CVE-2021-33056Network attack vector

Unauthenticated crash via null URI scheme dereference

Published Aug 12, 2021 · Updated Aug 3, 2024

Denial of service in Belledonne Belle-sip before 4.5.20 allows remote attackers to crash consuming applications via a crafted SIP From header. The addr_spec_with_generic_uri and fast_addr_spec_with_generic_uri parser rules pass a null URI scheme to strcasecmp while parsing the invalid header. Exposure requires an application to accept untrusted SIP messages through the affected parser; the reported effect is an application crash.

CVSS severity7.5
High
EPSS probability1.29%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Belledonne Belle-sip before 4.5.20 allows remote attackers to crash consuming applications via a crafted SIP From header. The addr_spec_with_generic_uri and fast_addr_spec_with_generic_uri parser rules pass a null URI scheme to strcasecmp while parsing the invalid header. Exposure requires an application to accept untrusted SIP messages through the affected parser; the reported effect is an application crash.

The record

CVE
CVE-2021-33056
Published
Aug 12, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-444, CAPEC-33, CAPEC-273, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 12, 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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.

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-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
CAPEC-33HTTP Request Smuggling
CAPEC-273HTTP Response Smuggling
T1499.004Application or System Exploitation

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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