Adding and distrusting certificates

8 min read

Two things can be decided about this machine's trust, and both are values under Machine\System\Trust\Certificates. Change them with trust on a terminal, or Security Policy on the desktop (type security in the launcher), which takes the same steps.

Before making a change, check trust status and trust list. You need write access to the relevant registry key; by default SYSTEM and Administrators may write, and everybody may read. See who may change it.

In Security Policy #

Security Policy's Certificates sections show what trust status, trust list and trust list --distrusted show, and change it.

  • Trusted lists every certificate authority in force, those shipped with Peios and those added on this machine, with a search above them. Opening one shows who issued it, when it is valid, its key, serial number and SHA-256 fingerprint, and what it is trusted for. Export… saves it as a PEM file. Distrust… stops trusting it, and Remove… takes back one this machine added; both ask first, under their row.
  • Add Certificate… opens a PEM or DER file and shows what is in it before anything is trusted. A file that holds several certificates, a certificate that isn't a CA, one that has expired, or one already trusted or distrusted is shown with the reason and can't be added. Otherwise give it a name and choose its purposes, then Add Certificate.
  • Distrusted lists what is refused, with the reason it was given, and names each certificate where this machine has a copy. Trust Again… restores one after asking. Distrust by Fingerprint… refuses a certificate the machine doesn't have, by its whole fingerprint, or by a prefix of one it does.
  • Settings shows whether trustd is up to date, with Reload for those trustd lets reload it; turns the files under /etc/ssl on and off; and opens the permissions editor on Machine\System\Trust (Changing Trust) and on trustd's ControlSecurity (Trust Service).

The window follows trustd, so a change made elsewhere, with trust or reg, appears in it within a few seconds. What you may not change is shown read-only, with the reason said once.

Trusting a certificate authority #

An internal CA, a test CA, a partner's CA:

$ trust add corp-ca /path/to/corp-root.pem
added corp-ca
  CN=Corp Issuing CA,O=Example Ltd,C=GB
  SHA-256 4f2a…c19b

The name is yours to choose — it names the decision, not the certificate, so corp-ca is a better name than 4f2a. PEM or DER is accepted; a file holding more than one certificate is refused, because a bundle added under one name would be one decision covering several authorities.

The certificate is checked before it is written: it must parse, it must be a CA (basicConstraints), and it must not have expired. trustd checks it again when it reads it — it never trusts the writer — but doing it here means a mistake is a message rather than a line in a log.

To withdraw this named addition:

$ trust remove corp-ca

Purposes #

By default an addition is trusted for ServerAuth — validating a TLS server. Say otherwise if you mean otherwise:

$ trust add corp-ca corp-root.pem --purposes ServerAuth,CodeSigning

The purpose is recorded with the addition and can filter socket queries. Do not assume it limits the compatibility files. The inspected trustd 0.1.6 renderer writes every composed root to both PEM bundles and to the hashed directory, including an addition whose purposes omit ServerAuth. This differs from the intended ServerAuth-only rendering described in the source.

Do not use a non-ServerAuth purpose as an isolation boundary for a machine-wide addition on that implementation. The certificate's DER is preserved; this finding does not establish whether a particular TLS consumer accepts it, which also depends on the certificate and that consumer's validation rules. This is source inspection, not a live TLS test or a claim about every installed version.

Distrusting a certificate authority #

This is the one that has to work in a hurry, and it works on any certificate — one Mozilla shipped or one somebody added:

$ trust distrust 018e13f0772532cf --reason "withdrawn, see CA/B incident 2026-08"
distrusted 018e13f0772532cf809bd1b17281867283fc48c6e13be9c69812854a490c1b05
  was: CN=DigiCert TLS ECC P384 Root G5,O=DigiCert\, Inc.,C=US

After trustd observes the decision and successfully composes and renders the new set, the root is absent from its socket set, the rendered bundles and the hashed directory. Withdrawal does not require a package update. Check the result below: a composition failure retains the old set, while a rendering failure can leave the socket and files different. Neither a registry write nor socket readback proves that every running application has stopped using a cached copy.

A certificate is named by its SHA-256 fingerprint — never by subject name, which is forgeable and reused. Any of these work:

018e13f0772532cf                      the prefix `trust list` prints
018E13F0…C1B05                        upper case
01:8e:13:f0:…                          colon-separated, as other tools print it
/path/to/the-certificate.pem          the file itself

Distrusting a certificate this machine does not have is legitimate and is recorded rather than refused — the entry sits there, and if that root ever arrives in a bundle upgrade or an addition, it is already refused.

To see what has been decided, and to undo it:

$ trust list --distrusted
018e13f0772532cf  withdrawn, see CA/B incident 2026-08

1 distrusted

$ trust restore 018e13f0772532cf
restored 018e13f0772532cf809bd1b17281867283fc48c6e13be9c69812854a490c1b05

Verify the change #

A successful write records a decision. Check that trustd has applied it:

  1. Run trust status. If health is degraded, follow the recovery steps below. If entries were skipped, read the warnings in trustd's log; a skipped entry does not establish trust.
  2. Run trust list to inspect the effective set. For an addition, use trust show <fingerprint> to check its full fingerprint, source and purposes. trust list --purpose ServerAuth filters the socket reply; it does not inventory or constrain the compatibility files. See the purpose limitation.
  3. For a distrust, check that the certificate is absent from trust list and that the decision appears in trust list --distrusted. The latter reads the registry, so it is not enough on its own. After trust restore, check the effective set again: removing a distrust does not supply a certificate that is no longer shipped or added.

Retest the affected application. Programs with private trust stores use their own policy; a successful trust command is not an application TLS test.

Doing it from the registry #

trust writes registry values and nothing else, so reg does the same job and is subject to exactly the same access check:

Machine\System\Trust\Certificates\
  Add\<name>\
    Certificate   REG_BINARY    the certificate, DER
    Purposes      REG_MULTI_SZ  default ServerAuth
  Distrust\
    <fingerprint> REG_SZ        why, for whoever reads it later

Write Certificate last when creating an entry by hand. Until it exists the entry is incomplete, and trustd deliberately skips incomplete entries rather than acting on half of one.

This is also how a domain distributes trust: pushing a value to Trust\Certificates\Add on every member is an ordinary policy write, not a special mechanism.

Packages may offer trust, never take it #

A package must never widen what the whole machine trusts by being installed — peipkg install quietly adding a CA is a supply-chain hole, and every system that allowed it eventually closed it.

A package that comes with a CA ships a seed (/usr/share/regim/…), inert until an image names it in [registry] autoapply or an administrator applies it. Installing the package grants nothing.

A package shipping a trust store for its own private use — a browser, a language runtime — is unaffected by any of this. Those are ordinary files that only that program reads.

When the store is degraded #

$ trust status
health       degraded — cannot read /usr/share/ca-certificates/mozilla.crt: …

degraded can report either a composition failure or a rendering failure. They need different readback. In the inspected trustd 0.1.6 refresh path, a composition failure leaves the previous set and files untouched. A rendering failure happens after the socket set has changed; it can leave old, new or missing compatibility artifacts. Do not assume all consumers still use one last-good store.

Use the error in trust status to choose the next step:

  • For a missing or damaged bundle, check the installed package and repair the package problem. Do not edit the generated /etc/ssl files.
  • For a distrust list that would empty the store, review trust list --distrusted and correct unintended decisions. Do not restore an intentionally distrusted CA just to clear the health warning.
  • For render failed, preserve the operation and path in the error and investigate that filesystem failure. Compare the socket set with the files the affected program actually uses; the rendered paths can be retained from the last successful render and are not a fresh file inventory after failure. Do not hand-edit the generated files or assume a successful trust list proves they were updated. See compatibility rendering limits.

Once the cause is fixed, ask trustd to recompose:

$ trust reload

Reload requires permission to control trustd; Security Policy exposes its ControlSecurity permissions under Settings → Trust Service. If reload is refused, have an authorized administrator perform it.

The inspected daemon replies successfully to an authorized trust reload even if recomposition or rendering failed. Run trust status again and confirm health is ok, then verify the intended change and retest the affected application. Until recovery succeeds, do not assume a newly recorded addition or distrust is in force for every consumer.

Skipped entries are different and are not a degraded state: trust status counts them and the log names each one. Correct the named entry, then check the effective set again.

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