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Fencer Vulns
medium 6.2

CVE-2026-39245 json

decompress before 4.2.2 contains an improper path containment check that enables directory traversal and arbitrary file write.

published 2026-07-09 4 mo ago modified 2026-07-13 3 mo ago also EUVD-2026-42769

cvss
6.2
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.4%
EPSS 0.4% chance of exploitation in 30 days, percentile 31%
public exploits
0
none known

Summary

decompress before 4.2.2 contains an improper path containment check that enables directory traversal and arbitrary file write. The safeMakeDir function (index.js line 29) and the extraction path validation (index.js line 106) use String.indexOf() to verify the resolved path is within the output directory: realDestinationDir.indexOf(realOutputPath) !== 0. This check is flawed because it does not enforce a path separator boundary. For example, "/tmp/app_config".indexOf("/tmp/app") returns 0, incorrectly passing the check even though /tmp/app_config is outside /tmp/app. Combined with the unvalidated symlink creation in the same package, an attacker can write arbitrary files to directories adjacent to the extraction target. This is a bypass of the fix for CVE-2020-12265. The correct check requires appending a path separator: realParentPath.indexOf(realOutputPath + path.sep) !== 0.

Patch / fix

  • dotnet8.0 hummingbird 1 8.0.128-1.1.hum1
  • dotnet-sdk-8.0 hummingbird 1 8.0.128-1.1.hum1
  • dotnet-hostfxr-8.0 hummingbird 1 8.0.28-1.1.hum1
  • dotnet-runtime-8.0 hummingbird 1 8.0.28-1.1.hum1
  • dotnet-sdk-dbg-8.0 hummingbird 1 8.0.128-1.1.hum1
  • dotnet-templates-8.0 hummingbird 1 8.0.128-1.1.hum1
  • aspnetcore-runtime-8.0 hummingbird 1 8.0.28-1.1.hum1
  • dotnet-runtime-dbg-8.0 hummingbird 1 8.0.28-1.1.hum1

red hat advisories RHSA-2026:37577

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-39245 updated cvss 6.2

decompress: decompress: path traversal via indexOf containment bypass allows arbitrary file write (bypass of CVE-2020-12265 fix)

  • Red Hat Build of Keycloak decompress 4.2.1
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Hardened Images dotnet8.0 8.0.128-1.1.hum1, aspnetcore-runtime-8.0 8.0.28-1.1.hum1
    fixed: vendor fix RHSA-2026:37577
  • Red Hat Advanced Cluster Management for Kubernetes 2 rhacm2/volsync-rhel9, rhacm2/volsync-operator-bundle
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
4 mo ago

Exploit prediction

0.38% chance of exploitation in the next 30 days, higher than 31% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/kevva/decompress/issues/115 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CISA ADP 3.1 6.2 CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
CNA 3.1 6.2 CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Classification

Weakness
  • CWE-22 Improper Limitation of a Pathname to a Restricted Directory

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.