Allele Security Alert
ASA-2019-00531
Identifier(s)
ASA-2019-00531, CVE-2019-15718
Title
Missing access controls on systemd-resolved’s D-Bus interface
Vendor(s)
The systemd project
Product(s)
systemd
Affected version(s)
systemd versions before v243
Fixed version(s)
systemd version v243
Proof of concept
Unknown
Description
It was discovered that systemd-resolved does not enforce appropriate access controls on its D-Bus interface and allows unprivileged users to execute methods that are meant to be available only to privileged users. This can be exploited by local users to modify the system’s DNS resolver settings.
Technical details
The function manager_connect_bus() in src/resolve/resolved-bus.c opens a connection to the system bus using the bus_open_system_watch_bind_with_description() helper function, which is defined in src/shared/bus-util.c.
This helper function calls sd_bus_set_trusted(). This has the effect of disabling access controls, even for members that are defined without the SD_BUS_VTABLE_UNPRIVILEGED flag – the absence of which should deny access from unprivileged clients. See check_access() in src/libsystemd/sd-bus/bus-objects.c:
static int check_access(sd_bus *bus, sd_bus_message *m, struct
vtable_member *c, sd_bus_error *error) {
uint64_t cap;
int r;
assert(bus);
assert(m);
assert(c);
/* If the entire bus is trusted let's grant access */
if (bus->trusted)
return 0;
/* If the member is marked UNPRIVILEGED let's grant access */
if (c->vtable->flags & SD_BUS_VTABLE_UNPRIVILEGED)
return 0;
...
timesyncd and networkd both use the same helper function to connect to the system bus, but both of these are unaffected by this bug. In timesyncd’s case, it only exposes some read-only properties and these don’t have access controls. In networkd’s case, all methods are annotated with SD_BUS_VTABLE_UNPRIVILEGED and it uses policykit for enforcing access controls.
Credits
Nadav Markus (Palo Alto Networks)
Reference(s)
CVE-2019-15718: Missing access controls on systemd-resolved’s D-Bus interface
https://seclists.org/oss-sec/2019/q3/191
Resolved issue by keszybz · Pull Request #13457 · systemd/systemd
https://github.com/systemd/systemd/pull/13457
shared/but-util: drop trusted annotation from bus_open_system_watch_bind_with_description()
https://github.com/systemd/systemd/commit/35e528018f315798d3bffcb592b32a0d8f5162bd
CVE-2019-15718 - Red Hat Customer Portal
https://access.redhat.com/security/cve/CVE-2019-15718
https://people.canonical.com/~ubuntu-security/cve/CVE-2019-15718.html
CVE-2019-15718
https://security-tracker.debian.org/tracker/CVE-2019-15718
CVE-2019-15718 | SUSE
https://www.suse.com/security/cve/CVE-2019-15718
CVE-2019-15718
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-15718
CVE-2019-15718
https://nvd.nist.gov/vuln/detail/CVE-2019-15718
If there is any error in this alert or you wish a comprehensive analysis, let us know.
Last modified: September 3, 2019