CVE-2025-4894: Django-sso-server RCE Vulnerability

CVE-2025-4894 Overview

CVE-2025-4894 is an inadequate encryption strength vulnerability in the calmkart Django-sso-server project, a single sign-on implementation built on the Django framework. The flaw resides in the gen_rsa_keys function within the common/crypto.py file. The function generates RSA key pairs with insufficient cryptographic strength, weakening the confidentiality guarantees of the SSO tokens and credentials it protects. The issue affects code up to commit 057247929a94ffc358788a37ab99e391379a4d15. Because the project uses a rolling release model, no discrete affected or fixed version numbers exist. The vulnerability is remotely reachable but requires high attack complexity, making practical exploitation difficult.

Critical Impact
Attackers who successfully break the weakened RSA keys can decrypt SSO traffic and compromise authentication material, undermining confidentiality across all applications that rely on the SSO server.

Affected Products

calmkart Django-sso-server (rolling release, up to commit 057247929a94ffc358788a37ab99e391379a4d15)
Deployments using the gen_rsa_keys function in common/crypto.py
Downstream applications relying on this SSO server for authentication

Discovery Timeline

2025-05-18 - CVE-2025-4894 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in the NVD database

Technical Details for CVE-2025-4894

Vulnerability Analysis

The vulnerability is categorized under [CWE-310] (Cryptographic Issues) and [CWE-326] (Inadequate Encryption Strength). The gen_rsa_keys function in common/crypto.py generates RSA key material used to protect SSO tokens exchanged between the identity provider and relying applications. When RSA keys are generated with an insufficient modulus size or with weak parameters, the resulting keys become tractable to factor or otherwise attack using modern cryptanalysis and computing resources.

An attacker who obtains the public key can attempt offline factorization to recover the private key. With the private key, the attacker decrypts intercepted SSO exchanges, forges signed tokens, and impersonates users across all federated services. Because the SSO server sits at the trust boundary of every integrated application, key compromise cascades across the entire authentication ecosystem.

The attack is network-reachable and requires no authentication or user interaction, but its high complexity reflects the computational effort needed to break even a weak RSA key.

Root Cause

The root cause lies in the RSA key generation routine using cryptographic parameters that do not meet current strength recommendations. Modern guidance requires RSA keys of at least 2048 bits, with 3072 bits or greater recommended for long-lived keys. Any key generation path that falls below these thresholds, or that relies on weak entropy sources, produces keys vulnerable to factorization or related-key attacks.

Attack Vector

The attack vector is remote over the network. An adversary retrieves the SSO server's public key through its standard discovery endpoint or by capturing SSO traffic. The attacker then performs offline cryptanalysis to recover the private key. Once the private key is compromised, the attacker decrypts session material, forges assertions, and gains unauthorized access to any downstream application that trusts the SSO server. No prior authentication or user interaction is required.

No verified public exploit code is available for this vulnerability. Technical submissions are tracked in the VulDB entry #309448 and the associated VulDB submission #578019.

Detection Methods for CVE-2025-4894

Indicators of Compromise

Presence of RSA key files generated by the vulnerable gen_rsa_keys function with modulus sizes below 2048 bits
Anomalous SSO token issuance or replay of previously valid tokens after key material has been exposed
Unexpected successful authentications from unusual source addresses immediately following public-key disclosure events

Detection Strategies

Audit the common/crypto.py file in every Django-sso-server deployment and confirm the exact commit hash against 057247929a94ffc358788a37ab99e391379a4d15
Inspect generated RSA public keys and verify modulus length using tooling such as openssl rsa -in key.pem -text -noout
Correlate SSO issuance logs with downstream application authentication logs to identify tokens accepted without a corresponding legitimate login

Monitoring Recommendations

Monitor Django-sso-server processes for calls to gen_rsa_keys and alert on new key generation events outside change windows
Track SSO endpoint access patterns for automated harvesting of public keys or JWKS documents
Aggregate authentication telemetry from the SSO server and all relying parties into a central data lake for cross-source correlation of anomalous token use

How to Mitigate CVE-2025-4894

Immediate Actions Required

Pull the latest commit from the calmkart Django-sso-server repository and verify that gen_rsa_keys uses a minimum modulus of 2048 bits, preferably 3072 or 4096 bits
Rotate all RSA key pairs currently in use and invalidate SSO sessions and refresh tokens issued under the previous keys
Restrict network exposure of the SSO server to trusted networks until keys have been regenerated with adequate strength

Patch Information

The project follows a rolling release model, so no versioned patch is available. Operators must review upstream commits after 057247929a94ffc358788a37ab99e391379a4d15 and confirm that the RSA generation routine has been updated to use a secure key length and a cryptographically strong random source. Track updates through the VulDB CTI entry #309448.

Workarounds

Replace calls to the built-in gen_rsa_keys with a hardened wrapper that invokes cryptography.hazmat.primitives.asymmetric.rsa.generate_private_key with a public_exponent of 65537 and a key_size of at least 3072 bits
Terminate TLS at a reverse proxy in front of the SSO server and use externally managed keys stored in a hardware security module (HSM) or a managed KMS
Enforce short token lifetimes and require re-authentication for sensitive operations to reduce the window of exposure if keys are compromised

bash

# Verify RSA key strength on the SSO server
openssl rsa -in /path/to/sso_private_key.pem -text -noout | grep 'Private-Key'

# Generate a replacement key with adequate strength
openssl genpkey -algorithm RSA \
  -pkeyopt rsa_keygen_bits:3072 \
  -out sso_private_key_new.pem

# Restart the SSO service after installing the new key
systemctl restart django-sso-server