CVE-2021-26691: Apache HTTP Server mod_session response handling heap overflow
Vulnerability summary
CVE-2021-26691 is a critical (CVSS 9.8) Heap-based Buffer Overflow vulnerability in apache software foundation apache http server. EPSS estimates a 68.29% chance of exploitation in the next 30 days, higher than 99.3% of scored vulnerabilities.
Severe, CVSS 9.8 (NVD); no exploitation reported by the 5 checked sources; EPSS 68% (FIRST, 03-11-2026)
Source freshness unavailable
How is this decided?Advisory lineage
Upstream 0 · Downstream 13 · Related 7Timeline
- 10-06-2021PublishedVulnerability first disclosed.
- 03-08-2024Last modifiedVulnerability information updated.
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
CVSS metrics
5 scores · 4 sources · 2 versions- OSV · AlpineEcosystem advisory · calculated9.8CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- OSV · UbuntuEcosystem advisory · calculated9.8CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
- OSV · DebianEcosystem advisory · calculated9.8CriticalCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS trend
Probability
68.29%
Chance of exploitation in the next 30 days
Percentile
99%
Versus all other CVEs
30-day change
No change 0.00 pp
From 68.29% on 04-10-2026
Known exploited
Compare KEV catalogs
CISA KEVNot in the CISA KEV catalog.
ENISA CNWNot in the ENISA CNW catalog.
CIRCLNot in the CIRCL catalog.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root causeAffected systems
Known conditions: network-reachable, no authentication, no user interaction (CVSS vector). Other conditions: not stated in public data.
Oracle4 products
- enterprise_manager_ops_center12.4.0.0
- instantis_enterprisetrack17.1 | 17.2 | 17.3
- secure_backup< 18.1.0.1.0
- zfs_storage_appliance_kit8.8
Debian2 products
- apache2< 2.4.46-6
- debian_linuxEOL9.0 | 10.0
Alpine1 product
- apache2< 2.4.48-r0
Apache1 product
- http_server≥ 2.4.0, ≤ 2.4.46
Apache software foundation1 product
- apache http server2.4.46 | 2.4.43 | 2.4.41 | 2.4.39 | 2.4.38 | 2.4.37 | 2.4.35 | 2.4.34 | 2.4.33 | 2.4.29 | 2.4.28 | 2.4.27 | 2.4.26 | 2.4.25 | 2.4.23 | 2.4.20 | 2.4.18 | 2.4.17 | 2.4.16 | 2.4.12 | 2.4.10 | 2.4.9 | 2.4.7 | 2.4.6 | 2.4.4 | 2.4.3 | 2.4.2 | 2.4.1 | 2.4.0
Fedoraproject1 product
- fedoraEOL34 | 35
Netapp1 product
Ubuntu1 product
- apache2< 2.4.7-1ubuntu4.22+esm1 | < 2.4.18-2ubuntu3.17+esm1 | < 2.4.29-1ubuntu4.16 | < 2.4.41-4ubuntu3.3
8 vendors · 12 products
References
22 links- www.openwall.com/lists/oss-security/2021/06/10/7Mailing ListX Refsource MLISTREPORTThird Party Advisory
lists.debian.org/debian-lts-announce/2021/07/msg00006.htmlMailing ListX Refsource MLISTThird Party Advisory- www.debian.org/security/2021/dsa-4937Vendor AdvisoryX Refsource DEBIANMailing ListThird Party Advisory
