CVE-2021-39275: Apache HTTP Server Buffer Overflow Flaw

CVE-2021-39275 Overview

CVE-2021-39275 is a critical out-of-bounds write vulnerability in the Apache HTTP Server's ap_escape_quotes() function. When this function is provided with malicious input, it may write beyond the end of an allocated buffer, potentially leading to memory corruption. While Apache's included modules do not pass untrusted data to this function, third-party or external modules may inadvertently do so, creating an exploitable condition.

Critical Impact
This buffer overflow vulnerability in Apache HTTP Server could allow remote attackers to cause memory corruption, potentially leading to denial of service or arbitrary code execution on affected systems running vulnerable third-party modules.

Affected Products

Apache HTTP Server 2.4.48 and earlier
Fedora 34 and 35
Debian Linux 9.0, 10.0, and 11.0
NetApp Cloud Backup, Clustered Data ONTAP, and StorageGRID
Oracle HTTP Server 12.2.1.3.0 and 12.2.1.4.0
Oracle Instantis EnterpriseTrack 17.1, 17.2, and 17.3
Oracle ZFS Storage Appliance Kit 8.8
Siemens SINEC NMS and SINEMA Server 14.0

Discovery Timeline

September 16, 2021 - CVE CVE-2021-39275 published to NVD
May 1, 2025 - Last updated in NVD database

Technical Details for CVE-2021-39275

Vulnerability Analysis

This vulnerability stems from improper bounds checking in the ap_escape_quotes() function within Apache HTTP Server. The function is designed to escape quote characters in strings, but it fails to properly validate the size of the destination buffer relative to the input data. When specially crafted input containing an excessive number of quote characters is processed, the function can write data beyond the allocated buffer boundaries.

The vulnerability is categorized as CWE-787 (Out-of-bounds Write), which represents one of the most dangerous classes of memory safety vulnerabilities. The network-accessible nature of the Apache HTTP Server combined with the potential for remote exploitation without authentication makes this vulnerability particularly concerning for internet-facing systems.

Root Cause

The root cause lies in the ap_escape_quotes() function's buffer size calculation logic. When escaping quotes in a string, the function must allocate additional space for escape characters. However, the function does not properly account for edge cases where the input contains a high density of quote characters requiring escaping. This miscalculation can result in the output buffer being too small to hold the escaped string, leading to a heap-based buffer overflow when the function attempts to write beyond the allocated memory region.

Attack Vector

An attacker can exploit this vulnerability by sending specially crafted HTTP requests to a server running Apache HTTP Server with vulnerable third-party modules that pass untrusted data to ap_escape_quotes(). The attack requires network access to the target server but does not require authentication or user interaction. Successful exploitation could allow attackers to corrupt memory structures, potentially achieving denial of service or, in some configurations, arbitrary code execution with the privileges of the Apache HTTP Server process.

The exploitation scenario involves:

Identifying an Apache HTTP Server with a third-party module that passes user-controlled input to ap_escape_quotes()
Crafting a malicious request containing strings with carefully constructed quote character patterns
Sending the request to trigger the buffer overflow condition
Leveraging the memory corruption for denial of service or potential code execution

Detection Methods for CVE-2021-39275

Indicators of Compromise

Unexpected Apache HTTP Server crashes or segmentation faults in error logs
Memory corruption errors referencing ap_escape_quotes() or related string handling functions
Anomalous HTTP requests containing unusually long strings with high concentrations of quote characters
Core dumps indicating heap corruption in Apache httpd processes

Detection Strategies

Monitor Apache error logs for segmentation faults, memory allocation failures, or unexpected process terminations
Implement web application firewall (WAF) rules to detect and block requests with excessive quote characters in parameters
Deploy intrusion detection signatures to identify exploitation attempts targeting ap_escape_quotes()
Conduct regular vulnerability scans to identify Apache HTTP Server instances running versions 2.4.48 or earlier

Monitoring Recommendations

Enable verbose logging for Apache HTTP Server to capture detailed request information
Configure real-time alerting for Apache process crashes or restarts
Monitor system resources for unusual memory consumption patterns that may indicate exploitation attempts
Review and audit third-party Apache modules to determine if they pass untrusted data to vulnerable functions

How to Mitigate CVE-2021-39275

Immediate Actions Required

Upgrade Apache HTTP Server to version 2.4.49 or later immediately
Review all third-party modules installed on Apache HTTP Server instances to assess exposure
Implement network segmentation to limit exposure of vulnerable Apache servers to untrusted networks
Deploy WAF rules as a temporary mitigation while patching is completed

Patch Information

Apache Software Foundation has addressed this vulnerability in Apache HTTP Server version 2.4.49 and later releases. Organizations should consult the Apache HTTPD Security Vulnerabilities page for the latest security updates and detailed patch information.

Additional vendor-specific patches are available:

Debian users should apply updates referenced in DSA-4982 and the Debian LTS Announcement
Fedora users should apply updates via Fedora Package Announcements
Oracle customers should reference the Oracle January 2022 and April 2022 Critical Patch Updates
Siemens customers should consult Siemens Security Advisory SSA-685781
NetApp customers should review NetApp Security Advisory NTAP-20211008-0004

Workarounds

Audit and remove or disable unnecessary third-party Apache modules that may pass untrusted data to ap_escape_quotes()
Implement strict input validation at the application layer to filter potentially malicious strings before they reach Apache
Use reverse proxy configurations to inspect and sanitize incoming requests before they reach vulnerable Apache instances
Consider deploying Apache with reduced privileges to minimize the impact of potential exploitation

bash

# Verify Apache HTTP Server version
httpd -v
# or
apache2 -v

# Check for installed third-party modules
httpd -M
# or
apache2ctl -M

# Update Apache on Debian/Ubuntu systems
sudo apt update && sudo apt upgrade apache2

# Update Apache on RHEL/CentOS/Fedora systems
sudo dnf update httpd