CVE-2021-39275
📋 TL;DR
CVE-2021-39275 is a critical buffer overflow vulnerability in Apache HTTP Server's ap_escape_quotes() function that could allow remote code execution or denial of service. The vulnerability affects Apache HTTP Server 2.4.48 and earlier versions. While no included Apache modules pass untrusted data to this function, third-party or external modules could be vulnerable.
💻 Affected Systems
- Apache HTTP Server
📦 What is this software?
Fedora by Fedoraproject
Fedora by Fedoraproject
Sinec Nms by Siemens
⚠️ Risk & Real-World Impact
Worst Case
Remote code execution with SYSTEM/root privileges leading to complete server compromise, data theft, and lateral movement within the network.
Likely Case
Denial of service causing Apache server crashes, potentially leading to service disruption and availability issues.
If Mitigated
Limited impact if no third-party modules use the vulnerable function or if proper input validation is implemented.
🎯 Exploit Status
Exploitation requires third-party modules that use the vulnerable function. Public proof-of-concept exists but requires specific module configurations.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Apache HTTP Server 2.4.49 and later
Vendor Advisory: https://httpd.apache.org/security/vulnerabilities_24.html
Restart Required: Yes
Instructions:
1. Download Apache HTTP Server 2.4.49 or later from official Apache website. 2. Backup current configuration files. 3. Stop Apache service. 4. Install updated version. 5. Restore configuration files. 6. Start Apache service. 7. Verify successful update.
🔧 Temporary Workarounds
Disable vulnerable third-party modules
allIdentify and disable any third-party modules that might use ap_escape_quotes() function with untrusted input
# Check loaded modules: httpd -M
# Disable module in httpd.conf: LoadModule module_name modules/mod_module_name.so
Implement WAF rules
allAdd Web Application Firewall rules to detect and block buffer overflow attempts
# Example ModSecurity rule: SecRule ARGS "@rx .{1000,}" "id:1001,phase:2,deny,msg:'Possible buffer overflow attempt'"
# Add to modsecurity.conf or appropriate WAF configuration
🧯 If You Can't Patch
- Implement strict input validation and sanitization in all third-party modules
- Deploy network segmentation and restrict Apache server access to trusted sources only
🔍 How to Verify
Check if Vulnerable:
Check Apache version and review third-party modules for use of ap_escape_quotes() functionCheck Version:
httpd -vVerify Fix Applied:
Verify Apache version is 2.4.49 or later and test server functionality📡 Detection & Monitoring
Log Indicators:
- Apache segmentation fault errors in error_log
- Unexpected Apache process crashes
- Large input strings in access logs
Network Indicators:
- Unusual HTTP requests with extremely long parameter values
- Multiple connection attempts followed by service disruption
SIEM Query:
source="apache_error.log" AND ("segmentation fault" OR "core dumped" OR "SIGSEGV")🔗 References
- https://cert-portal.siemens.com/productcert/pdf/ssa-685781.pdf
- https://httpd.apache.org/security/vulnerabilities_24.html
- https://lists.apache.org/thread.html/r3925e167d5eb1c75def3750c155d753064e1d34a143028bb32910432%40%3Cusers.httpd.apache.org%3E
- https://lists.apache.org/thread.html/r61fdbfc26ab170f4e6492ef3bd5197c20b862ce156e9d5a54d4b899c%40%3Cusers.httpd.apache.org%3E
- https://lists.apache.org/thread.html/r82838efc5fa6fc4c73986399c9b71573589f78b31846aff5bd9b1697%40%3Cusers.httpd.apache.org%3E
- https://lists.apache.org/thread.html/r82c077663f9759c7df5a6656f925b3ee4f55fcd33c889ba7cd687029%40%3Cusers.httpd.apache.org%3E
- https://lists.debian.org/debian-lts-announce/2021/10/msg00001.html
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SPBR6WUYBJNACHKE65SPL7TJOHX7RHWD/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZNCYSR3BXT36FFF4XTCPL3HDQK4VP45R/
- https://security.gentoo.org/glsa/202208-20
- https://security.netapp.com/advisory/ntap-20211008-0004/
- https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-apache-httpd-2.4.49-VWL69sWQ
- https://www.debian.org/security/2021/dsa-4982
- https://www.oracle.com/security-alerts/cpuapr2022.html
- https://www.oracle.com/security-alerts/cpujan2022.html
- https://cert-portal.siemens.com/productcert/pdf/ssa-685781.pdf
- https://httpd.apache.org/security/vulnerabilities_24.html
- https://lists.apache.org/thread.html/r3925e167d5eb1c75def3750c155d753064e1d34a143028bb32910432%40%3Cusers.httpd.apache.org%3E
- https://lists.apache.org/thread.html/r61fdbfc26ab170f4e6492ef3bd5197c20b862ce156e9d5a54d4b899c%40%3Cusers.httpd.apache.org%3E
- https://lists.apache.org/thread.html/r82838efc5fa6fc4c73986399c9b71573589f78b31846aff5bd9b1697%40%3Cusers.httpd.apache.org%3E
- https://lists.apache.org/thread.html/r82c077663f9759c7df5a6656f925b3ee4f55fcd33c889ba7cd687029%40%3Cusers.httpd.apache.org%3E
- https://lists.debian.org/debian-lts-announce/2021/10/msg00001.html
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SPBR6WUYBJNACHKE65SPL7TJOHX7RHWD/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZNCYSR3BXT36FFF4XTCPL3HDQK4VP45R/
- https://security.gentoo.org/glsa/202208-20
- https://security.netapp.com/advisory/ntap-20211008-0004/
- https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-apache-httpd-2.4.49-VWL69sWQ
- https://www.debian.org/security/2021/dsa-4982
- https://www.oracle.com/security-alerts/cpuapr2022.html
- https://www.oracle.com/security-alerts/cpujan2022.html
