External risk intelligence

libtar Crafted Tar File Out-of-Bounds Read Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2021-33643

The vulnerability affects libtar, a C library for manipulating tar files. As a low-level utility library used by other software to process file archives, it is not a network service or internet-facing application itself. Exposure is dependent on local file processing or specific downstream applications that might handle untrusted input, making direct public network reachability very unlikely.

Out-of-bounds Read

Feep Libtar

before 1.2.2120.0322.03353637

Halo Surface Signal: 1 out of 5 — much less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability has been identified in systems processing tar archives. This issue could allow an attacker to read unintended memory, potentially impacting system stability or leading to further security risks. The main concern is confirming if your environment utilizes the affected library for handling tar files.

The issue involves unexpected memory access during file processing.
Leadership should recall this due to potential system instability.
Confirm relevance and exposure within your operations.

Attack Path

How an attacker could exploit the issue

An attacker can exploit this vulnerability by submitting a specially crafted tar file. This file's header will contain a size of zero for a specific field, triggering a memory allocation issue. This can lead to an out-of-bounds read, potentially exposing sensitive information or causing denial of service.

Crafted archive file submission.
Processing a tar file with a zero-size header.
Information disclosure or denial of service.

Live Threat

Current exploitation, exposure, and threat context

When a crafted tar file is processed, an out-of-bounds read could occur. This may impact the integrity of the system processing the file.

System data during tar processing.
Processing a crafted tar file.
Potential for data corruption or instability.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in libtar can be exploited by attackers submitting a crafted tar file, leading to an out-of-bounds read. Real-world ownership typically falls to the platform or infrastructure teams managing the systems where libtar is utilized, potentially involving vendor management if it's part of a third-party application. The first practical step is to identify all instances of the affected library, assess their exposure and criticality, and then coordinate remediation with accountable owners, prioritizing business-critical systems.

Platform and infrastructure teams own remediation.
Verify affected systems and data criticality.
Plan and execute remediation per risk.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is libtar?

libtar is a C programming library specifically designed for managing and manipulating tar archive files. Developers integrate this library into their applications to handle the tasks of creating, extracting, and reading archive contents. Because it acts as a foundational building block for file operations, it is frequently embedded within various software distributions and utility packages used to process structured data.

What does CVE-2021-33643 mean?

CVE-2021-33643 describes an out-of-bounds read vulnerability, which falls under the CWE-125 weakness class. In technical terms, this happens when software reads data past the end or before the beginning of the intended memory buffer. In this specific case, the weakness is triggered when the library processes a malformed file header, potentially leading to unauthorized memory access or service instability.

How is this vulnerability triggered?

The flaw is triggered when the library parses a specially crafted tar file containing a header structure where the defined size field is set to zero. This specific condition causes the software to incorrectly invoke a memory allocation function. Note that standard, correctly formatted archive files do not trigger this memory error, as the issue is strictly tied to this specific, malformed header input.

Do I need to worry about this if I don't have internet-facing services?

While Halo Surface Signal notes that libtar is not a network service itself, you should still evaluate where your systems process untrusted tar files. If a local system or internal application accepts and unpacks archives from external or untrusted sources, it remains a potential vector. Access is not limited to remote internet-facing interfaces; any workflow processing archive files is a candidate for review.

What is the first step to address CVE-2021-33643?

Begin by auditing your infrastructure to locate every instance where libtar is installed or bundled within your applications. Since libtar is a library, it is often a dependency for other software rather than a standalone program, so check software manifests or package managers. Once identified, consult your operating system or software vendor to determine if a patched version is available for your specific environment.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.