CVE-2021-20835Network attack vector

Access-token disclosure via unrestricted custom URL handler

Published Nov 24, 2021 · Updated Aug 3, 2024

Improper authorization in Mercari before 4.49.1 allows remote attackers to obtain account access tokens through a malicious webpage. The application's custom URL scheme handler fails to restrict which web-originated Intents can launch arbitrary application Activities. Exploitation requires a user with an affected Android app to visit a malicious page, and the documented consequence is access-token disclosure.

CVSS severity7.4
High
EPSS probability1.57%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Improper authorization in Mercari before 4.49.1 allows remote attackers to obtain account access tokens through a malicious webpage. The application's custom URL scheme handler fails to restrict which web-originated Intents can launch arbitrary application Activities. Exploitation requires a user with an affected Android app to visit a malicious page, and the documented consequence is access-token disclosure.

The record

CVE
CVE-2021-20835
Published
Nov 24, 2021
Updated
Aug 3, 2024
Vendor
Mercari, Inc.
Product
Mercari
Classifications
CWE-939, T1189, T1528
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Nov 24, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=versions prior to 4.49.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mercari, Inc. · Mercariversion=versions prior to 4.49.1

Published CVSS scores

7.4JPCERT/CCCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:N/A:N
4.3JPCERT/CCAV:N/AC:M/Au:N/C:P/I:N/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-939Improper Authorization in Handler for Custom URL Scheme
T1189Drive-by Compromise
T1528Steal Application Access Token

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo