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borgbackup (1.2.8-1) unstable; urgency=high
**** Important notice ****
- Fix for CVE-2023-36811
**** Please follow the upgrade procedure below to avoid any risk of data
loss!!! ****
A flaw in the cryptographic authentication scheme in Borg allowed an attacker to
fake archives and potentially indirectly cause backup data loss in the repository.
The attack requires an attacker to be able to
1. insert files (with no additional headers) into backups
2. gain write access to the repository
This vulnerability does not disclose plaintext to the attacker, nor does it
affect the authenticity of existing archives.
Creating plausible fake archives may be feasible for empty or small archives,
but is unlikely for large archives.
The fix enforces checking the TAM authentication tag of archives at critical
places. Borg now considers archives without TAM as garbage or an attack.
We are not aware of others having discovered, disclosed or exploited this vulnerability.
Below, if we speak of borg 1.2.8, we mean a borg version >= 1.2.8 **or** a
borg version that has the relevant security patches for this vulnerability applied
(could be also an older version in that case).
Steps you must take to upgrade a repository (this applies to all kinds of repos no
matter what encryption mode they use, including “none”):
1. Upgrade all clients using this repository to borg 1.2.8. Note: it is not
required to upgrade a server, except if the server-side borg is also used as a
client (and not just for “borg serve”).
Do *not* run `borg check` with borg > 1.2.4 before completing the upgrade steps:
* borg check would complain about archives without a valid archive TAM.
* borg check --repair would remove such archives!
2. Do this step on every client using this repo:
# borg upgrade --show-rc --check-tam <repo>
This will check the manifest TAM authentication setup in the repo and on this
client. The command will exit with rc=0 if all is OK, otherwise with rc=1.
a. If you get “Manifest authentication setup OK for this client and this
repository.” and rc=0, continue with 3.
b. If you get some warnings and rc=1, run:
# borg upgrade --tam --force <repository>
3. Run:
# borg upgrade --show-rc --check-archives-tam <repo>
This will create a report about the TAM status for all archives. In the last
line(s) of the report, it will also report the overall status. The command
will exit with rc=0 if all archives are TAM authenticated or with rc=1 if
there are some archives with TAM issues.
If there are no issues and all archives are TAM authenticated, continue with 5.
Archive TAM issues are expected for:
* archives created by borg <1.0.9.
* archives resulting from a borg rename or borg recreate operation (see #7791)
But, important, archive TAM issues could also come from archives created by
an attacker. You should verify that archives with TAM issues are authentic
and not malicious (== have good content, have correct timestamp, can be
extracted successfully). In case you find crappy/malicious archives, you must
delete them before proceeding.
In low-risk, trusted environments, you may decide on your own risk to skip
step 3 and just trust in everything being OK.
4. If there are no archives with TAM issues left at this point, you can skip this step.
Run: # borg upgrade --archives-tam <repo>
This will unconditionally add a correct archive TAM to all archives not
having one. borg check would consider TAM-less or invalid-TAM archives as
garbage or a potential attack.
To see that all archives are OK now, you can optionally repeat the command
from step 3.
5. Done. Manifest and archives are TAM authenticated now.
Vulnerability time line:
* 2023-06-13: Vulnerability discovered during code review by Thomas Waldmann
* 2023-06-13…: Work on fixing the issue, upgrade procedure, docs.
* 2023-06-30: CVE was assigned via Github CNA
* 2023-06-30…2023-08-29: Fixed issue, code review, docs, testing.
* 2023-08-30: Released fixed version 1.2.5 (broken upgrade procedure for some repos)
* 2023-08-31: Released fixed version 1.2.6 (fixes upgrade procedure)
(Source: upstream docs/changes.rst, formatted for plain-text clarity)
* Note: borg supports the "compact" feature!
- compact: "borg compact" needs to be used to free repository space by
compacting the segments (reading sparse segments, rewriting still needed
data to new segments, deleting the sparse segments).
Borg < 1.2 invoked compaction automatically at the end of each repository
writing command.
Borg >= 1.2 does not do that any more to give better speed, more control,
more segment file stability (== less stuff moving to newer segments) and
more robustness.
See the docs about "borg compact" for more details.
- "borg compact --cleanup-commits" is to cleanup the tons of 17byte long
commit-only segment files caused by borg 1.1.x issue #2850.
Invoke this once after upgrading (the server side) borg to 1.2.
Compaction now automatically removes unneeded commit-only segment files.
-- Nicholas D Steeves <sten@debian.org> Sun, 15 Jun 2025 11:17:33 -0400
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