PYSEC-2026-4012

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Import Source
https://github.com/pypa/advisory-database/blob/main/vulns/virtualenv/PYSEC-2026-4012.yaml
JSON Data
https://api.test.osv.dev/v1/vulns/PYSEC-2026-4012
Aliases
Published
2026-09-17T16:42:47Z
Modified
2026-09-30T16:45:02Z
Summary
virtualenv writes prompt values into pyvenv.cfg without sanitizing line boundaries, allowing configuration injection
Details

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.

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. 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.

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.

Fixed in 21.7.11: 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.

References
Credits
    • Bernát Gábor - FINDER

Affected packages

PyPI / virtualenv

Package

Name
virtualenv
View open source insights on deps.dev
Purl
pkg:pypi/virtualenv

Affected ranges

Type
ECOSYSTEM
Events
Introduced
0 Unknown introduced version / All previous versions are affected
Fixed
21.7.11

Affected versions

0.*
0.8
0.8.1
0.8.2
0.8.3
0.8.4
0.9
0.9.1
0.9.2
1.*
1.0
1.1
1.2
1.3
1.3.1
1.3.2
1.3.3
1.3.4
1.4rc1
1.4
1.4.1
1.4.2
1.4.3
1.4.4
1.4.5
1.4.6
1.4.7
1.4.8
1.4.9
1.5
1.5.1
1.5.2
1.6
1.6.1
1.6.2
1.6.3
1.6.4
1.7
1.7.1
1.7.1.1
1.7.1.2
1.7.2
1.8
1.8.1
1.8.2
1.8.3
1.8.4
1.9
1.9.1
1.10
1.10.1
1.11
1.11.1
1.11.2
1.11.3
1.11.4
1.11.5
1.11.6
12.*
12.0
12.0.1
12.0.2
12.0.4
12.0.5
12.0.6
12.0.7
12.1.0
12.1.1
13.*
13.0.0
13.0.1
13.0.2
13.0.3
13.1.0
13.1.1
13.1.2
14.*
14.0.0
14.0.1
14.0.2
14.0.3
14.0.4
14.0.5
14.0.6
15.*
15.0.0
15.0.1
15.0.2
15.0.3
15.1.0
15.2.0
16.*
16.0.0
16.1.0
16.2.0
16.3.0
16.3.1.dev0
16.4.0
16.4.1
16.4.3
16.4.4.dev0
16.5.0
16.6.0
16.6.1
16.6.2
16.7.0
16.7.1
16.7.2
16.7.3
16.7.4
16.7.5
16.7.6
16.7.7
16.7.8
16.7.9
16.7.10
16.7.11
16.7.12
20.*
20.0.0b1
20.0.0b2
20.0.0
20.0.1
20.0.2
20.0.3
20.0.4
20.0.5
20.0.6
20.0.7
20.0.8
20.0.9
20.0.10
20.0.11
20.0.12
20.0.13
20.0.14
20.0.15
20.0.16
20.0.17
20.0.18
20.0.19
20.0.20
20.0.21
20.0.22
20.0.23
20.0.24
20.0.25
20.0.26
20.0.27
20.0.28
20.0.29
20.0.30
20.0.31
20.0.32
20.0.33
20.0.34
20.0.35
20.1.0
20.2.0
20.2.1
20.2.2
20.3.0
20.3.1
20.4.0
20.4.1
20.4.2
20.4.3
20.4.4
20.4.5
20.4.6
20.4.7
20.5.0
20.6.0
20.7.0
20.7.1
20.7.2
20.8.0
20.8.1
20.9.0
20.10.0
20.11.0
20.11.1
20.11.2
20.12.0
20.12.1
20.13.0
20.13.1
20.13.2
20.13.3
20.13.4
20.14.0
20.14.1
20.15.0
20.15.1
20.16.0
20.16.1
20.16.2
20.16.3
20.16.4
20.16.5
20.16.6
20.16.7
20.17.0
20.17.1
20.18.0
20.19.0
20.20.0
20.21.0
20.21.1
20.22.0
20.23.0
20.23.1
20.24.0
20.24.1
20.24.2
20.24.3
20.24.4
20.24.5
20.24.6
20.24.7
20.25.0
20.25.1
20.25.2
20.25.3
20.26.0
20.26.1
20.26.2
20.26.3
20.26.4
20.26.5
20.26.6
20.27.0
20.27.1
20.28.0
20.28.1
20.29.0
20.29.1
20.29.2
20.29.3
20.30.0
20.31.0
20.31.1
20.31.2
20.32.0
20.33.0
20.33.1
20.34.0
20.35.0
20.35.1
20.35.2
20.35.3
20.35.4
20.36.0
20.36.1
20.38.0
20.39.0
20.39.1
21.*
21.0.0
21.1.0
21.2.0
21.2.1
21.2.2
21.2.3
21.2.4
21.3.0
21.3.1
21.3.2
21.3.3
21.4.0
21.4.1
21.4.2
21.4.3
21.5.0
21.5.1
21.5.2
21.6.0
21.6.1
21.7.0
21.7.1
21.7.2
21.7.3
21.7.4
21.7.5
21.7.6
21.7.7
21.7.8
21.7.9
21.7.10

Database specific

source 
"https://github.com/pypa/advisory-database/blob/main/vulns/virtualenv/PYSEC-2026-4012.yaml"