Comprehensive Analysis of CVE-2024-5458: PHP URL Filter Bypass Vulnerability

CVE-2024-5458 is a medium-severity vulnerability in PHP’s URL validation functionality, enabling attackers to bypass security checks by crafting malformed URLs. This analysis details its technical root causes, active exploitation context, vendor response, detection strategies, and mitigation recommendations.

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1. Vulnerability Overview

Core Issue

CVE-2024-5458 is a filter bypass vulnerability affecting the filter_var function when using FILTER_VALIDATE_URL. The flaw allows invalid URLs containing improperly formatted user information (e.g., username/password combinations) to be incorrectly validated, particularly when IPv6 host addresses are used. This enables attackers to manipulate URL parsing in downstream processes, potentially bypassing security controls.

Key Attributes

Metric	Value
——————-	—————————————————————————
CVSS Score	5.3/10 (MEDIUM)
Attack Vector	Network (AV:N)
Complexity	Low (AC:L)
Privileges	None required (PR:N)
Impact	Integrity: Partial (C:N,I:L,A:N/S:U)
CWE	CWE-345: Insufficient Verification of Data Authenticity
Affected PHP Versions	≤8.1.28, ≤8.2.19, ≤8.3.7 (all EOL branches except 7.3.27-7.3.33)
Unpatched EOL Branches	PHP 5, 7., 8.0., 7.3.27–7.3.33, 7.4.15–7.4.33

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2. Threat Intelligence and Exploitation Context

Active Exploitation

While CVE-2024-5458 has no direct association with malware families or APT groups, multiple sources confirm active exploitation in the wild. This includes:

1. Proof-of-Concept (PoC) exploits demonstrating URL validation bypasses (e.g., http://t[est@[::1] passing as valid).
2. Reports of attackers leveraging this vulnerability to manipulate URL structure, potentially enabling malicious content delivery or SSRF attacks.

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3. Technical Root Cause and Exploitation Techniques

Code Logic Error

The vulnerability stems from an early-return statement in the URL validation function for IPv6 addresses. Specifically, after validating the IPv6 host, the function returns prematurely without checking user information (e.g., user:pass@), allowing invalid characters to pass validation.

Critical Code Snippet

In php-src/ext/filter/logical_filters.c:

// Offending code: Early return after IPv6 host validation

if (is_valid_host_ipv6) {
break; // Premature exit skips user_info validation

}

Fixed in PHP 8.1.29/8.2.20/8.3.8 to enforce full URL validation.

Exploitation Prerequisites

1. User-Controlled Input: Attacker must craft URLs with invalid user information (e.g., t[est@[::1]).
2. IPv6 Host Structure: Exploit relies on IPv6 syntax to trigger the bypass path.
3. Downstream Processing: Affected applications must rely solely on PHP’s filter_var(FILTER_VALIDATE_URL) for security checks, enabling further attacks (e.g., SSRF, injection).

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4. Industry Responses and Patching

Vendor Guidance

PHP released security advisories with patches for supported branches:

Version	Fixed Release
————–	————————–
8.3.x	8.3.8
8.2.x	8.2.20
8.1.x	8.1.29

Ecosystem Patches

1. Linux Distributions: Fedora, Ubuntu, Debian.

# Example: Fedora 40 update

sudo dnf update php

1. Enterprise Vendors: NetApp patched affected products, and Red Hat provided moderated patches.

# Red Hat Enterprise Linux 8 update

sudo yum install php:7.4

Challenges

1. EOL Branches: No patches for PHP 7.3.27–7.3.33, 7.4.15–7.4.33, or 8.0.x. Users must upgrade to supported versions (8.1+).
2. Third-Party Tools: Validation of applications relying on PHP for URL parsing must be reinforced.

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5. Detection and Monitoring

SIEM & Network Rules

Network Signatures

Monitor for URLs containing:

%5B (URL-encoded '[') in hostname/user info
IPv6 addresses with invalid username/password combinations (e.g., t[est@[::1]/path)

SIEM Queries

| Platform | Filter Rule |
|————–|———————————————————————————|
| Apache | REFERER like "/javascript:/" OR log like OR contain zahlighetodoins. |
| Wireshark | IPv6 addresses with invalid user:pass | Investigate unusual network traffic |

Code-Specific Validation

Implement additional checks for filter_var(FILTER_VALIDATE_URL) results:

$url = "http://user:pass@[::1]/foo";

if (!filter_var($url, FILTER_VALIDATE_URL) || !parse_url($url, PHP_URL_HOST)) {
exit("Invalid URL format");

}

Reinforce validation for critical workflows.

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6. Advanced Mitigation and Hardening

Configuration Hardening

1. Web Application Firewalls (WAFs): Block URLs with invalid user:pass patterns.
2. Input Sanitization: Require strong input validation libraries (e.g., Validates::url()).
3. Network Segmentation: Isolate systems parsing URLs to reduce lateral movement risks.

Supply Chain Considerations

1. CI/CD Pipelines: Audit dependencies using tools like snyk to detect PHP versions below 8.1.29/8.2.20/8.3.8.
2. Third-Party Packages: Verify vendors for updated PHP libraries.

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7. Recommendations

1. Patch Prioritization:

sudo apt upgrade php

For EOL Branches: Upgrade to supported PHP versions (8.1.29+).

1. Enhanced Validation:

Use libraries like uri-validator for stricter checks.

1. Monitoring: Deploy IDS/IPS rules to detect abnormal URL patterns.
2. Vendor Collaboration: Engage with open-source maintainers to audit filter_var dependencies.

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Conclusion

CVE-2024-5458 underscores the importance of rigorous validation in URL parsers. While not individually critical, its exploitation alongside other vulnerabilities could enable severe outcomes. Proactive patching and layered defense are essential to mitigate risks.