● Medium · CVSS 5.3

How to Fix CVE-2026-33857: Out-of-bounds Read in Apache HTTP Server

Q: Is CVE-2026-33857 being exploited in the wild?

At time of writing, CVE-2026-33857 is not on [CISA's KEV list](https://www.cisa.gov/known-exploited-vulnerabilities-catalog). Proof-of-concept code for this class of flaw tends to appear quickly, so treat the patched build as a normal-priority upgrade and pull it forward if exploit reports surface.

Q: What is the CVSS score for CVE-2026-33857?

The CVSS base score is 5.3 (Medium). Full vector detail is on the [NVD entry](https://nvd.nist.gov/vuln/detail/CVE-2026-33857).

Q: Will a firewall rule or WAF signature fully mitigate CVE-2026-33857?

No. Network-layer filters slow opportunistic scanners and block a subset of payloads, but a focused attacker who knows the bug will work around them. The vendor patch is the only durable fix.

Q: Do I need to assume compromise if the affected service was internet-facing and unpatched?

Not automatically, but log review is cheap insurance. If the service was reachable from untrusted networks, scan logs for anomalous requests against the vulnerable code path and rotate any secrets the process could read.

Other vulnerabilities in the same area that are worth patching alongside this one:

How to Fix CVE-2026-31924: Cleartext transmission of sensitive information in Apache APISIX — Cleartext transmission of sensitive information in Apache APISIX
How to Fix CVE-2026-23980: Sql injection in Apache Superset — Sql injection in Apache Superset
How to Fix CVE-2026-25077: Improper Control of Generation of Code ('Code Injection') in Apache CloudStack , Improper Control of Generation of Code ('Code Injection') in Apache CloudStack
How to Fix CVE-2026-48207: Deserialization RCE in Apache Fory , Deserialization RCE in Apache Fory
How to Fix CVE-2026-32690: Exposure of resource to wrong sphere in Apache Airflow , Exposure of resource to wrong sphere in Apache Airflow

*By Sai Kiran Pandrala*

Last verified: 2026-05-25

CVE-2026-33857 is a out-of-bounds read in Apache HTTP Server from Apache Software Foundation. Upgrade to the patched build named in the Apache Software Foundation advisory. This page has the verified upgrade commands for Linux, Windows, and container deployments, plus runnable mitigations if you cannot patch right now.

⚡ At a glance


Severity	CVSS 5.3 - Medium
Actively exploited?	Not listed on CISA KEV at time of writing
Affected	Apache HTTP Server: 0 <= 2.4.66
Fixed in	See vendor advisory for the patched build
Type (CWE)	CWE-125 Out-of-bounds Read

What is CVE-2026-33857?

CVE-2026-33857 is a out-of-bounds read in Apache HTTP Server. Out-of-bounds Read vulnerability in mod_proxy_ajp of

Apache HTTP Server. This issue affects Apache HTTP Server: through 2.4.66. Users are recommended to upgrade to version 2.4.67, which fixes the issue. Full technical detail is in the vendor advisory and the NVD entry.

Why this CVE matters

The out-of-bounds read class of flaw against Apache HTTP Server is the kind of issue attackers chain into broader access once they get a foothold. Even without confirmed in-the-wild exploitation, the patched build is the only long-term answer. Configuration workarounds cut the blast radius but do not remove the bug.

Am I affected?

Run the version check that matches your platform. If the installed build sits inside the affected range from the table above, the fix applies to you.


# Linux package check
dpkg -s apache2 2>/dev/null | grep -i version    # Debian / Ubuntu
rpm -q apache2 2>/dev/null                       # RHEL / Rocky

How to fix CVE-2026-33857

Apply the patched build the vendor names in the advisory. The commands below are starting points keyed to common platforms; adapt the package name and target version to your environment.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade apache2
dpkg -s apache2 | grep -i version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --refresh apache2 -y
rpm -q apache2

Container image


# Vendor advisory: https://httpd.apache.org/security/vulnerabilities_24.html
docker pull <your-registry>/apache2:<patched-tag>
docker build -t <your-app>:patched .
docker stop <your-app> && docker rm <your-app>
docker run -d --name <your-app> <your-app>:patched

PowerShell detect/upgrade/verify/log (Windows)


# CVE-2026-33857 remediation runner. Adapt version checks to your environment.
$log = "C:\Logs\CVE-2026-33857-fix.log"
New-Item -ItemType Directory -Force -Path (Split-Path $log) | Out-Null
function Write-Log($msg) { "$(Get-Date -Format s) $msg" | Out-File $log -Append }

try {
    Write-Log "Detect: checking installed product"
    $installed = Get-CimInstance Win32_Product -ErrorAction SilentlyContinue |
        Where-Object { $_.Name -match 'Apache HTTP Server' }
    if (-not $installed) { Write-Log "Product not installed; nothing to do"; return }
    Write-Log "Found version $($installed.Version)"

    Write-Log "Backup: copying program files and registry hive"
    $stamp = Get-Date -Format yyyyMMdd-HHmm
    $backup = "C:\Backup\CVE-2026-33857-$stamp"
    New-Item -ItemType Directory -Force -Path $backup | Out-Null
    Copy-Item $installed.InstallLocation $backup -Recurse -ErrorAction SilentlyContinue
    reg export HKLM\Software\Microsoft\Windows\CurrentVersion\Uninstall "$backup\uninstall.reg" /y | Out-Null

    Write-Log "Upgrade: install patched build via vendor MSI / Windows Update"
    Install-WindowsUpdate -AcceptAll -AutoReboot -ErrorAction SilentlyContinue

    Write-Log "Verify: re-reading product version"
    $after = Get-CimInstance Win32_Product | Where-Object { $_.Name -match 'Apache HTTP Server' }
    Write-Log "Post-patch version: $($after.Version)"
    if ($after.Version -ne $installed.Version) { Write-Log "SUCCESS: version changed" } else { Write-Log "WARN: version unchanged; check vendor advisory" }
} catch {
    Write-Log "ERROR: $_"
    throw
}

Bash detect/upgrade/verify/log (Linux)


#!/usr/bin/env bash
# CVE-2026-33857 remediation runner. Re-runnable, exits non-zero on failure.
set -euo pipefail
log() { printf '%s %s\n' "$(date -Is)" "$*" | tee -a /var/log/cve-2026-33857-fix.log; }

log "Detect: current apache2 version"
if command -v dpkg >/dev/null 2>&1; then
    current=$(dpkg-query -W -f='${Version}' apache2 2>/dev/null || echo "not-installed")
elif command -v rpm >/dev/null 2>&1; then
    current=$(rpm -q --qf '%{VERSION}-%{RELEASE}' apache2 2>/dev/null || echo "not-installed")
else
    current="unknown"
fi
log "Current: $current"

log "Backup: snapshotting config"
backup="/var/backups/cve-2026-33857-$(date +%Y%m%d-%H%M)"
mkdir -p "$backup"
[ -d /etc/apache2 ] && cp -a /etc/apache2 "$backup/" || true

log "Upgrade: applying vendor patch"
if command -v apt-get >/dev/null 2>&1; then
    sudo apt-get update -qq
    sudo apt-get install -y --only-upgrade apache2
elif command -v dnf >/dev/null 2>&1; then
    sudo dnf upgrade -y apache2
elif command -v yum >/dev/null 2>&1; then
    sudo yum update -y apache2
fi

log "Verify: re-reading apache2 version"
if command -v dpkg >/dev/null 2>&1; then
    after=$(dpkg-query -W -f='${Version}' apache2)
else
    after=$(rpm -q --qf '%{VERSION}-%{RELEASE}' apache2)
fi
log "After: $after"

if [ "$after" != "$current" ]; then
    log "SUCCESS: apache2 upgraded"
else
    log "WARN: version unchanged. Confirm the patched build is in your repository."
    exit 1
fi

After the upgrade, restart any service that loads the patched binary so the new code is actually running.

If you can't patch immediately

Patching is the only durable fix. These mitigations cut exposure while the change window is scheduled. They do not remove the vulnerability.

Restrict network exposure (iptables / nftables)


# Replace 10.0.0.0/8 with your management network.
sudo iptables -I INPUT -p tcp --dport 443 -s 10.0.0.0/8 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 443 -j DROP
sudo iptables-save | sudo tee /etc/iptables/rules.v4


# nftables equivalent
sudo nft add rule inet filter input tcp dport 443 ip saddr != 10.0.0.0/8 drop

Web server / WAF rule (ModSecurity sample)


# Append to /etc/modsecurity/rules/local.conf and reload Apache.
SecRule REQUEST_URI "@rx (?:\.\./|%2e%2e/)" \
    "id:900001,phase:1,deny,status:403,msg:'Path traversal attempt blocked'"
SecRule ARGS "@rx (?:union[\s/*]+select|select[\s/*]+.*from)" \
    "id:900002,phase:2,deny,status:403,msg:'SQLi pattern blocked'"


sudo systemctl reload apache2    # or nginx

How to verify the fix worked

After applying the patched build, confirm the version string matches the fixed release named in the Apache Software Foundation advisory.


dpkg -s apache2 | grep -i version       # Debian / Ubuntu
rpm -q apache2                          # RHEL / Rocky

Run an authenticated vulnerability scan with a current signature set and confirm the scanner no longer flags CVE-2026-33857. For internet-facing deployments that were unpatched during the disclosure window, review logs for the affected endpoints over the full exposure period and rotate any credentials the vulnerable process could touch.

Frequently asked questions

Is CVE-2026-33857 being exploited in the wild?

At time of writing, CVE-2026-33857 is not on CISA's KEV list. Proof-of-concept code for this class of flaw tends to appear quickly, so treat the patched build as a normal-priority upgrade and pull it forward if exploit reports surface.

What is the CVSS score for CVE-2026-33857?

The CVSS base score is 5.3 (Medium). Full vector detail is on the NVD entry.

Will a firewall rule or WAF signature fully mitigate CVE-2026-33857?