CVE-2023-36811

Archive spoofing vulnerability in borgbackup

Workarounds

Data loss after being attacked can be avoided by reviewing the archives (timestamp and contents valid and as expected) after any "borg check --repair" and before "borg prune".

References

https://github.com/borgbackup/borg/blob/1.2.6/docs/changes.rst#pre-125-archives-spoofing-vulnerability-cve-2023-36811

A flaw in the cryptographic authentication scheme in borgbackup 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.

Affected are all borgbackup releases prior to 1.2.5.

Note: CVSS scoring model seemed to badly fit for this case, thus I manually set score to "moderate".

CVE-2023-36811 has a CVSS score of 4.7 (Medium). The vector is requires local access, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.2.5); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

The issue has been fixed in borgbackup 1.2.5.
But there was a bug in 1.2.5 upgrade instructions, thus 1.2.6 with an important fix in docs and code was released a day afterwards.

Additionally to installing the fixed code, users must follow the upgrade procedure as documented in the latest version of the change log.

1. What is CVE-2023-36811? CVE-2023-36811 is a medium-severity security vulnerability in borgbackup (pip), affecting versions < 1.2.5. It is fixed in 1.2.5.
2. How severe is CVE-2023-36811? CVE-2023-36811 has a CVSS score of 4.7 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of borgbackup are affected by CVE-2023-36811? borgbackup (pip) versions < 1.2.5 is affected.
4. Is there a fix for CVE-2023-36811? Yes. CVE-2023-36811 is fixed in 1.2.5. Upgrade to this version or later.
5. Is CVE-2023-36811 exploitable, and should I be worried? Whether CVE-2023-36811 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2023-36811 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2023-36811? Upgrade borgbackup to 1.2.5 or later.

Other vulnerabilities in borgbackup