ReDoS in LDAP schema parser

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

6.5 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2021-46823: 1.94% estimated exploitation activity in the next 30 days; percentile 80.17 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

https://github.com/python-ldap/python-ldap/issues/424 Impact The LDAP schema parser of python-ldap 3.3.1 and earlier are vulnerable to a regular expression denial-of-service attack. The issue affects clients that use ldap.schema package to parse LDAP schema definitions from an untrusted source. Patches The upcoming release of python-ldap 3.4.0 will contain a workaround to prevent ReDoS attacks. The schema parser refuses schema definitions with an excessive amount of backslashes. Workarounds As a workaround, users can check input for excessive amount of backslashes in schemas. More than a dozen backslashes per line are atypical. References CWE-1333 For more information If you have any questions or comments about this advisory: Open an issue in python-ldap tracker

Published by the source

19 Jun 2022, 00:00 UTC

Last modified by the source

8 Jul 2026, 06:49 UTC

Source last checked

26 Oct 2026, 13:08 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-r8wq-qrxc-hmcm

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-r8wq-qrxc-hmcm

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-qfr5-wjpw-q4c4

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# Patch Next advisory brief: CVE-2021-46823

Advisory: ReDoS in LDAP schema parser
IDs: CVE-2021-46823, GHSA-qfr5-wjpw-q4c4
Record: https://patchnext.com/advisories/cve-2021-46823 (revision 3, source checked 26 Oct 2026, 13:08 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: python-ldap 3.4.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI python-ldap: affected < 3.4.0; fixed in 3.4.0

## Suggested inspection (read-only)
- `python -m pip show python-ldap`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on python-ldap.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-qfr5-wjpw-q4c4 (OSV record GHSA-qfr5-wjpw-q4c4 lists affected ranges: PyPI python-ldap >=0 <3.4.0 (fixed))
- Verified: https://osv.dev/vulnerability/GHSA-r8wq-qrxc-hmcm (OSV record GHSA-r8wq-qrxc-hmcm lists affected ranges: PyPI python-ldap >=0 <3.4.0 (fixed))
- Verified: https://github.com/advisories/GHSA-r8wq-qrxc-hmcm (GitHub-reviewed advisory GHSA-r8wq-qrxc-hmcm lists affected ranges: PyPI python-ldap >=0 <3.4.0)

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