CVE-2020-1927: Apache HTTP Server Redirect Vulnerability

CVE-2020-1927 Overview

CVE-2020-1927 is an open redirect vulnerability (CWE-601) affecting Apache HTTP Server versions 2.4.0 through 2.4.41. The vulnerability exists in the mod_rewrite module, where redirects configured to be self-referential can be manipulated using encoded newlines, causing the server to redirect users to unexpected URLs within the request URL. This flaw enables attackers to redirect victims to malicious websites, potentially facilitating phishing attacks or credential theft.

Critical Impact

Attackers can exploit this vulnerability to redirect users from trusted Apache web servers to malicious external sites, enabling phishing campaigns, credential harvesting, and delivery of malware through trusted domain reputation abuse.

Affected Products

• Apache HTTP Server 2.4.0 to 2.4.41
• Fedora 31 and 32
• Debian Linux 9.0 and 10.0
• Canonical Ubuntu Linux 16.04 ESM, 18.04 LTS, 20.04 LTS
• openSUSE Leap 15.1
• NetApp OnCommand Unified Manager Core Package
• Broadcom Brocade Fabric Operating System
• Oracle Communications Element Manager 8.1.1, 8.2.0, 8.2.1
• Oracle Communications Session Report Manager 8.1.1, 8.2.0, 8.2.1
• Oracle Communications Session Route Manager 8.1.1, 8.2.0, 8.2.1
• Oracle Enterprise Manager Ops Center 12.4.0.0
• Oracle Instantis EnterpriseTrack
• Oracle SD-WAN Aware 8.2
• Oracle ZFS Storage Appliance Kit 8.8

Discovery Timeline

• April 2, 2020 - CVE-2020-1927 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-1927

Vulnerability Analysis

This open redirect vulnerability stems from improper handling of encoded newline characters within the mod_rewrite module. When Apache HTTP Server processes redirect rules intended to be self-referential (redirecting back to the same server), the URL parsing logic fails to properly validate encoded newline sequences such as %0d (carriage return) and %0a (line feed).

Attackers can inject these encoded characters into request URLs to break out of the intended redirect target. The parser misinterprets the URL structure after encountering these characters, allowing the attacker to specify an arbitrary external destination. This bypass technique exploits the difference between how the server validates the redirect target versus how browsers interpret the resulting Location header.

The vulnerability requires user interaction—a victim must click a maliciously crafted link pointing to the vulnerable server. However, since the link originates from a trusted domain, users are more likely to trust and click such links, making this an effective vector for phishing attacks.

Root Cause

The root cause is insufficient input validation in the mod_rewrite module's redirect handling logic. The module fails to properly sanitize and validate URL-encoded characters, particularly newline sequences (%0d%0a), before constructing redirect responses. Self-referential redirect rules using the RewriteRule directive with redirect flags ([R] or [L,R]) do not adequately verify that the resulting redirect URL matches the intended target domain.

Attack Vector

The attack is network-based and requires user interaction. An attacker crafts a malicious URL targeting a vulnerable Apache server with mod_rewrite enabled and self-referential redirect rules configured. The URL contains encoded newline characters followed by the attacker's malicious destination.

When a victim clicks this link, the Apache server processes the request and generates a redirect response. Due to the vulnerability, the Location header contains the attacker-controlled URL instead of the intended self-referential target. The victim's browser follows the redirect to the malicious site, where the attacker can harvest credentials or deliver malware.

The vulnerability exploits mod_rewrite redirect rules by injecting encoded newline characters (%0d%0a) into the request URL. When Apache processes the redirect, the encoded newlines cause the URL parser to misinterpret the redirect target, allowing an attacker to redirect users to arbitrary external URLs. A typical attack payload would include sequences like /%0d%0aLocation:%20https://malicious-site.com appended to legitimate URLs, causing the server to generate an unintended redirect response.

Detection Methods for CVE-2020-1927

Indicators of Compromise

• HTTP access logs containing URL-encoded newline sequences (%0d, %0a, %0D, %0A) in request URIs
• Unexpected 301/302 redirect responses with Location headers pointing to external domains
• User reports of being redirected to unexpected or suspicious websites after clicking internal links
• Network traffic showing redirect chains originating from Apache servers to known malicious domains

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block requests containing encoded CRLF sequences in URLs
• Configure intrusion detection systems (IDS) to alert on HTTP requests with %0d%0a patterns targeting Apache servers
• Deploy log analysis rules to identify unusual redirect patterns in Apache access logs
• Monitor for anomalous outbound redirect traffic from web servers to external destinations

Monitoring Recommendations

• Enable detailed Apache access logging including full request URIs with query parameters
• Implement real-time log monitoring for requests containing URL-encoded control characters
• Configure SentinelOne Singularity Platform to monitor Apache processes for suspicious redirect behavior
• Establish baseline redirect patterns and alert on deviations indicating potential exploitation attempts

How to Mitigate CVE-2020-1927

Immediate Actions Required

• Upgrade Apache HTTP Server to version 2.4.42 or later, which contains the fix for this vulnerability
• Review and audit all mod_rewrite redirect rules for self-referential configurations
• Implement input validation at the application layer to sanitize URL-encoded characters
• Deploy WAF rules to block requests containing CRLF injection patterns

Patch Information

Apache Software Foundation addressed this vulnerability in Apache HTTP Server version 2.4.42 and later releases. The patch improves URL validation in the mod_rewrite module to properly handle encoded newline characters and prevent open redirect attacks. Detailed information is available in the Apache HTTPD Vulnerability List.

For downstream distributions, consult the respective security advisories:

• Debian Security Advisory DSA-4757
• Ubuntu Security Notice USN-4458-1
• NetApp Security Advisory
• Oracle Security Alert July 2020

Workarounds

• If immediate patching is not possible, disable mod_rewrite or remove self-referential redirect rules temporarily
• Implement strict URL validation at a reverse proxy or WAF layer to block encoded newline characters
• Configure Apache to reject requests containing %0d or %0a sequences using mod_security or similar modules
• Use explicit absolute URLs in redirect rules rather than relying on self-referential patterns

# Example mod_security rule to block CRLF injection attempts
SecRule REQUEST_URI "@rx %0[dDaA]" \
    "id:1001,phase:1,deny,status:403,msg:'CRLF Injection Attempt Blocked'"

# Alternatively, update Apache to patched version
# For Debian/Ubuntu:
sudo apt update && sudo apt install apache2

# For RHEL/CentOS:
sudo yum update httpd

# Verify Apache version after update
httpd -v