CVE-2025-1861: PHP HTTP Redirect Truncation Vulnerability

CVE-2025-1861 Overview

CVE-2025-1861 is an Input Validation Error vulnerability affecting PHP's HTTP redirect handling mechanism. When PHP parses HTTP redirects in response to HTTP requests, a buffer size limitation of 1024 bytes causes URLs to be improperly truncated. According to RFC9110, the recommended limit for location values should be 8000 bytes. This discrepancy may lead to incorrect URL truncation, potentially redirecting users to unintended locations.

Critical Impact

Applications using affected PHP versions may experience incorrect redirect behavior when processing HTTP responses with long location headers, potentially leading to security issues such as open redirect vulnerabilities or misdirected traffic.

Affected Products

• PHP versions 8.1.* before 8.1.32
• PHP versions 8.2.* before 8.2.28
• PHP versions 8.3.* before 8.3.19
• PHP versions 8.4.* before 8.4.5
• NetApp ONTAP 9

Discovery Timeline

• 2025-03-30 - CVE-2025-1861 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-1861

Vulnerability Analysis

This vulnerability stems from an Improper Calculation of Buffer Size (CWE-131) within PHP's HTTP redirect parsing functionality. The issue occurs when PHP processes HTTP redirect responses containing a Location header value. The internal buffer allocated for storing the location value is limited to 1024 bytes, which falls significantly short of the 8000-byte recommendation specified in RFC9110.

When a redirect URL exceeds this 1024-byte boundary, PHP silently truncates the value, potentially resulting in malformed or incomplete URLs being used for the redirect destination. This behavior can be exploited in scenarios where attackers can influence the redirect location through long URLs.

Root Cause

The root cause is an insufficient buffer allocation in PHP's HTTP redirect handling code. The fixed 1024-byte buffer was implemented without considering the RFC9110 specification, which recommends supporting location values up to 8000 bytes. This buffer size miscalculation causes longer URLs to be silently truncated rather than being handled appropriately or generating an error.

Attack Vector

The attack vector is network-based and requires specific conditions to exploit. An attacker would need to either:

1. Control or manipulate a server that returns HTTP redirect responses to a PHP application
2. Craft redirect URLs that, when truncated at 1024 bytes, point to an attacker-controlled domain

The vulnerability exploitation occurs during the processing of the HTTP Location header in redirect responses. The truncation could result in the redirect pointing to a different domain than intended, particularly if an attacker can craft a URL where the truncation point falls in a way that changes the effective destination.

The vulnerability can manifest in any PHP application that follows HTTP redirects automatically, such as when using functions like file_get_contents() with HTTP stream wrappers or the cURL extension when configured to follow redirects.

Detection Methods for CVE-2025-1861

Indicators of Compromise

• Unexpected redirect behavior in PHP applications when processing HTTP responses
• Application logs showing truncated URLs in redirect chains
• HTTP requests being sent to unexpected destinations after redirect processing
• Unusual network traffic patterns from PHP applications following truncated redirect URLs

Detection Strategies

• Monitor PHP application logs for redirect-related warnings or errors
• Implement network monitoring to detect unexpected redirect destinations
• Use application-level logging to track all HTTP redirects and their full location values
• Audit PHP applications for usage of HTTP redirect functionality with external URLs

Monitoring Recommendations

• Enable verbose logging for HTTP client operations in PHP applications
• Deploy network monitoring to track outbound connections from PHP processes
• Implement URL validation before and after redirect processing
• Set up alerts for PHP applications connecting to unexpected external domains

How to Mitigate CVE-2025-1861

Immediate Actions Required

• Upgrade PHP to patched versions: 8.1.32, 8.2.28, 8.3.19, or 8.4.5
• Review applications that process HTTP redirects with external URLs
• Implement input validation for redirect URLs in application code
• Consider disabling automatic redirect following where not strictly necessary

Patch Information

The PHP development team has addressed this vulnerability in the following versions:

• PHP 8.1.32
• PHP 8.2.28
• PHP 8.3.19
• PHP 8.4.5

Organizations should upgrade to these versions or later. For detailed information, refer to the PHP Security Advisory on GitHub. Additional advisories have been published by Debian LTS and NetApp.

Workarounds

• Implement application-level URL length validation before processing redirects
• Use custom redirect handling that validates the full URL rather than relying on PHP's built-in behavior
• Disable automatic redirect following and manually handle redirects with URL validation
• Deploy network-level controls to restrict outbound connections from PHP applications

# Check current PHP version
php -v

# Verify PHP version is patched (should be >= 8.1.32, 8.2.28, 8.3.19, or 8.4.5)
php -r "echo 'PHP Version: ' . PHP_VERSION . PHP_EOL;"

# Update PHP on Debian/Ubuntu systems
sudo apt update && sudo apt upgrade php

# Update PHP on RHEL/CentOS systems
sudo yum update php