PyCA Cryptography vulnerable to GCM tag forgery

Exploitation

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

CVSS 4.0

8.7 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2018-10903: 3.20% estimated exploitation activity in the next 30 days; percentile 88.13 (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.

A flaw was found in python-cryptography versions between =1.9.0 and <2.3. The finalizewithtag API did not enforce a minimum tag length. If a user did not validate the input length prior to passing it to finalizewithtag an attacker could craft an invalid payload with a shortened tag (e.g. 1 byte) such that they would have a 1 in 256 chance of passing the MAC check. GCM tag forgeries can cause key leakage.

Published by the source

31 Jul 2018, 18:28 UTC

Last modified by the source

13 Sep 2024, 18:23 UTC

Source last checked

26 Oct 2026, 13:17 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-fcf9-3qw3-gxmj

2. Revision 2 · amended · 24 Oct 2026

   Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-fcf9-3qw3-gxmj

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# Patch Next advisory brief: CVE-2018-10903

Advisory: PyCA Cryptography vulnerable to GCM tag forgery
IDs: CVE-2018-10903, GHSA-fcf9-3qw3-gxmj
Record: https://patchnext.com/advisories/cve-2018-10903 (revision 3, source checked 26 Oct 2026, 13:17 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: cryptography 2.3. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI cryptography: affected >= 1.9.0, < 2.3; fixed in 2.3

## Suggested inspection (read-only)
- `python -m pip show cryptography`: 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 cryptography.
- 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-fcf9-3qw3-gxmj (OSV record GHSA-fcf9-3qw3-gxmj lists affected ranges: PyPI cryptography >=1.9.0 <2.3 (fixed))
- Verified: https://github.com/advisories/GHSA-fcf9-3qw3-gxmj (GitHub-reviewed advisory GHSA-fcf9-3qw3-gxmj lists affected ranges: PyPI cryptography >=1.9.0 <2.3)

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