Table of Contents

1) Introduction

The Kea DHCP distribution is the next generation DHCP server suite offered by the Internet Systems Consortium (ISC). It replaces the traditional ISC DHCP software which has reached its end of life.

Since SUSE is also going to ship Kea DHCP in its products, we performed a routine review of its code base. Even before checking the network security of Kea, we stumbled over a range of local security issues, among them a local root exploit which is possible in many default installations of Kea on Linux and BSD distributions. This, as well as some other issues and security recommendations for Kea follow below in a detailed report.

This report is based on Kea release 2.6.1. Any source code references in this report relate to this version. Many systems still ship older releases of Kea, but we believe they are all affected as well by the issues described in this report.

In the next section an overview of the Kea design, as far as it is relevant for the issues in this report, is provided. In section 3) the security issues we found will be discussed in detail. In section 4) further hardening suggestions are provided. In section 5) the upstream bugfixes for the issues are discussed. In section 6) the packaging properties and affectedness of Kea in widespread Linux and UNIX systems are documented. Finally in section 7) an overview of the CVE assignments is given.

2) Overview of Kea Design

This section provides a short overview of the involved components, to allow readers that are unfamiliar with Kea to better understand the rest of this report.

Kea offers three separate services for dhcp4, dhcp6 and dhcp-ddns. A kea-ctrl-agent is active by default in most Kea installations and offers an HTTP REST API listening on localhost:8000. This REST API is based on JSON requests that are either processed by kea-ctrl-agent itself or forwarded to one of the Kea services it controls. To allow forwarding, each Kea service listens on a UNIX domain socket that is only accessible to kea-ctrl-agent.

In most installations, the REST API is by default accessible to all users in the system without authentication. Many installations run the Kea services with full root privileges. On Linux systems that use a dedicated service user account instead, the Linux capability CAP_NET_BIND_SERVICE is assigned to all of the Kea services. The dhcp4 service additionally needs CAP_NET_RAW to function.

The default configuration and the packaging of Kea are important aspects to judge the exploitability of the issues described in this report. In some sense, the issues could be considered vendor-specific problems and not upstream issues (some ISC engineers argued in this direction when we first reported these issues). The number of affected Kea packages and the fact that the default configuration installed by the Kea build system also enables an unauthenticated REST API make these seem like overarching upstream issues to us, however.

3) Security Issues

3.1) Local Privilege Escalation by Injecting a Hook Library via the set-config Command (CVE-2025-32801)

The set-config REST API command allows to completely control the configuration of kea-ctrl-agent itself, as well as of the individual Kea services. A trivial local privilege escalation is possible by configuring a hook library under control of an unprivileged user. The following example uses the curl utility to perform the exploit.

someuser$ curl -X POST -H "Content-Type: application/json" \
    -d '{ "command": "config-set", "arguments":
          { "Control-agent": {"hooks-libraries": [{"library": "/home/someuser/libexploit.so"}] }}}' \
    localhost:8000

By placing a constructor function into libexploit.so, attacker controlled code will be executed by kea-ctrl-agent upon dlopen() of the library. The impact is arbitrary code execution with full root privileges on installations that run Kea services as root. On systems that use a dedicated service user for Kea, the impact will be full control over the Kea processes and also escalated networking privileges.

Hook libraries can be configured for any of the other Kea services as well, thus code execution can be achieved in the context of each of the Kea daemons this way.

We offer a simple Python script kea-hook-lib-exploit.py for download which can be used to reproduce the issue.

3.2) Arbitrary File Overwrite via config-write Command (CVE-2025-32802)

The config-write REST API command instructs a Kea service to write out its configuration to an arbitrary file path:

curl -X POST -H "Content-Type: application/json" \
    -d '{ "command": "config-write", "arguments": { "filename": "/etc/evil.conf" } }' \
    localhost:8000

The file write happens via a regular C++ std::ofstream with trunc setting, i.e. the target file will be truncated and overwritten if it already exists. The configuration content that is written to disk can as well be controlled by the attacker, but the JSON format and configuration sanity checks that are enforced by Kea restrict the degree of freedom of what will eventually be written out.

If the Kea services run with full root privileges, then this is a local denial-of-service bordering on a local root exploit. By embedding shell code, a crafted JSON configuration written e.g. to a file in /etc/profile.d could trigger a local root exploit upon root logging in, for example.

If the Kea services are running as dedicated service users, then this attack vector can be used to corrupt Kea-owned configuration, log and state files, thus resulting in integrity violation and denial-of-service limited to the scope of the Kea services.

3.3) Redirection of Log Files to Arbitrary Paths (shared CVE with 3.2)

This is similar to issue 3.2): an arbitrary new logfile path can be configured to be used by Kea services. This is an example JSON configuration that demonstrates the problem:

{
    "command": "config-set",
        "arguments": {
             "Control-agent": {
                 "loggers": [{
                     "name": "kea-ctrl-agent",
                     "output-options": [{
                         "output": "/root/bad.log"
                     }],
                     "severity": "DEBUG"
                 }]
             }
         }
     }
}

This configuration causes kea-ctrl-agent to create the file /root/bad.log and also to change logging severity to DEBUG, potentially exposing sensitive internal program state. Also a lock file will be created in /root/bad.log.lock.

This attack vector poses another local denial-of-service vulnerability bordering on a local root exploit, similar to what is outlined in section 3.2).

3.4) Service Spoofing with Sockets in /tmp (shared CVE with 3.2)

For the purposes of forwarding a REST API request to one of the Kea services, kea-ctrl-agent attempts to connect to the service’s UNIX domain socket. If a legit Kea admin tries to send a command to a Kea service that is not currently running, like the kea-dhcp-ddns service (which isn’t configured by default on most distributions), then the admin can fall victim to a local service spoofing attack.

Whether this is possible depends upon the directory into which the UNIX domain sockets are placed. Many distributions use the public /tmp directory for this. In this case an unprivileged local user can create the UNIX domain socket in question on its own, for example in /tmp/kea-ddns-ctrl-socket. If this succeeds, then API requests will be forwarded to a spoofed service that can respond with a crafted reply. With this a local attacker can attempt to trick the admin into performing dangerous actions, or might be able to intercept sensitive data contained in the request forwarded to the spoofed service.

We reproduced this attack vector for example on Fedora 41 as follows:

curl -X POST -H "Content-Type: application/json" \
    -d '{ "command": "config-get", "service": [ "d2" ], "arguments": { "secret": "data" } }' \
    localhost:8000

The d2 service socket is configured by default in kea-ctrl-agent and refers to the kea-dhcp-ddns service. When running strace on kea-ctrl-agent then the following connect() attempt is observed during this request:

connect(18, {sa_family=AF_UNIX, sun_path="/tmp/kea-ddns-ctrl-socket"}, 27) = -1 ENOENT (No such file or directory)

A local unprivileged user can bind a UNIX domain socket in /tmp/kea-ddns-ctrl-socket to intercept any such requests.

This attack type also affects Kea services that are configured but not yet running, e.g. before the Kea service unit or init script is started, or when Kea services are restarted. Upon startup each service will attempt to unlink() the UNIX domain socket path before binding to it, but this is subject to a race condition that unprivileged users can win by rebinding a socket in this location before the legit service has a chance to do so. The legit service will then fail to start, while the unprivileged user will be able to intercept REST API requests that are forwarded to the spoofed service by kea-ctrl-agent.

3.5) Denial-of-Service issues with Sockets in /tmp (shared CVE with 3.2)

The use of the /tmp directory for the Kea service sockets is generally problematic. The Kea services create lock files in the socket directory that are derived from the socket names. Any local user can pre-create either the UNIX domain sockets or the associated lock files to prevent Kea services from starting.

3.6) World-Readable DHCP Lease Files in /var/lib/kea/*.cvs (CVE-2025-32803)

Many of the distributions we checked grant read access to the state data of the default Kea in-memory database, which is in most cases found in /var/lib/kea. This means all local users will be able to access this information and thereby this poses a local information leak. Whether DHCP leases are private data is debatable. More sensitive data might be stored in these files (in a future implementation), however.

We don’t recommend to allow general read access to this data. We originally only reported this as a hardening recommendation, but upstream decided to assign a CVE for it anyway.

3.7) World-Readable Kea Log Files (shared CVE with 3.6)

On most systems we checked, the Kea log files found in /var/log/kea or /var/log/kea*.log are world-readable. As a hardening measure we recommend to restrict access to this data.

We originally only reported this as a hardening recommendation, but upstream decided to assign a CVE for it anyway.

4) Hardening Suggestions

This section contains further hardening suggestions about issues that we don’t consider high severity at the moment.

4.1) Possible Timing Attack against the HTTP Basic Auth Implementation

kea-ctrl-agent uses the HTTP Basic Auth mechanism to implement authentication on the REST API interface. In this scheme the string "<username>:<password>" is base64 encoded and placed into an "Authorization:" HTTP header.

The verification of these credentials happens in BasicHttpAuthConfig::checkAuth(). The code maintains a std::unordered_map<std::string, std::string>, where the keys consist of the base64 encoded "<username>:<password>" combinations found in the Kea configuration. The values are the cleartext usernames that can be authenticated with the credentials found in the key.

In basic_auth_config.cc:365 the credentials provided by the REST API client are looked up directly in this map data structure to verify them. The verification of cleartext passwords can suffer from timing attack weaknesses when the passwords are compared using optimized string comparison routines. Attackers can perform statistical analysis of the time required by a service to report an authentication failure to construct, little by little, a valid user/password combination.

In the case of kea-ctrl-agent it isn’t plaintext passwords that are compared, but the base64 encoded "<username>:<password>" strings. This adds a bit of complexity but does not prevent a timing attack from succeeding. A bigger hurdle is the use of the std::unordered_map, however, which uses a hash function to lookup elements in the map. When using the gcc compiler suite and the libstdc++ standard library, then the default hash function used for std::string is MurmurHash2 with a static seed. While the hash lookup complicates a possible timing attack, it is still a deterministic algorithm and an attacker might be able to choose input values in a way that causes kea-ctrl-agent to produce hash values for the hash map lookup that are suitable for a timing attack.

To be on the safe side we suggest to supply a custom KeyEqual template parameter to the std::unordered_map. This key comparison function should implement a constant-time comparison of the input data to avoid any observable timing differences.

Since the complexity of such a timing attack, given the circumstances, will be very high, we don’t see this as a relevant security issue at the moment. A dedicated attacker might be willing to make an attempt at this and succeed, however.

4.2) API Credentials Leak via ‘get-config’ Command

When API authorization is enabled in the REST API, then the configuration potentially contains cleartext user names and passwords that can be used for authentication. A user that already has a valid set of credentials can discover the credentials of other users by retrieving the configuration via the API, even if the configuration file would otherwise not be world-readable in the system.

This means that any user with valid credentials can impersonate any other user. This can also be problematic when the credentials of a user are revoked at some point. By storing the credentials of other users, such a user could still access the API even after being denied access to Kea. Another issue could be that users might be reusing the same credentials for other, unrelated services.

Cleartext credentials should never be exposed on the API level, except maybe if the client is root anyway. Even then it could be a source of information leaks, for example if an admin shares a Kea configuration dump (e.g. for debugging purposes), unaware of the fact that cleartext credentials are contained in the data.

Users of Kea can circumvent this problem by avoiding storing cleartext credentials in the Kea configuration and instead referring to credential files on disk that are only accessible to privileged users.

5) Bugfixes

In our initial report we suggested to upstream to restrict paths from where hook libraries are loaded (issue 3.1) and also paths where configuration and log files are written to (issues 3.2, 3.3). It is obvious, however, that the unauthenticated REST API is problematic beyond the concrete exploit scenarios we explored. Arbitrary users in the system should not be able to fully control Kea’s configuration, for example. Thus we advised to enforce authentication on REST API level by default.

To fix the issues described in this report, upstream published bugfix releases for all currently supported versions of Kea:

We looked into release 2.6.3 and believe the bugfixes are thorough. As is also documented in the upstream release notes, the following changes have been introduced:

  • For many operations only safe directories are allowed for reading from or writing to. Among others this covers the following aspects:
    • Hook libraries can only be loaded from a trusted system directory (addresses issue 3.1).
    • Configuration files can only be written to the trusted system configuration directory (addresses issue 3.2).
    • Logfiles can only be written to the log directory determined during build time (addresses issue 3.3).
  • The default configuration files installed by Kea now enforce authentication of the REST API.
  • The log, state and socket directories are now installed without the world readable / world writable bits set (addresses issues 3.5, 3.6, 3.7).
  • Sockets are now placed under /var/run/kea by default. This directory must not be world-writable (addresses issue 3.5).
  • The documentation and example files have been updated to avoid issues like discussed in this report.

The hardenings for the issues described in section 4) are not yet available, but upstream intends to address them in the near future.

6) Affectedness of Kea Configurations on Common Linux and UNIX Systems

Kea is a cross-platform project that also targets traditional UNIX systems, which might be the reason why there are no well established standards for the packaging of Kea. Every distribution integrates Kea in its own way, leading to a complex variety of outcomes with regards to affectedness. The defaults and the resulting affectedness on a range of current well-known Linux and BSD systems are documented in detail in this section.

All systems we looked at have been updated to the most recent package versions on 2025-05-23.

6.1) Arch Linux

   
System Release rolling release (as of 2025-05-23)
Kea Version 2.6.1
Kea Credentials root:root
Kea Socket Dir /tmp
Kea Log Dir /var/log/kea-*.log, mode 0644
Kea State Dir /var/lib/kea, mode 0755
Affected By 3.1 through 3.7

Arch Linux is affected by all the issues.

6.2) Debian Linux

   
System Release 12.10, 12.11 (Bookworm)
Kea Version 2.2.0
Kea Credentials _kea:_kea
Kea Socket Dir /run/kea, owned by _kea:_kea mode 0755
Kea Log Dir /var/log/kea, owned by _kea:_kea mode 0750
Kea State Dir /var/lib/kea, mode 0755
Affected By 3.2 (partially), 3.3 (partially), 3.6

When we first discovered these issues we looked into Debian 12.10. Meanwhile Debian 12.11 has been released. The situation seems to be the same in both versions, however.

No local root exploit is possible here, because the services run as non-root. Debian also applies an AppArmor profile to Kea services. This makes the hook library injection (3.1) difficult. For the injection to succeed, a directory would be needed that can be written to by the attacker and from where the Kea service is allowed to read and map a library. This seems not possible in the current AppArmor profile used for Kea. Due to this, Debian is not affected by 3.1) at all.

3.2) and 3.3) only affect files owned by _kea and that are allowed to be written to according to AppArmor configuration. This still allows to corrupt the log, lock and state files owned by _kea.

The only information leak is found in the state directory (3.6); logs are protected.

AppArmor Security

We checked more closely if there is a loophole in the Kea AppArmor profiles to make arbitrary code execution (3.1) possible after all. The profiles for the dhcp4, dhcp6 and ddns Kea services allow reading and mapping of files found in /home/*/.Private/**, with the restriction that the files must be owned by _kea. An attacker with a home directory can place an injection library in its $HOME/.Private/libexploit.so. Only the ownership of the file is preventing the exploit from succeeding.

By leveraging issue 3.2), the Kea services can be instructed to create _kea owned files in the attacker’s $HOME/.Private. The content of the created files is not fully attacker controlled, however, so it will not be possible to craft a valid ELF object for loading via dlopen() this way. By placing a setgid-directory in $HOME/.Private/evil-dir, any files created in this directory will even have the group-ownership of the attacker. The file mode will be 0644, however, so the attacker is still not able to write to the file. Our research shows that there is only a very thin line of defense left against this arbitrary code execution in _kea:_kea context on Debian, but it seems to hold.

6.3) Ubuntu Linux

   
System Release 24.04.02 LTS
Kea version 2.4.1
Kea Credentials _kea:_kea
Kea Socket Dir /run/kea, owned by _kea:_kea mode 0755
Kea Log Dir /var/log/kea, owned by _kea:_kea mode 0750
Kea State Dir /var/lib/kea, mode 0755
Affected By 3.6

Ubuntu is mostly equivalent to the situation on Debian Linux with one major difference: REST API access authentication is enforced either by configuring a custom “user:password” pair, or by generating a random password. If no password is configured, kea-ctrl-agent will not start.

Due to this, Ubuntu is not affected by 3.1 and 3.2 at all. Only the information leak in the state directory (3.6) exists.

6.4) Fedora Linux

  Fedora 41 Fedora 42
Kea Version 2.6.1 2.6.2
Kea Credentials kea:kea ”
Kea Socket Dir /tmp ”
Kea Log Dir /var/log/kea, mode 0755 /var/log/kea mode 0750
Kea State Dir /var/lib/kea, mode 0750 ”
Affected By (all limited to the kea:kea credentials) 3.1, 3.2, 3.3, 3.4, 3.5, 3.7 (all limited to the kea:kea credentials) 3.1, 3.2, 3.3, 3.4, 3.5

When we first discovered these issues we looked into Fedora 41. Meanwhile Fedora 42 has been released. There are some changes found in Fedora 42, most notably a safer mode for /var/log/kea.

No local root exploit is possible on Fedora, because the services run as non-root.

Items 3.1 and 3.2 only affect Kea integrity and escalation of the CAP_NET_RAW and CAP_NET_BIND_SERVICE capabilities. There is no SELinux policy in effect for Kea, thus there are no additional protection layers present that would prevent arbitrary code execution in the context of kea:kea (3.1).

Fedora is affected by 3.3, 3.4 and 3.5 within the constraints of the kea:kea credentials. On Fedora 41 there exists an information leak in the log directory (3.7). The state directory is safe on both Fedora versions.

6.5) Gentoo Linux

   
System Release rolling release (as of 2025-05-23)
Kea Version 2.4.1
Kea Credentials root:root
Kea Socket Dir /run/kea owned by dhcp:dhcp mode 0750
Kea Log Dir /var/log/kea, owned by root:dhcp mode 0750
Kea State Dir /var/lib/kea, owned by root:dhcp mode 0750
Affected By if kea-ctrl-agent is manually enabled: 3.1, 3.2, 3.3

On Gentoo Linux Kea is only available as an unstable ~amd64 ebuild. It seems still incomplete, because the default configuration is broken (wrong paths) and the services won’t start. Also the kea-ctrl-agent is not part of the default configuration.

The directory permissions are inconsistent with the root:root credentials the Kea services are running with. This creates opportunities for a compromised dhcp user/group to stage symlink attacks in /run/kea, for example.

There are no information leaks and the /tmp directory is not used for sockets. Since the agent is not configured by default at all, we consider that Gentoo is not affected by any of the issues.

When kea-ctrl-agent is actively added to the mix and authorization is not enabled on the REST API, then Gentoo would be affected by issues 3.1, 3.2 and 3.3.

6.6) openSUSE Tumbleweed

System Release rolling release (as of 2025-04-01) rolling release (as of 2025-05-23)
Kea Version 2.6.1 2.6.2
Kea Credentials root:root keadhcp:keadhcp
Kea Socket Dir /tmp /tmp
Kea Log Dir /var/log/kea, owned by keadhcp:keadhcp mode 0755 mode changed to 0750
Kea State Dir /var/lib/kea, owned by root:root mode 0755 /var/lib/kea, owned by keadhcp:keadhcp mode 0750
Affected By 3.1 through 3.7 (all limited to keadhcp credentials) 3.1, 3.2, 3.3, 3.4, 3.5

When we first discovered these issues, openSUSE Tumbleweed was fully affected by all of them. We asked our Kea maintainer to harden the packaging already before the publication of these issues, which was possible without disclosing any information about the vulnerabilities. In the current packaging on openSUSE Tumbleweed Kea no longer runs as root and the systemd unit has ProtectSystem=full enabled, which adds another layer of defense. The information leaks in /var/log/kea and /var/lib/kea have been fixed as well.

The more disruptive changes have been delayed until the general publication of these issues and will soon be addressed as well.

6.7) FreeBSD

   
System Release 14.2
Kea Version 2.6.1
Kea Credentials root:root
Kea Socket Dir /tmp
Kea Log Dir /var/log/kea-*.log, owned by root:root mode 0644
Kea State Dir /var/db/kea, owned by root:wheel mode 0755
Affected By 3.1 through 3.7

FreeBSD is affected by all the issues.

6.8) NetBSD (pkgsrc binary)

   
System Release 10.1
Kea Version 2.6.1
Kea Credentials root:root
Kea Socket Dir /tmp
Kea Log Dir /var/log/kea-*.log, owned by root:wheel mode 0644
Kea State Dir /var/lib/kea, owned by root:wheel mode 0755
Affected By if example configuration is used unmodified: 3.1 through 3.7

NetBSD supports the installation of a pkgsrc binary distribution of Kea, which is also available on some other systems like MacOS. This distribution of Kea is affected by all the issues.

By default no configuration is active, however. Admins have to copy over the configuration from example files found in /usr/pkg/share/examples/kea. Thus it is debatable whether NetBSD is affected in default installations of Kea.

6.9) OpenBSD

     
System Release 7.6 7.7
Kea Version 2.4.1 ”
Kea Credentials root:root ”
Kea Socket Dir /var/run/kea, owned by root:_kea mode 0775 ”
Kea Log Dir redirected to syslog (world-readable) ”
Kea State Dir /var/lib/kea, owned by root:_kea mode 0775 mode 0750
Affected By 3.1, 3.2, 3.3, 3.6, 3.7 3.1, 3.2, 3.3, 3.7

When we first discovered these issues we looked into OpenBSD 7.6. Meanwhile OpenBSD 7.7 has been released. As far as we can see only the mode of the /var/lib/kea directory changed in this release.

OpenBSD is affected by issues 3.1, 3.2 and 3.3. Sockets are placed in a dedicated directory, thus 3.4 and 3.5 do not apply here. There exist information leaks for log and state data (the latter only in release 7.6).

The _kea group ownership for the socket and state dir is inconsistent with the actual daemon credentials. A compromised _kea group could stage symlink attacks in these directories.

7) CVE Assignments

Kea upstream assigned the following CVEs. Some of them are cumulative and cover multiple of the issues found in this report.

CVE Corresponding Issues Description
CVE-2025-32801 3.1 Loading a malicious hook library can lead to local privilege escalation.
CVE-2025-32802 3.2, 3.2, 3.4, 3.5 Insecure handling of file paths allows multiple local attacks.
CVE-2025-32803 3.6, 3.7 Insecure file permissions can result in confidential information leakage.

Timeline

2025-04-01 We reported the findings via a private issue in the ISC GitLab.
2025-04-02 After some initial controversial discussions, Kea upstream decided to accept the offer for coordinated disclosure and to work on bugfixes.
2025-04-10 Upstream assigned CVEs for the issues.
2025-04-29 Upstream communicated a coordinated release date of 2025-05-28 and their intention to involve the distros mailing list 5 days earlier. Given the range of affected distributions and the severity of the issues, we suggested to involve the distros mailing list already 10 days before publication.
2025-05-15 Kea upstream pre-disclosed the vulnerabilities to the distros mailing list.
2025-05-22 Kea upstream shared links to private bugfix releases 2.4.2, 2.6.3, and 2.7.9, containing fixes for the issues, both with the distros mailing list and in the private GitLab issue.
2025-05-26 We inspected the differences between version 2.6.2 and version 2.6.3 and found the bugfixes to be thorough.
2025-05-28 Publication happened as planned.

References