GHSA-vxj4-q4g9-crj7: OS Command Injection
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 VIRTUALENVPROMPT 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
console $ 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 VIRTUALENVDISABLEPROMPT=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
- [x] 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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/virtualenvto a version that resolves this vulnerability.Fixed in 21.14.7 - Upgrade
Upgrade
virtualenvto a version that resolves this vulnerability.Fixed in 21.14.7 - Configuration
Set VIRTUAL_ENV_DISABLE_PROMPT=1 before sourcing activate; alternatively, if the prompt does not need expansion, run shopt -u promptvars.
virtualenv bash activate script VIRTUAL_ENV_DISABLE_PROMPT = 1 - Compensating control
Check the name of an environment you did not create before sourcing its activate script.
Event History
Frequently Asked Questions
Who is most likely to be exposed to attacker-controlled environment names?
Users are exposed when a CI job, wrapper tool, cloned repository, or unpacked archive can influence the virtual environment directory name or the value passed through --prompt or VIRTUALENV_PROMPT. Using --prompt . also makes the current directory name relevant.
What does an attacker need for code execution?
The attacker needs to influence the environment name and include a backslash followed by command-substitution syntax such as \$(...). No privileges are required, but the user must activate the environment and bash must draw the prompt.
How can I identify potentially dangerous environments before activating them?
Inspect environment directory names and any --prompt or VIRTUALENV_PROMPT values supplied by automation or wrapper scripts. Treat values containing a backslash and command-substitution syntax, particularly \$(...), as suspicious.
When does the injected command run?
It runs in the user's shell each time bash renders the prompt after the affected activation script has prepended the environment name to PS1.