# CVE-2026-55735: Guardian.revoke/3 acts on unverified token claims, allowing forged-token session revocation

> An unauthenticated attacker can revoke victim sessions by submitting forged tokens to the Guardian.revoke/3 function, causing a denial of service.

- **Severity:** High 8.2 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 1 Aug 2026
- **Updated:** 3 Aug 2026
- **Also known as:** EEF-CVE-2026-55735, GHSA-7975-hp3r-5qhv

## What is CVE-2026-55735?

CVE-2026-55735 is a high-severity vulnerability in ueberauth guardian, published on 1 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

Improper Verification of Cryptographic Signature in ueberauth guardian allows an unauthenticated attacker to revoke a victim's session with a forged token.

Guardian.revoke/3 in lib/guardian.ex decodes the supplied token with peek/1, which performs no signature verification (it only base64-decodes the JWT header and payload). The resulting unverified claims are forwarded directly to the configured token module's revoke callback and the implementation's on_revoke callback, a state-mutating sink. The sibling operations refresh/2 and exchange/4 both call decode_and_verify first, so the signature is checked before anything acts on the claims; revoke/3 is the only state-mutating path that acts on claims without verifying the signature.

An attacker who knows or guesses a victim's identifying claim values (jti, sub) can forge a JWT carrying those claims, sign it with an arbitrary key, and submit it to any endpoint that funnels a caller-supplied token into Guardian.revoke/3 (the standard logout / session-revocation pattern). When the token module mutates state keyed by the claims (whitelist deletion or blacklist insertion, for example a GuardianDb-style store), the victim's legitimate session is evicted. This is an unauthenticated session-revocation denial of service; the attacker never needs the signing secret.

This issue affects guardian: from 1.0.0 before 2.4.1.

Weakness: CWE-347 (Improper Verification of Cryptographic Signature)

## Which versions are affected by CVE-2026-55735?

1 open source package ship an affected version, including guardian. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `guardian` | Hex | From 1.0.0 | 2.4.1 |

Products: Ueberauth Guardian.

## How do I fix CVE-2026-55735?

Upgrade guardian to 2.4.1 or later.

## Is CVE-2026-55735 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55735 exists.

## Other vulnerabilities in ueberauth guardian

- [CVE-2026-54894](https://insights.sbomify.com/CVE-2026-54894/): Atom-table exhaustion denial of service in Guardian via unbounded atom creation from binary keys (medium 6.9)
- [CVE-2026-55733](https://insights.sbomify.com/CVE-2026-55733/): Atom-table exhaustion denial of service in Guardian permissions AtomEncoding via unbounded atom creation (medium 6.9)
- [CVE-2026-55734](https://insights.sbomify.com/CVE-2026-55734/): guardian atom exhaustion in Guardian.Permissions.encode_permissions!/1 (medium 6.9)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55735 and fix it if it is affected.

What it is: CVE-2026-55735 is a high-severity vulnerability in ueberauth guardian, published on 1 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-55735 exists.

Affected packages and fixed versions:
- guardian (Hex): affected from 1.0.0; fixed in 2.4.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-55735/ (Markdown: https://insights.sbomify.com/CVE-2026-55735/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55735
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55735
- OSV: https://osv.dev/vulnerability/CVE-2026-55735

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55735/
Summaries are generated and may contain mistakes; the official description above is the record.