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PythonApplication

CVE-2021-29921

CRITICAL · 9.8 CVSS v3.1 Published 2021-05-06
Fix available
A fix is available. Upgrade to 3.8.12 / 3.9.5 or later.
See remediation →
100/100
Remediation priority · Urgent
Public exploit Remotely reachable No privileges Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
In Python before 3,9,5, the ipaddress library mishandles leading zero characters in the octets of an IP address string. This (in some situations) allows attackers to bypass access control that is based on IP addresses.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis

A detailed technical summary for this CVE is being prepared.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
PythonApplication
Affected:>= 3.8.0, < 3.8.12>= 3.9.0, < 3.9.5
Communications Cloud Native Core Automated Test SuiteApplication
Affected:= 1.8.0
Communications Cloud Native Core Binding Support FunctionApplication
Affected:= 1.11.0
Communications Cloud Native Core Network Slice Selection FunctionApplication
Affected:= 1.8.0
GraalvmApplication
Affected:= 20.3.2= 21.1.0
Zfs Storage Appliance KitApplication
Affected:= 8.8

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 3.8.12 / 3.9.5 or later
Fixed in 3.8.123.9.5
Vendor patch bugs.python.org →
Recommended fix High confidence

Python 3.8.12+ or Python 3.9.5+ (or 3.10+)

  1. Upgrade Python to version 3.8.12 or later for the 3.8.x series
  2. Alternatively, upgrade Python to version 3.9.5 or later for the 3.9.x series
  3. If using Python 3.10+, verify you are on a version that includes the fix (3.10.0 and later include the fix)
  4. For other affected products (Graalvm, Communications Cloud Native Core, Zfs Storage Appliance Kit), contact the respective vendors for patched versions as specific fixes are not detailed in available sources
Caveat The fix changes IP address parsing behavior; leading zeros in octets are now consistently interpreted as invalid (e.g., '010.1.1.1' will raise an error rather than being interpreted as '10.1.1.1'). Applications relying on the previous lenient parsing may need updates.

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $2,300. Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-29921 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data