Comprehensive Analysis of PHP Header Parsing Vulnerability CVE-2025-1736

Executive Summary

CVE-2025-1736 represents a moderate-severity input validation vulnerability in PHP’s HTTP stream wrapper implementation, scoring 7.3 CVSS v3.1 and 6.3 CVSS v4.0. The flaw enables header manipulation through insufficient validation of end-of-line characters in user-supplied headers, potentially causing authorization headers to be omitted or misinterpreted. First patched in March 2025, this vulnerability affects PHP versions 8.1. (<8.1.32), 8.2. (<8.2.28), 8.3. (<8.3.19), and 8.4. (<8.4.5). While no in-the-wild exploitation has been observed as of July 2025, the potential for authorization bypass and denial-of-service scenarios necessitates prompt mitigation. The vulnerability stems from inadequate validation of carriage return (\r) characters in header parsing logic, creating opportunities for header injection attacks when malicious input contains carefully crafted newline sequences.

Technical Analysis

Root Cause Mechanism

The vulnerability originates in PHP’s check_has_header() function within the HTTP stream wrapper implementation. This function improperly validates end-of-line characters by exclusively checking for line feed (\n) characters while neglecting carriage return (\r) validation. This oversight enables attackers to inject malicious headers through user-controlled input like cookies or POST parameters. For instance, an attacker could supply the payload Cookie: x=y\nauhtorization:x\r\n where the \n triggers header truncation and the \r facilitates header misinterpretation. The parser’s failure to properly handle \r characters allows bypass of header integrity checks, potentially omitting critical headers like Authorization.

Exploitation Constraints

Successful exploitation requires:

1. User-controllable input passed into HTTP headers
2. Application reliance on HTTP stream wrappers
3. Unpatched PHP versions within specified ranges

The attack surface is limited to PHP applications processing user-supplied header values, particularly those employing basic authentication mechanisms. Network-based exploitation doesn’t require authentication, but effectiveness depends on application-specific header usage patterns.

Attack Sequence

1. Initialization: Attacker identifies user-controllable header parameter (e.g., cookie value)
2. Payload Crafting: Injects \n followed by spoofed header and \r\n termination
3. Header Processing: PHP’s parser fails to validate \r, truncating original headers
4. Impact Realization: Authorization header omission causes authentication failure.

// Simplified vulnerable parsing logic
function check_has_header($value) {
if (strpos($value, "\n") !== false) {
// Malicious \r remains unvalidated
return false;
}
return true;

}

Vendor Response and Patching

Patch Development Timeline

| Date | Milestone |
|——|———–|
| 2025-03-13 | Vulnerability reported via GitHub Security Advisory (GHSA-hgf5-96fm-v528) |
| 2025-03-14 | PHP Group releases patches across all active branches: 8.1.32, 8.2.28, 8.3.19, 8.4.5 |
| 2025-03-30 | CVE formally assigned and published in NVD |
| 2025-05-23 | NetApp releases advisory (NTAP-20250523-0006) for ONTAP 9 |

The primary patch modified the check_has_header() function to include comprehensive carriage return validation, ensuring both \r and \n characters trigger proper header validation. The patch commit specifically added:

1. Explicit \r detection in header values
2. Unified end-of-line character handling
3. Strict header format enforcement

Distribution-Specific Responses

1. Debian: Released updates via DLA-4088-1 (bullseye) and DSA-5878-1 (bookworm)
2. Ubuntu: Patched in noble (8.3.6-0ubuntu0.24.04.4) and oracular (8.3.11-0ubuntu0.24.10.5)
3. NetApp: Issued security advisory with patching guidance for ONTAP 9

Threat Intelligence

Exploitation Likelihood

Current threat intelligence indicates:

1. EPSS Score: 0.15% probability of exploitation within 30 days
2. Proof-of-Concept Availability: No public exploit code observed
3. In-the-Wild Exploitation: Zero confirmed incidents as of July 2025
4. Malware Integration: No evidence of incorporation into exploit kits or ransomware

The absence of exploitation correlates with:

1. Complex vulnerability preconditions requiring specific application functionality
2. Widespread patching adoption within major distributions
3. Limited attack surface for header manipulation vulnerabilities

Supply Chain Implications

Dependency Analysis

While not a direct supply chain vulnerability, CVE-2025-1736 impacts:

1. Web Applications: PHP-based systems processing user-supplied headers
2. CI/CD Pipelines: PHP runtime dependencies in build environments
3. Container Ecosystems: Docker images using vulnerable PHP versions

Mitigation for Development Pipelines

1. Dependency Pinning: Enforce exact PHP version constraints (e.g., 8.4.5) in composer.json
2. CI Hardening:

- name: Audit PHP Dependencies
uses: step-security/harden-runner@v2
with:

php-version: '8.4.5' # Enforce patched version

1. Artifact Scanning: Integrate SCA tools like Dependabot with rulesets targeting PHP vulnerabilities

Detection Methodologies

Signature-Based Detection

YARA Rule for Compromised Headers:

rule PHP_Header_Injection_Attempt {

meta:
description = "Detects potential CVE-2025-1736 exploitation"
reference = "https://github.com/php/php-src/security/advisories/GHSA-hgf5-96fm-v528"
strings:
$header_injection = /[\r\n].+?:/ nocase
condition:
$header_injection and filesize < 10KB

}

Behavioral Indicators

1. Abnormal \r sequences in HTTP headers
2. Unexpected authentication failures in PHP applications
3. Mismatched content-length versus actual body size
4. Unusual header casing variations

SIEM Queries

# Splunk SPL

index=web_logs sourcetype=php_error
| regex _raw="(\\r|%0D).+?:"

| stats count by src_ip, uri

Mitigation Strategies

Configuration Hardening

1. Input Validation: Implement regex filtering for headers:

if (preg_match('/[\r\n]/', $header_value)) {
throw new InvalidHeaderException();

}

1. Content Security: Enable header_remove() for sensitive headers before processing user input
2. Defense-in-Depth: Deploy WAF rules blocking requests containing \r in headers

Compensating Controls

| Control Type | Implementation | Effectiveness |
|————-|—————-|————–|
| Web Application Firewall | Block requests containing \r or \n in headers | High |
| PHP Configuration | Set filter.default = "special_chars" in php.ini | Medium |
| Runtime Protection | Install Suhosin patch with header integrity checks | High |

Related Vulnerabilities

This vulnerability exists within a cluster of PHP header processing flaws:

1. CVE-2025-1734: Streams HTTP wrapper doesn’t fail for headers without colon
2. CVE-2025-1861: Redirect location truncation to 1024 bytes
3. CVE-2025-1217: Improper handling of folded headers

Attackers could chain these vulnerabilities to:

1. Bypass authentication via header omission (CVE-2025-1736)
2. Manipulate redirect locations (CVE-2025-1861)
3. Inject malicious content through header folding (CVE-2025-1217)

Conclusion and Recommendations

CVE-2025-1736 presents a moderate-risk header parsing vulnerability requiring specific preconditions for successful exploitation. While no active exploitation has been observed, organizations should prioritize:

1. Immediate Patching: Upgrade PHP to 8.1.32+, 8.2.28+, 8.3.19+, or 8.4.5+
2. Input Sanitization: Implement rigorous validation of user-supplied header values
3. Defense Layering: Deploy WAF rules blocking carriage returns in headers
4. Monitoring Enhancements: Create SIEM alerts for header manipulation patterns

The vulnerability underscores the critical importance of robust input validation in HTTP header processing. Organizations maintaining PHP applications should conduct targeted audits of header handling mechanisms, particularly in authentication flows.