HTTP ServerWeb server / proxy · Apache
CVE-2021-26690
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 · uneditedApache HTTP Server versions 2.4.0 to 2.4.46 A specially crafted Cookie header handled by mod_session can cause a NULL pointer dereference and crash, leading to a possible Denial Of Service
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 · high confidenceIn Apache HTTP Server versions 2.4.0 to 2.4.46, the mod_session module fails to properly handle specially crafted Cookie headers, leading to a NULL pointer dereference that causes the server process to crash and result in a Denial of Service condition.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
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
>= 2.4.0, <= 2.4.46
Debian LinuxOperating system
= 9.0= 10.0
FedoraOperating system
= 34= 35= 12.4.0.0= 17.1= 17.2= 17.3= 8.8CVSS 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
- None
- Integrity
- None
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
-
Check Apache HTTP Server versionRun 'httpd -v' or 'apache2 -v' to display the server version. On Debian/Ubuntu, also check 'apache2ctl -V'. Compare the displayed version against the affected range 2.4.0 to 2.4.46.Affected if The installed version is 2.4.0 through 2.4.46 inclusive.
-
Verify mod_session is loadedRun 'httpd -M 2>/dev/null | grep session' or 'apache2ctl -M 2>/dev/null | grep session' to list loaded modules. On some systems, check /etc/httpd/conf.modules.d/ or /etc/apache2/mods-enabled/ for session-related .load files.Affected if mod_session or mod_session_crypto appears in the loaded modules list.
-
Inspect Apache configuration for session directivesSearch configuration files (httpd.conf, apache2.conf, included .conf files) for 'Session' directives such as Session, SessionEnv, SessionHeader, or SessionCookieName. Use 'grep -r Session /etc/apache2/' or 'grep -r Session /etc/httpd/' to find active session configuration.Affected if Configuration files contain active Session, SessionEnv, or SessionHeader directives that enable session handling.
-
Confirm Cookie-based sessions are in useCheck configuration for 'SessionCookieName' or 'SessionHeader' directives that process Cookie headers. Look for 'SessionCookieName name' or 'SessionHeader X-Forwarded-User' in config files.Affected if SessionCookieName or SessionHeader is configured to read session data from Cookie headers.
You are affected if Apache HTTP Server version is 2.4.0-2.4.46 AND mod_session is loaded AND session handling via Cookie headers is actively configured.
Generated from the published advisory. Verify against your own configuration.
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.
From vendor dataUpgrade Apache HTTP Server to version 2.4.47 or later to obtain the patch. If immediate patching is not feasible, consider disabling mod_session or implementing rate limiting on Cookie-based requests as a temporary workaround.
Fix this in HTTP Server
Scoped from the published advisory
- Consultation2.0 h
- Implementation2.0 h
- Testing2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,696.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-26690 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-26690 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.
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.
- 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