# CVE-2021-4456: Net::CIDR versions before 0.24 for Perl mishandle leading zeros in IP CIDR addresses, which may have unspecified impact

> Attackers can bypass IP-based access controls by exploiting leading zero mishandling in Net::CIDR Perl library functions.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Feb 2026
- **Updated:** 27 Feb 2026
- **Also known as:** DEBIAN-CVE-2021-4456, GHSA-6xjh-63ff-92mc, UBUNTU-CVE-2021-4456, USN-8110-1, openSUSE-SU-2026:10756-1, openSUSE-SU-2026:20773-1

## What is CVE-2021-4456?

CVE-2021-4456 is a medium-severity vulnerability in MRSAM Net::CIDR, published on 27 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Net::CIDR versions before 0.24 for Perl mishandle leading zeros in IP CIDR addresses, which may have unspecified impact.

The functions `addr2cidr` and `cidrlookup` may return leading zeros in a CIDR string, which may in turn be parsed as octal numbers by subsequent users. In some cases an attacker may be able to leverage this to bypass access controls based on IP addresses.

The documentation advises validating untrusted CIDR strings with the `cidrvalidate` function. However, this mitigation is optional and not enforced by default. In practice, users may call `addr2cidr` or `cidrlookup` with untrusted input and without validation, incorrectly assuming that this is safe.

Weakness: CWE-704 (Incorrect Type Conversion or Cast)

## Which versions are affected by CVE-2021-4456?

It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `perl-Net-CIDR` | openSUSE:Tumbleweed | All versions | 0.270.0-2.1 |

Products: Mrsam Net\.

## How do I fix CVE-2021-4456?

Upgrade to a fixed version.

## Is CVE-2021-4456 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-4456 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-4456 and fix it if it is affected.

What it is: CVE-2021-4456 is a medium-severity vulnerability in MRSAM Net::CIDR, published on 27 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2021-4456 exists.

Affected product: MRSAM Net::CIDR. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-4456/ (Markdown: https://insights.sbomify.com/CVE-2021-4456/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-4456
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-4456
- OSV: https://osv.dev/vulnerability/CVE-2021-4456

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-4456/
Summaries are generated and may contain mistakes; the official description above is the record.