PatchTheCVECVE-2015-7871: Crypto-NAK packets in ntpd in NTP 4.2.x before 4.2.8p4, and 4.3.x before 4.3.77 allows remote attackers to byp
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CRITICAL 9.8REMOTEWATCHLISTUPGRADEMITIGATEMITIGATE
CVE-2015-7871 is a critical-severity flaw (CVSS 9.8) affecting ntp, debian linux, oncommand balance. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
Crypto-NAK packets in ntpd in NTP 4.2.x before 4.2.8p4, and 4.3.x before 4.3.77 allows remote attackers to bypass authentication.
Why this priority
- Critical severity
- EPSS top 1%: very likely to be exploited
- Public exploit reference
- Remotely exploitable, no login, no user action
- On your watchlist
A fixed release or distro patch exists.
Risk facts
| CVSS score | 9.8 (Critical) |
| Attack vector | Network (remote) |
| Attack complexity | Low |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 81.8% (top 1%) |
| CISA exploited list | No |
| Weakness | CWE-287 |
| Published | 2017-08-07 |
| Last modified | 2026-06-17 |
Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
ntp ntp UPGRADE
Affected: >= 4.2.6 and < 4.2.8 · >= 4.3.0 and < 4.3.77 · 4.2.5 p186 · 4.2.5 p187 · 4.2.5 p188 · 4.2.5 p189 · 4.2.5 p190 · 4.2.5 p191
Fixed in: 4.2.8, 4.3.77
Safe to downgrade to: a release before 4.2.5
1. Check installed version
apt-cache policy ntp || rpm -q ntp || ntp --version
2. UPGRADE to 4.2.8 (Debian / Ubuntu)
sudo apt update && sudo apt install --only-upgrade ntp
2. UPGRADE (RHEL / Amazon Linux / Fedora)
sudo dnf upgrade ntp
Note
Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.
3. If no update is available yet, DOWNGRADE to a release before 4.2.5 (Debian / Ubuntu)
apt-cache madison ntp
sudo apt install ntp=<VERSION_BEFORE_4.2.5>
sudo apt-mark hold ntp
3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)
dnf --showduplicates list ntp
sudo dnf downgrade ntp-<VERSION_BEFORE_4.2.5>
sudo dnf versionlock add ntp
Downgrade risk
Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.
After downgrade
Restart the service, and keep the package held until a fixed release ships.
debian debian linux MITIGATE
Affected: 7.0 · 8.0 · 9.0
Fixed in: no fixed release known
Safe to downgrade to: a release before 7.0
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
netapp oncommand balance MITIGATE
Affected: all versions
Fixed in: no fixed release known
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy oncommand-balance || rpm -q oncommand-balance || oncommand-balance --version
No fixed or unaffected release known
Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.
netapp oncommand performance manager MITIGATE
Affected: all versions
Fixed in: no fixed release known
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy oncommand-performance-manager || rpm -q oncommand-performance-manager || oncommand-performance-manager --version
No fixed or unaffected release known
Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.
netapp oncommand unified manager MITIGATE
Affected: all versions
Fixed in: no fixed release known
Safe to downgrade to: no unaffected older release
1. Check installed version
apt-cache policy oncommand-unified-manager || rpm -q oncommand-unified-manager || oncommand-unified-manager --version
No fixed or unaffected release known
Apply the vendor mitigation from the advisory, disable the affected feature, or restrict network access.
netapp clustered data ontap MITIGATE
Affected: all versions
Fixed in: no fixed release known
Safe to downgrade to: no unaffected older release
Check your OS release
cat /etc/os-release; uname -r
Debian / Ubuntu
sudo apt update && sudo apt full-upgrade
RHEL / Amazon Linux / Fedora
sudo dnf upgrade --refresh
Windows
Install the latest cumulative update from Windows Update
If no patch exists yet
OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.
Exact fixes by distribution
A distro patch exists: prefer upgrading over downgrading.
Fedora 46 · root · FIXED · 6.40.04-7.fc46 · FEDORA-2026-7329868400 · source
sudo dnf upgrade --advisory=FEDORA-2026-7329868400
RHEL 5 · ntp · NOT AFFECTED
RHEL 6 · ntp · NOT AFFECTED
RHEL 7 · ntp · NOT AFFECTED
sudo yum update --advisory=ALAS-2015-607
Public exploits and detection
Public code exists for this CVE. Treat unpatched systems as at risk, and only run PoCs in a lab.
- Metasploit: NTP "NAK to the Future"
Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2015-7871?
ntp: upgrade to 4.2.8, 4.3.77; debian linux: mitigate or apply the listed mitigation; oncommand balance: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.
Is CVE-2015-7871 being exploited?
CVE-2015-7871 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2015-7871 affect?
CVE-2015-7871 affects ntp, debian linux, oncommand balance. Severity is critical (CVSS 9.8). Crypto-NAK packets in ntpd in NTP 4.2.x before 4.2.8p4, and 4.3.x before 4.3.77 allows remote attackers to bypass authentication.
How urgent is CVE-2015-7871?
Priority 74/100. Critical severity; EPSS top 1%: very likely to be exploited; Public exploit reference; Remotely exploitable, no login, no user action; On your watchlist
Software pages
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