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CVE Record

CVE-2021-29922: library/std/src/net/parser.rs in Rust before 1.53.0 does not properly consider extraneous zero characters a...

library/std/src/net/parser.rs in Rust before 1.53.0 does not properly consider extraneous zero characters at the beginning of an IP address string, which (in some situations) allows attackers to bypass access control that is based on IP addresses, because of unexpected octal interpretation.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

Rust before 1.53.0 could interpret IPv4 addresses with leading zeroes unexpectedly. If an application used that parsing result for IP allowlists or blocklists, an attacker might bypass the intended access decision by supplying an ambiguous address string.

Executive priority

Treat this as a targeted remediation item, not an emergency by default. Prioritize internet-facing or multi-tenant services where IP-based controls protect sensitive functions.

Technical view

The issue is in Rust standard library IPv4 parsing, specifically library/std/src/net/parser.rs. Extra leading zero characters could lead to unexpected octal interpretation. The vulnerable condition matters when parsed addresses drive access-control logic. The supplied sources identify Rust before 1.53.0 as affected.

Likely exposure

Exposure is likely limited to Rust applications built with affected Rust versions that parse user-supplied IPv4 strings and use those results for IP-based authorization, filtering, or routing decisions.

Exploitation context

The source bundle does not show CISA KEV listing or confirmed active exploitation. Public advisories describe a bypass class, but exploitation depends on application-specific IP parsing and access-control behavior.

Researcher notes

Key uncertainty is application reachability. The CVE describes a parser flaw, but impact requires downstream ACL logic. Validate exposure through code review and build provenance rather than assuming every Rust application is affected.

Mitigation direction

  • Upgrade Rust toolchains and rebuild affected applications with Rust 1.53.0 or later.
  • Review vendor or distribution guidance for packaged Rust versions.
  • Avoid accepting ambiguous IPv4 text in security decisions.
  • Normalize and validate IP address input before authorization checks.
  • Reassess IP-based access controls that depend on Rust parsing.

Validation and detection

  • Inventory Rust services built with toolchains older than 1.53.0.
  • Identify code paths parsing user-controlled IPv4 address strings.
  • Check whether parsed addresses influence allowlists, blocklists, or trust boundaries.
  • Test ambiguous leading-zero IPv4 inputs against defensive validation only.
  • Confirm rebuilt binaries use a fixed Rust standard library.
Prepared
Confidence
medium
Sources
7

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
7Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/an/an/aListed
Weakness

CWE details

No CWE listed

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