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PYSEC-2026-1814

LowCVSS 2.2 / 10
Published Jul 7, 2026·Last modified Jul 7, 2026
Affected Components(0)

No affected components available

Description

Summary

The wrong string if check is run for iss checking, resulting in "acb" being accepted for "_abc_".

Details

This is a bug introduced in version 2.10.0: checking the "iss" claim changed from isinstance(issuer, list) to isinstance(issuer, Sequence).

-        if isinstance(issuer, list):
+        if isinstance(issuer, Sequence):
            if payload["iss"] not in issuer:
                raise InvalidIssuerError("Invalid issuer")
        else:

Since str is a Sequnce, but not a list, in is also used for string comparison. This results in if "abc" not in "__abcd__": being checked instead of if "abc" != "__abc__":.

PoC

Check out the unit tests added here: https://github.com/jpadilla/pyjwt-ghsa-75c5-xw7c-p5pm

        issuer = "urn:expected"

        payload = {"iss": "urn:"}

        token = jwt.encode(payload, "secret")

        # decode() succeeds, even though `"urn:" != "urn:expected". No exception is raised.
        with pytest.raises(InvalidIssuerError):
            jwt.decode(token, "secret", issuer=issuer, algorithms=["HS256"])

Impact

I would say the real world impact is not that high, seeing as the signature still has to match. We should still fix it.

Risk Scores
Base Score
2.2

The vulnerability can be exploited over the network without needing physical access. It is difficult for an attacker to exploit this vulnerability and may require special conditions. An attacker needs high-level or administrative privileges. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a low impact on the integrity of the data.

Threat Intelligence
2.1

Limited exploitation activity has been observed. Close monitoring and planned remediation are recommended.

EPSS
0.81%

The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.

Exploit
Not available

We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.

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