---
title: "CVE-2021-26691: Apache HTTP Server mod_session response… | CubePath"
description: "In Apache HTTP Server versions 2.4.0 to 2.4.46 a specially crafted SessionHeader sent by an origin server could cause a heap overflow"
language: en
source: https://cubepath.com/security/cve/CVE-2021-26691
---

# CVE-2021-26691: Apache HTTP Server mod_session response… | CubePath

## Description

In Apache HTTP Server versions 2.4.0 to 2.4.46 a specially crafted SessionHeader sent by an origin server could cause a heap overflow

Official description from the CVE record, published by apache. Copyright © 1999-2026 The MITRE Corporation. See [/security/cve/legal](https://cubepath.com/security/cve/legal) for the CVE license. [View the record on cve.org](https://www.cve.org/CVERecord?id=CVE-2021-26691)

## Key data

### EPSS

Exploit Prediction Scoring System by FIRST: an estimate of the probability that exploitation activity will be observed in the next 30 days.

Score

0.68067 (68.1% probability)

Percentile

99.3%

Scores of

Nov 10, 2026

Model

v2026.06.15

### Weakness (CWE)

CWE catalog version 5.0.

CWE names: Copyright © 2006-2026, The MITRE Corporation. See [/security/cve/legal](https://cubepath.com/security/cve/legal) for the CWE license.

- [CWE-122](https://cwe.mitre.org/data/definitions/122.html) Heap-based Buffer Overflow

## Affected products

Products listed in the CVE record, as the CNA published them.

- Apache HTTP Server

  - affected: 2.4.46
  - affected: 2.4.43
  - affected: 2.4.41
  - affected: 2.4.39
  - affected: 2.4.38
  - affected: 2.4.37
  - affected: 2.4.35
  - affected: 2.4.34
  - affected: 2.4.33
  - affected: 2.4.29
  - affected: 2.4.28
  - affected: 2.4.27
  - affected: 2.4.26
  - affected: 2.4.25
  - affected: 2.4.23
  - affected: 2.4.20
  - affected: 2.4.18
  - affected: 2.4.17
  - affected: 2.4.16
  - affected: 2.4.12
  - affected: 2.4.10
  - affected: 2.4.9
  - affected: 2.4.7
  - affected: 2.4.6
  - affected: 2.4.4
  - and 4 more

## References

Links listed in the CVE record. They lead to other sites, which CubePath does not control.

- [httpd.apache.org/security/vulnerabilities\_24.html](http://httpd.apache.org/security/vulnerabilities%5F24.html)
- [lists.apache.org/thread.html/re026d3da9d7824bd93b9f871c0fdd…](https://lists.apache.org/thread.html/re026d3da9d7824bd93b9f871c0fdda978d960c7e62d8c43cba8d0bf3%40%3Ccvs.httpd.apache.org%3E)
- [\[httpd-announce\] 20210609 CVE-2021-26691: mod\_session response handling heap overflow](https://lists.apache.org/thread.html/r50cae1b71f1e7421069036b213c26da7d8f47dd59874e3bd956959fe%40%3Cannounce.httpd.apache.org%3E)  (lists.apache.org) mailing-list
- [\[httpd-dev\] 20210610 Re: svn commit: r1890598 - in /httpd/site/trunk/content/security/json: CVE-2019-17567.json CVE-2020-13938.json CVE-2020-13950.json CVE-2020-35452.json CVE-2021-26690.json CVE-2021-26691.json CVE-2021-30641.json CVE-2021-31618.json](https://lists.apache.org/thread.html/r7f2b70b621651548f4b6f027552f1dd91705d7111bb5d15cda0a68dd%40%3Cdev.httpd.apache.org%3E)  (lists.apache.org) mailing-list
- [\[oss-security\] 20210609 CVE-2021-26691: Apache httpd: mod\_session response handling heap overflow](http://www.openwall.com/lists/oss-security/2021/06/10/7)  (openwall.com) mailing-list
- [\[debian-lts-announce\] 20210709 \[SECURITY\] \[DLA 2706-1\] apache2 security update](https://lists.debian.org/debian-lts-announce/2021/07/msg00006.html)  (lists.debian.org) mailing-list
- [DSA-4937](https://www.debian.org/security/2021/dsa-4937)  (debian.org) vendor-advisory
- [GLSA-202107-38](https://security.gentoo.org/glsa/202107-38)  (security.gentoo.org) vendor-advisory
- [FEDORA-2021-dce7e7738e](https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SPBR6WUYBJNACHKE65SPL7TJOHX7RHWD/)  (lists.fedoraproject.org) vendor-advisory
- [FEDORA-2021-e3f6dd670d](https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZNCYSR3BXT36FFF4XTCPL3HDQK4VP45R/)  (lists.fedoraproject.org) vendor-advisory
- [oracle.com/security-alerts/cpuoct2021.html](https://www.oracle.com/security-alerts/cpuoct2021.html)
- [security.netapp.com/advisory/ntap-20210702-0001](https://security.netapp.com/advisory/ntap-20210702-0001/)
- [oracle.com/security-alerts/cpujan2022.html](https://www.oracle.com/security-alerts/cpujan2022.html)

## Timeline

1. ID reserved Feb 4, 2021
2. Record published Jun 10, 2021
3. Record last updated Aug 3, 2024

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## About CubePath

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Data from the CVE Program (MITRE), CISA KEV and FIRST EPSS. CVSS is owned by FIRST and used by permission. Not affiliated with or endorsed by MITRE, CISA or FIRST. Data as of Nov 10, 2026, 20:05 UTC. [Sources and licenses](https://cubepath.com/security/cve/legal)