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HTTP ServerWeb server / proxy · Apache

CVE-2021-26691

CRITICAL · 9.8 CVSS v3.1 Published 2021-06-10
Fix available
A fix is available. Upgrade to 18.1.0.1.0 or later.
See remediation →
100/100
Remediation priority · Urgent
High EPSS Remotely reachable No privileges Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis
How this class of weakness works · CWE-122

Data overflows a buffer allocated on the heap, corrupting neighbouring heap structures and allocator metadata that a patient attacker can groom into control of execution. It is subtler than a stack overflow but just as dangerous. The fix is validating lengths before every write and using safe allocators and bounded operations.

General guidance for the heap-based buffer overflow class — the official description and references above are authoritative for this specific CVE. Want a bespoke review and a reviewed fix? Ask our team →

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
HTTP ServerWeb server / proxy
Affected:>= 2.4.0, <= 2.4.46
Debian LinuxOperating system
Affected:= 9.0= 10.0
FedoraOperating system
Affected:= 34= 35
Enterprise Manager Ops CenterApplication
Affected:= 12.4.0.0
Instantis EnterprisetrackApplication
Affected:= 17.1= 17.2= 17.3
Secure BackupApplication
Affected:< 18.1.0.1.0
Zfs Storage Appliance KitApplication
Affected:= 8.8
Cloud BackupApplication
Affected:all versions

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 18.1.0.1.0 or later
Fixed in 18.1.0.1.0
Vendor patch www.oracle.com →
Recommended fix High confidence

Apache HTTP Server 2.4.47 or later

  1. Identify current Apache HTTP Server version using 'httpd -v' or 'apache2 -v'
  2. Stop the Apache service using 'systemctl stop httpd' or 'systemctl stop apache2'
  3. Upgrade Apache HTTP Server to version 2.4.47 or later via package manager (e.g., 'apt update && apt upgrade apache2' for Debian/Ubuntu, or 'yum update httpd' for RHEL/CentOS)
  4. Alternatively, download and compile Apache HTTP Server 2.4.47+ from apache.org if using source installation
  5. Start the Apache service using 'systemctl start httpd' or 'systemctl start apache2'
  6. Verify the upgrade was successful by running 'httpd -v' or 'apache2 -v' and confirming version is 2.4.47 or higher

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $2,300. Get the upgrade done

Scan for this in your stack

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-26691 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data