Access-control bypass via leading-zero CIDR confusion
Published Feb 27, 2026 · Updated Feb 27, 2026
Incorrect IP conversion in Net::CIDR before 0.24 allows remote attackers to bypass IP-based access controls through crafted CIDR input. addr2cidr preserves leading zeros in IPv4 octets, while cidrlookup treats the resulting text as decimal even when later consumers interpret it as octal. Exploitation requires a caller to accept untrusted CIDR text without cidrvalidate; the mismatch can cause downstream code to trust the wrong network identity.
Summary
What happened
Incorrect IP conversion in Net::CIDR before 0.24 allows remote attackers to bypass IP-based access controls through crafted CIDR input. addr2cidr preserves leading zeros in IPv4 octets, while cidrlookup treats the resulting text as decimal even when later consumers interpret it as octal. Exploitation requires a caller to accept untrusted CIDR text without cidrvalidate; the mismatch can cause downstream code to trust the wrong network identity.
The record
- CVE
- CVE-2021-4456
- Published
- Feb 27, 2026
- Updated
- Feb 27, 2026
- Vendor
- Sam Varshavchik
- Product
- Net::CIDR
- Classifications
- CWE-704
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Feb 27, 2026CVE publishedPublication date reported by the CVE source.
- Feb 27, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=0 <0.24
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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