CVE-2021-21704Public exploitNetwork attack vector

Service interruption via malformed Firebird responses

Published Oct 4, 2021 · Updated Sep 17, 2024

Memory-safety flaws in PHP before 7.3.29, 7.4.21, and 8.0.8 allow remote attackers controlling a Firebird server to crash PHP. The PDO Firebird driver trusts response lengths and type codes: firebird_info_cb appends an unbounded server-version string to a fixed stack buffer, while statement and BLOB parsers advance or copy using unchecked server-supplied sizes. Exposure requires a PDO Firebird connection to an attacker-controlled or intercepted database server; crafted replies cause a hang or crash and can corrupt memory.

CVSS severity5.9
Medium
EPSS probability1.80%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Memory-safety flaws in PHP before 7.3.29, 7.4.21, and 8.0.8 allow remote attackers controlling a Firebird server to crash PHP. The PDO Firebird driver trusts response lengths and type codes: firebird_info_cb appends an unbounded server-version string to a fixed stack buffer, while statement and BLOB parsers advance or copy using unchecked server-supplied sizes. Exposure requires a PDO Firebird connection to an attacker-controlled or intercepted database server; crafted replies cause a hang or crash and can corrupt memory.

The record

CVE
CVE-2021-21704
Published
Oct 4, 2021
Updated
Sep 17, 2024
Vendor
The PHP Group
Product
PHP
Classifications
CWE-787, CWE-190, CWE-125, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Oct 4, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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=7.3.x <7.3.29
  2. Affected versionversion=7.4.x <7.4.21
  3. Affected versionversion=8.0.X <8.0.8

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

The PHP Group · PHPversion=7.3.x <7.3.29; version=7.4.x <7.4.21; version=8.0.X <8.0.8

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:N/A:P
5.0PHP GroupCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:L
5.9NIST NVDCVSS:3.1/AV:N/AC:H/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-787Out-of-bounds Write
CWE-190Integer Overflow or Wraparound
CWE-125Out-of-bounds Read
T1499.004Endpoint Denial of Service: Application or System Exploitation

Public exploit references

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

Technologies

Your stack

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Your stack

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