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CVE-2020-28949 (PEAR Archive_Tar Deserialization of Untrusted Data)

CVE-2020-28949 is a deserialization of untrusted data vulnerability in PEAR Archive_Tar. CISA added it to the Known Exploited Vulnerabilities catalogue on 25 August 2022, which means attackers have used it in real attacks. PEAR Archive_Tar allows an unserialization attack because phar: is blocked but PHAR: is not blocked.

What this vulnerability means in practice

PEAR Archive_Tar is dangerous because archive handling can become code execution when untrusted serialized content is accepted or interpreted during extraction or parsing. In this case, the weakness is not the archive format itself, but the unsafe trust boundary around deserialization.

Attackers look for code paths that read archive metadata, object state, or embedded payload markers and then pass them into deserialization logic. A blocked phar: wrapper is not enough if uppercase or alternate handling such as PHAR: still reaches the vulnerable path.

Why deserialization flaws in archive libraries are high impact

Deserialization bugs are powerful because they can turn a file upload, vendor package, backup, or import workflow into a server-side execution path. That makes the issue especially serious when the library is used in build pipelines, web applications, automation, or any process that handles attacker-controlled archives.

Even when the immediate symptom appears to be file parsing, the security consequence can be much broader: arbitrary code execution, data theft, integrity loss, or the planting of persistence on the host. For vulnerability records and canonical identifier details, readers should verify the entry in the CVE Program and the NIST National Vulnerability Database.

How organisations assess exposure and reduce attack surface

Exposure assessment starts with asset inventory: find where PEAR Archive_Tar is installed, which applications bundle it, and which internet-facing services accept archives, packages, or uploads that may reach it. If a system processes untrusted archives, assume the library is part of the trust boundary until proven otherwise.

Remediation should prioritise vendor guidance, package updates, and removal of unnecessary archive-processing paths. Where patching is delayed, compensating controls should narrow who can supply archives, restrict upload sources, isolate processing, and reduce the permissions available to the process that handles the file.

Detection and post-exploitation review

Detection should focus on unusual archive parsing activity, unexpected process creation, web server anomalies, and follow-on behaviours such as credential use, webshell placement, or outbound connections from the affected host. If the vulnerability was reachable, review logs for archive upload events, extraction failures, and suspicious file writes in application directories.

After remediation, treat the environment as potentially compromised if exploitation was plausible. That means reviewing for persistence, checking whether secrets or session material were exposed, and validating that any service accounts used by the affected application have not been abused for lateral movement or additional execution.

Risk and Threat Considerations

This weakness matters because deserialization bugs often convert a routine file-handling feature into a direct execution path. In internet-facing software, that can let an attacker move from a harmless-looking archive to code execution with the privileges of the application or service account.

Failure mechanism: The attacker supplies a crafted archive or archive reference that reaches unsafe unserialization logic, bypassing superficial wrapper filtering and triggering execution or object manipulation.

Impact: Successful exploitation can lead to arbitrary code execution, data access, persistence, and further movement inside the environment.

Standards & Framework Alignment

This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.

MITRE ATT&CK addresses the attack and risk surface, while OWASP ASVS, NIST SP 800-53 Rev 5 and CIS Controls v8 set the governance and control requirements practitioners need to meet.

Framework Control / Reference Relevance
OWASP ASVS V5 — File Handling Archive parsing and upload handling can trigger unsafe deserialization paths.
Recommendation — Harden archive upload and extraction paths so untrusted files never reach unsafe deserialization logic.
NIST SP 800-53 Rev 5 SI-10 — Information Input Validation The flaw is reached through attacker-controlled archive input that must be validated and constrained.
RA-5 — Vulnerability Monitoring and Scanning CVE reachability and deployed-package discovery are central to exposure assessment for this issue.
Recommendation — Validate archive inputs before parsing and reject content that can reach unsafe deserialization. Scan assets for vulnerable Archive_Tar deployments and track remediation until exposure is removed.
CIS Controls v8 CIS-7 — Continuous Vulnerability Management The term is a real exploited CVE that requires inventory, prioritisation and remediation workflow.
Recommendation — Prioritise this exploited CVE in vulnerability management and verify remediation on all affected assets.
MITRE ATT&CK T1059 — Command and Scripting Interpreter Deserialization exploitation can be a precursor to arbitrary command execution on the target host.
Recommendation — Hunt for post-exploitation command execution after suspicious archive parsing events.

Practitioner Guidance

What to watch for: Treat any code path that parses attacker-controlled archives as sensitive, especially if it relies on blacklist-style blocking of only one wrapper variant. Filter design should assume alternate casing, alternate schemes, and indirect parser behaviour will be tested.

Practical takeaway: Prefer trusted parser behaviour, minimise archive privileges, and validate the full application path, not just the wrapper string.