CyberSecBulletin
CVE-2026-65927NVD status: Analyzed

Apache Tomcat: Off-by-one Error

Published Updated By the CyberSecBulletin desk

CVE-2026-65927 is a high-severity off-by-one error vulnerability in Apache Tomcat from Apache, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. Affected Apache Tomcat versions: 11.0.0-M1 through 11.0.24; 10.1.0-M1 through 10.1.57; 9.0.0.M1 through 9.0.120, and others listed below. The record does not name a fixed version, so check the vendor advisory. There is no confirmed exploitation so far. EPSS estimates a 0.68% chance of exploitation within 30 days, higher than 53% of scored CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update Tomcat to 9.0.121 or 10.1.58 or 11.0.25, whichever matches your release line, or a later version.
  2. Read the advisory on lists.apache.org for workarounds and exact fixed builds.
  3. Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.

Affected products and versions

VendorProductAffected versionsFixed in
ApacheApache Tomcat11.0.0-M1 through 11.0.24; 10.1.0-M1 through 10.1.57; 9.0.0.M1 through 9.0.120; 8.5.0 through 8.5.100Not stated
ApacheTomcat8.5.0 up to, but not including, 9.0.121; 10.1.0 up to, but not including, 10.1.58; 11.0.0 up to, but not including, 11.0.259.0.121, 10.1.58, 11.0.25

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
NoneNo data is disclosed.
Integrity
NoneNothing can be modified.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by CISA (ADP).

Official description

Off-by-one Error vulnerability in Apache Tomcat impacting the [N] flag on the rewrite valves causes rewrite processing to restart at the second rule rather than the first rule. This issue affects Apache Tomcat: from 11.0.0-M1 through 11.0.24, from 10.1.0-M1 through 10.1.57, from 9.0.0.M1 through 9.0.120. The following versions were EOL at the time the CVE was created but are known to be affected: from 8.5.0 through 8.5.100. Other unsupported versions may also be affected. Users are recommended to upgrade to version 11.0.25, 10.1.58 or 9.0.121 which fix the issue.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Off-by-one Error

A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value.

How developers prevent this class of flaw

  • When copying character arrays or using character manipulation methods, the correct size parameter must be used to account for the null terminator that needs to be added at the end of the array. Some examples of functions susceptible to this weakness in C include strcpy(), strncpy(), strcat(), strncat(), printf(), sprintf(), scanf() and sscanf().

More Off-by-one Error vulnerabilities (CWE-193)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
  3. Record last changed materially (score, affected products, status or references).

References

Third-party and government advisories

Other vulnerabilities in Apache Tomcat

All Apache Tomcat vulnerabilities

Recent Off-by-one Error vulnerabilities

Spotted an error? Tell us.