CVE-2020-13757: Rsa Denial of Service Vulnerability

HIGH
Advisory Identifier: GHSA-537h-rv9q-vvph
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CVSS 7.5
CWE-327
Published
Advisory Summary

CVE-2020-13757 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-327: Use of a Broken or Risky Cryptographic Algorithm) affecting Rsa libraries in the PyPI ecosystem. The issue involves python-rsa decryption of ciphertext leads to dos. Remediation is available by upgrading affected packages to version 4.1.

CVE-2020-13757 at a Glance

CVE Identifier CVE-2020-13757
GHSA Identifier GHSA-537h-rv9q-vvph
Severity HIGH
CVSS v3.1 Base Score 7.5
CWE Classification CWE-327: Use of a Broken or Risky Cryptographic Algorithm
Affected Product Rsa
Package Ecosystem PyPI
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality High (C:H)
Integrity None (I:N)
Availability None (A:N)

How to Fix CVE-2020-13757

To remediate this vulnerability, upgrade all affected Rsa dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

rsa fixed: 4.1 
pip install rsa>=4.1
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show rsa

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2020-13757, inspect your installed packages using pip:

pip show rsa

Analyze the terminal output to locate any version of Rsa in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 4.1), your application is affected.

Note: The pip show rsa command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-327: Use of a Broken or Risky Cryptographic Algorithm, where the product uses a broken or risky cryptographic algorithm or protocol. In Rsa, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

Python-RSA before 4.1 ignores leading '\0' bytes during decryption of ciphertext. This could conceivably have a security-relevant impact, e.g., by helping an attacker to infer that an application uses Python-RSA, or if the length of accepted ciphertext affects application behavior (such as by causing excessive memory allocation).

CVE-2020-13757 Affected Rsa Versions

1 component(s)
Fixed Versions:
4.1
Product / Package Ecosystem Affected Versions Fixed Version
rsa
pypi/rsa
PyPI
< 4.1
4.1

References

NVD — CVE-2020-13757 NVD
https://nvd.nist.gov/vuln/detail/CVE-2020-13757
Rsa Source Repository REPOSITORY
https://github.com/sybrenstuvel/python-rsa/issues/146
Rsa Source Repository REPOSITORY
https://github.com/sybrenstuvel/python-rsa/issues/146#issuecomment-641845667
Rsa Source Repository REPOSITORY
https://github.com/pypa/advisory-database/tree/main/vulns/rsa/PYSEC-2020-99.yaml
Rsa Source Repository REPOSITORY
https://github.com/sybrenstuvel/python-rsa
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/2KILTHBHNSDUCYV22ODLOKTICJJ7JQIQ
lists.fedoraproject.org/archives/list/packa... WEB
https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/ZYB65VNILRBTXL6EITQTH2PZPK7I23MW
usn.ubuntu.com/4478-1 WEB
https://usn.ubuntu.com/4478-1

Frequently Asked Questions

What is CVE-2020-13757?
CVE-2020-13757 is a security vulnerability in Rsa involving Python-RSA decryption of ciphertext leads to DoS under CWE-327: Use of a Broken or Risky Cryptographic Algorithm.
Which Rsa versions are affected?
Affected versions include releases prior to 4.1 across affected packages: rsa.
What version fixes CVE-2020-13757?
Upgrading to 4.1 (or the latest patched release for your respective branch) resolves the vulnerability.
Is rsa affected?
Yes, rsa is affected in releases prior to 4.1. Upgrade your dependency to 4.1 or later.
What is the CVSS score of CVE-2020-13757?
CVE-2020-13757 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N.
What CWE is associated with CVE-2020-13757?
CVE-2020-13757 is classified under CWE-327: Use of a Broken or Risky Cryptographic Algorithm (The product uses a broken or risky cryptographic algorithm or protocol.).
How can I check PyPI dependencies for CVE-2020-13757?
Run the dependency tree command: 'pip show rsa'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-537h-rv9q-vvph",
  "modified" : "2024-10-21T21:54:27Z",
  "published" : "2021-03-24T18:24:39Z",
  "aliases" : [ "CVE-2020-13757" ],
  "summary" : "Python-RSA decryption of ciphertext leads to DoS",
  "details" : "Python-RSA before 4.1 ignores leading '\\0' bytes during decryption of ciphertext. This could conceivably have a security-relevant impact, e.g., by helping an attacker to infer that an application uses Python-RSA, or if the length of accepted ciphertext affects application behavior (such as by causing excessive memory allocation).",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "rsa"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "4.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2020-13757"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/sybrenstuvel/python-rsa/issues/146"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/sybrenstuvel/python-rsa/issues/146#issuecomment-641845667"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/pypa/advisory-database/tree/main/vulns/rsa/PYSEC-2020-99.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/sybrenstuvel/python-rsa"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/2KILTHBHNSDUCYV22ODLOKTICJJ7JQIQ"
  }, {
    "type" : "WEB",
    "url" : "https://lists.fedoraproject.org/archives/list/package-announce@lists.fedoraproject.org/message/ZYB65VNILRBTXL6EITQTH2PZPK7I23MW"
  }, {
    "type" : "WEB",
    "url" : "https://usn.ubuntu.com/4478-1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-327" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-03-24T18:24:28Z",
    "nvd_published_at" : "2020-06-01T19:15:00Z"
  }
}