critical CVSS 9 EPSS 1.2%

CVE-2026-34356·Apache vulnerability

Heap-based Buffer Overflow vulnerability in Apache HTTP Server with malicious backend servers and ProxyPassReverseCookie* This issue affects Apache HTTP Server: from 2.4.0 through 2.4.67. Users are recommended to upgrade to version 2.4.68, which fixes the issue.

Severity
critical
Software
Apache
Fixed in
2.4.68-1~deb12u1
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at 1.2% — higher than 68% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Affected versions

Until: 2.4.68-1~deb12u1

Fixed in: 2.4.68-1~deb12u1

How to fix this CVE

Apache HTTP Server versions 2.4.0 through 2.4.67 contain a heap buffer overflow vulnerability triggered when processing specially crafted responses from malicious backend servers, particularly when ProxyPassReverseCookie directives are active. Upgrade to Apache 2.4.68 or later immediately to eliminate this memory corruption flaw that can lead to denial of service or potential code execution. Verify the patched version is running and review your proxy configurations to ensure reverse cookie rewriting is only enabled where necessary.

sudo dnf update httpd

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST/GET requests to proxy endpoints followed by abnormal backend responses with oversized Set-Cookie or Cookie headers; look for patterns like 'Set-Cookie:.*\x00.*\x00' or responses exceeding 8192 bytes in cookie-related headers in proxy logs

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement WAF rules to limit the size of Set-Cookie and Cookie headers from backend responses; block any responses with null bytes in cookie values; enforce strict Content-Length validation on upstream proxy responses

How to check if you are affected

  1. Check installed Apache version: apache2ctl -v | grep 'Server version'
  2. Verify if mod_proxy and ProxyPassReverseCookie directives are enabled: grep -r 'ProxyPassReverseCookie' /etc/apache2/sites-enabled/ /etc/apache2/conf-enabled/
  3. Search Apache error and access logs for unusual backend server responses or memory-related errors: grep -i 'segmentation\|buffer\|overflow' /var/log/apache2/error.log
  4. Confirm the fix by checking: apache2ctl -v and verifying version is >= 2.4.68

FAQ

What is CVE-2026-34356?

CVE-2026-34356 is a heap-based buffer overflow in Apache HTTP Server that occurs during the processing of cookie rewriting responses from backend proxy servers. When a malicious backend server sends specially crafted data, it can overflow heap memory in the reverse proxy handler, potentially causing a denial of service or information disclosure.

Is CVE-2026-34356 being actively exploited?

No, this vulnerability is not currently listed in CISA's Known Exploited Vulnerabilities catalog, and no public exploits have been disclosed. However, its critical CVSS score of 9.0 and denial-of-service impact warrant immediate patching.

What versions of Apache are affected by CVE-2026-34356?

Apache HTTP Server versions 2.4.0 through 2.4.67 are vulnerable. The vulnerability is fixed in version 2.4.68 and later.

How do I check if my server is vulnerable to CVE-2026-34356?

Run 'apache2ctl -v' or 'httpd -v' to display your Apache version. If it reports a version between 2.4.0 and 2.4.67, you are vulnerable. Also confirm that mod_proxy is loaded with 'apache2ctl -M | grep proxy' and check for ProxyPassReverseCookie usage in your configuration.

Does Defensia detect CVE-2026-34356?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Apache is installed on a monitored server, CVE-2026-34356 will appear in your dashboard with remediation steps.

Related Apache CVEs

CVE-2021-42013CVSS 10It was found that the fix for CVE-2021-41773 in Apache HTTP Server 2.4.50 was insufficient. An attacker could use a path traversal attack to map URLs to files outside the directories configured by Alias-like directives. If files outside of these directories are not protected by the usual default configuration "require all denied", these requests can succeed. If CGI scripts are also enabled for these aliased pathes, this could allow for remote code execution. This issue only affects Apache 2.4.49 and Apache 2.4.50 and not earlier versions.
CVE-2026-44631CVSS 10Buffer Underwrite vulnerability in Apache HTTP Server on crafted regular expressions in the configuration. This issue affects Apache HTTP Server: from 2.4.0 through 2.4.67. Users are recommended to upgrade to version 2.4.68, which fixes the issue.
CVE-2021-41773CVSS 10A flaw was found in a change made to path normalization in Apache HTTP Server 2.4.49. An attacker could use a path traversal attack to map URLs to files outside the directories configured by Alias-like directives. If files outside of these directories are not protected by the usual default configuration "require all denied", these requests can succeed. If CGI scripts are also enabled for these aliased pathes, this could allow for remote code execution. This issue is known to be exploited in the wild. This issue only affects Apache 2.4.49 and not earlier versions. The fix in Apache HTTP Server 2.4.50 was found to be incomplete, see CVE-2021-42013.
CVE-2026-29167CVSS 10Use After Free vulnerability in Apache HTTP Server with mod_ldap in per-directory configuration This issue affects Apache HTTP Server: from 2.4.0 through 2.4.67. Users are recommended to upgrade to version 2.4.68, which fixes the issue.
CVE-2021-44790CVSS 10A carefully crafted request body can cause a buffer overflow in the mod_lua multipart parser (r:parsebody() called from Lua scripts). The Apache httpd team is not aware of an exploit for the vulnerabilty though it might be possible to craft one. This issue affects Apache HTTP Server 2.4.51 and earlier.

References

Track CVEs across your fleet automatically

Defensia scans your Linux servers and tells you exactly which ones are running vulnerable versions — including CVE-2026-34356. Free for 1 server.

Get started free