CVE-2021-3751Network attack vector

Memory corruption via unchecked buffer move length

Published Sep 15, 2021 · Updated Aug 3, 2024

Out-of-bounds write in libmobi 0.6 and earlier allows attackers to corrupt memory by processing a crafted MOBI ebook document. The mobi_buffer_move function checked only whether a backward offset crossed the buffer start, then moved data without ensuring that the offset plus length stayed within maxlen. Exploitation requires an application to parse the crafted file, and the overwrite can crash that process or corrupt data in its address space.

CVSS severity9.8
Critical
EPSS probability1.24%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds write in libmobi 0.6 and earlier allows attackers to corrupt memory by processing a crafted MOBI ebook document. The mobi_buffer_move function checked only whether a backward offset crossed the buffer start, then moved data without ensuring that the offset plus length stayed within maxlen. Exploitation requires an application to parse the crafted file, and the overwrite can crash that process or corrupt data in its address space.

The record

CVE
CVE-2021-3751
Published
Sep 15, 2021
Updated
Aug 3, 2024
Vendor
bfabiszewski
Product
libmobi
Classifications
CWE-787, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 15, 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

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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=unspecified <=0.6

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

bfabiszewski · libmobiversion=unspecified <=0.6

Published CVSS scores

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
7.5NIST NVDAV:N/AC:L/Au:N/C:P/I:P/A:P
9.6huntr.devCVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H

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-787Out-of-bounds Write
T1203Exploitation for Client Execution

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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