PWNED

Impact

Arbitrary command execution in the shell of a user who sources the activate script of an environment whose name an attacker influenced, with that user's privileges, once per prompt draw.

Versions tested

21.14.6 with bash 5.2.15 on Debian bookworm, as an unprivileged user and as root.

Known mitigation

Set VIRTUAL_ENV_DISABLE_PROMPT=1 before you source activate, which leaves PS1 alone, or run shopt -u promptvars if your prompt does not need expansion. Check the name of an environment you did not create before you source its activate.

Latest release

• The problem reproduces on the latest virtualenv release.

21.14.6

Patches

Fixed in 21.14.7. Under bash, activate now escapes backslashes, dollar signs and backticks for both passes, so the example above runs nothing and the prompt shows the name as given. zsh keeps the single escaping it needs. Fix: https://github.com/pypa/virtualenv/commit/c3104e02474b652832a9bbaae79f6e1696296932

Credits

Found while running the virtualenv test suite as root inside the Anthropic OSS Scanner build image.