CVE-2026-49852 joserfc: HS256/HS384/HS512 verify accepts empty/nil HMAC key (cross-language sibling of CVE-2026-45363)
Attackers can forge HMAC-signed JWTs by using empty or nil keys, bypassing authentication in applications using the joserfc library.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-49852?
CVE-2026-49852 is a high-severity authentication bypass vulnerability in authlib joserfc, published on 17 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
joserfc is a Python library that provides an implementation of several JSON Object Signing and Encryption (JOSE) standards. Prior to 1.6.8, joserfc.jwt.decode accepts attacker-forged HMAC-signed tokens when the caller-supplied verification key is the empty string or None, because HMACAlgorithm.sign and HMACAlgorithm.verify in src/joserfc/_rfc7518/jws_algs.py pass the output of OctKey.get_op_key(...) to hmac.new(...) and OctKey.import_key in src/joserfc/_rfc7518/oct_key.py only emits a SecurityWarning for keys shorter than 14 bytes without rejecting zero-length input. This issue is fixed in version 1.6.8.
Which versions are affected by CVE-2026-49852?
3 open source packages ship an affected version, including joserfc.
| Package | Affected | Fixed in |
|---|---|---|
joserfc PyPI | All versions | 1.6.8 |
joserfc PyPI | All versions | 1.6.8 |
joserfc PyPI | All versions | 1.6.8 |
Linux distribution packages (3)
jaeger Red Hat:hummingbird:1 | All versions | 0:2.19.0-1.2.hum1 |
python-joserfc openSUSE:Leap 16.0 | All versions | 1.1.0-bp160.3.1 |
python-joserfc openSUSE:Tumbleweed | All versions | 1.7.2-2.1 |
How do I fix CVE-2026-49852?
Upgrade joserfc to 1.6.8 or later.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-49852 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-49852 exists.
Sources
Also known as CLEANSTART-2026-RP49018, DEBIAN-CVE-2026-49852, ECHO-9e9b-2656-ec45, GHSA-gg9x-qcx2-xmrh, MINI-f63x-qq5v-whfv, PYSEC-2026-2528 and 5 more.