CVE-2026-48028Network attack vector

Signed-activity content hiding via incomplete normalization

Published Jun 24, 2026 · Updated Jun 24, 2026

Integrity-check bypass in Mastodon 4.3.22 and earlier allows remote attackers to hide fields in signed ActivityPub activities. The same flaw affects 4.4.0-beta.1 through 4.4.16 and 4.5.0-beta.1 through 4.5.9; incoming-activity normalization accepts a Linked-Data-Signed object after selected JSON entries are removed. Exploitation requires access to the original signed activity through mechanisms such as inbox forwarding or a subscribed relay, and the receiving server treats the reduced object as complete.

CVSS severity6.5
Medium
EPSS probability0.18%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Integrity-check bypass in Mastodon 4.3.22 and earlier allows remote attackers to hide fields in signed ActivityPub activities. The same flaw affects 4.4.0-beta.1 through 4.4.16 and 4.5.0-beta.1 through 4.5.9; incoming-activity normalization accepts a Linked-Data-Signed object after selected JSON entries are removed. Exploitation requires access to the original signed activity through mechanisms such as inbox forwarding or a subscribed relay, and the receiving server treats the reduced object as complete.

The record

CVE
CVE-2026-48028
Published
Jun 24, 2026
Updated
Jun 24, 2026
Vendor
Mastodon
Product
Mastodon
Classifications
CWE-354, T1565.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 24, 2026CVE publishedPublication date reported by the CVE source.
  2. Jun 24, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

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

  1. Affected versionversion=< 4.3.23
  2. Affected versionversion=>= 4.4.0-beta.1, < 4.4.17
  3. Affected versionversion=>= 4.5.0-beta.1, < 4.5.10

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mastodon · Mastodonversion=< 4.3.23; version=>= 4.4.0-beta.1, < 4.4.17; version=>= 4.5.0-beta.1, < 4.5.10

Published CVSS scores

6.5GitHub CNACVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L

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.

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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-354Improper Validation of Integrity Check Value
T1565.002Transmitted Data Manipulation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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