Details
### Summary
`pyvenv.cfg` is a line-based format with no escape syntax. `PyEnvCfg.write()` wrote values verbatim, while `PyEnvCfg._read_values()` parses the file with `str.splitlines()`. A value containing a line boundary therefore became additional configuration lines, and because reading is last-wins, the injected keys replaced any key written earlier in the file.
### Impact
The `prompt` value is the reachable input: it is set by `--prompt`, by the `VIRTUALENV_PROMPT` environment variable, or from the config file, and `write()` emits `prompt` before `home`. A crafted prompt can therefore set `home` in the generated `pyvenv.cfg`:
```console
$ virtualenv --prompt $'x"\nhome = /attacker/path\nprompt = "z' venv
$ grep '^home' venv/pyvenv.cfg
home = /attacker/path
```
`home` is what tooling reads to locate the base interpreter, so a consumer that trusts it can be pointed elsewhere. `implementation`, `version_info`, `version`, `executable`, `command` and `virtualenv` are also written before `prompt` and can be replaced the same way.
This requires the prompt to come from somewhere other than the person running the command, for example a CI job templating a branch name into it, tooling deriving an environment name from user-supplied data, or an inherited `VIRTUALENV_PROMPT`. Where the operator supplies the prompt directly they already control the command line, and the effect is corruption rather than privilege gain: the value is truncated at the boundary and read back with a dangling quote.
### Details
The boundary set is the one `str.splitlines()` recognizes, which is wider than `\n`: `\r`, `\v`, `\f`, the file, group and record separators, `U+0085`, `U+2028` and `U+2029` were all written through unchanged and all split the line when read back.
### Patches
`PyEnvCfg.write()` now collapses those boundaries to spaces as it serializes each line, so it cannot emit a structurally invalid file regardless of what a caller places in `content`.
### Workarounds
Do not pass externally influenced data as the virtualenv prompt. Strip line boundaries from any value before using it as `--prompt` or `VIRTUALENV_PROMPT`.
