CVE-2021-22924VulnerabilityPublished 5 Aug 2021Updated 9 Jun 2025

CVE-2021-22924: https://github.com/curl/curl · Improper Input Validation

Affects https://github.com/curl/curlSource CVE Database V5

Not known to be exploited · CVSS 3.7 Low.

Data as of 26 Oct 2026, 06:26 UTC

CVSS 3.1 base score

3.7Low

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 6.2% chance of exploitation in 30 days – higher than 93% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: Low
Integrity: None
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: Low: There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 5 Aug 2021
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 2026-10-26.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Affected software

Product	Affected	Fixed in
GitHubcurl/curl`pkg:github/curl/curl`Other threats	Affected >=7.10.4 <=7.77.0	Fixed in–

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AI-suggested

To mitigate CVE-2021-22924, European organizations should: 1) Upgrade libcurl to versions later than 7.77.0 where this vulnerability has been fixed, ensuring the connection matching logic properly considers 'issuercert' and performs case-sensitive path comparisons where appropriate. 2) Audit and inventory all software and services using libcurl to identify vulnerable versions, including embedded devices and third-party applications. 3) Where immediate upgrades are not feasible, implement network-level controls such as strict TLS inspection and monitoring to detect anomalous connection reuse or certificate mismatches. 4) Enforce strict certificate pinning or validation policies in applications using libcurl to reduce reliance on connection reuse logic. 5) Conduct security testing and code reviews focusing on connection reuse and certificate validation mechanisms to identify similar logic flaws. 6) Educate developers and system administrators about the risks of improper input validation and the importance of keeping third-party libraries up to date.

Description

libcurl keeps previously used connections in a connection pool for subsequenttransfers to reuse, if one of them matches the setup.Due to errors in the logic, the config matching function did not take 'issuercert' into account and it compared the involved paths *case insensitively*,which could lead to libcurl reusing wrong connections.File paths are, or can be, case sensitive on many systems but not all, and caneven vary depending on used file systems.The comparison also didn't include the 'issuer cert' which a transfer can setto qualify how to verify the server certificate.

Impact

For European organizations, the impact of CVE-2021-22924 is generally low but context-dependent. Organizations using libcurl versions up to 7.77.0 in environments where secure HTTPS connections and certificate validation are critical could face risks of misrouted or improperly validated connections. This could potentially lead to minor confidentiality issues, such as data leakage or exposure to man-in-the-middle scenarios if connections are reused incorrectly. However, the vulnerability does not affect integrity or availability directly and requires a high attack complexity, reducing the likelihood of exploitation. European sectors with stringent data protection requirements, such as finance, healthcare, and government, might be more sensitive to any confidentiality lapses. Additionally, organizations deploying libcurl in large-scale or automated environments where connection reuse is frequent might experience subtle security risks. Overall, while the direct impact is limited, the vulnerability underscores the importance of correct certificate validation and connection management in secure communications.

Summary

CVE-2021-22924 is a vulnerability in libcurl, a widely used client-side URL transfer library, affecting versions from 7.10.4 up to and including 7.77.0. The issue arises from improper input validation related to connection reuse in libcurl's connection pooling mechanism. libcurl maintains a pool of previously used connections to optimize subsequent transfers by reusing connections that match the current setup. However, the logic responsible for matching configurations failed to consider the 'issuercert' parameter and performed file path comparisons in a case-insensitive manner. This is problematic because file paths can be case-sensitive depending on the operating system and file system used. Additionally, the omission of the 'issuer cert' in the comparison means that libcurl might incorrectly reuse a connection intended for a different server certificate verification context. This flaw could lead to libcurl reusing an incorrect connection, potentially causing data to be sent over an unintended connection or to an unintended server. The vulnerability is classified under CWE-20 (Improper Input Validation). The CVSS v3.1 base score is 3.7 (low severity), reflecting a network attack vector but requiring high attack complexity, no privileges, and no user interaction, with limited confidentiality impact and no integrity or availability impact. There are no known exploits in the wild, and no official patches were linked in the provided data, though later libcurl versions have addressed this issue. The vulnerability primarily affects applications and systems that rely on vulnerable libcurl versions for secure communications, especially where issuer certificate validation is critical.

CWE-20Improper Input Validation (opens in a new tab)
Class weaknessLikelihood of exploit: HighCWE Top 25 (2025): #18OWASP Top 10 A05:2025: Injection (opens in a new tab)
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Architecture and Design · Attack Surface Reduction
Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build "recognizers" for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of…
Architecture and Design · Libraries or Frameworks
Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).
Architecture and Design · Implementation · Attack Surface Reduction
Understand all the potential areas where untrusted inputs can enter the product, including but not limited to: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any…
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
Injection Prevention (opens in a new tab)
SQL Injection Prevention (opens in a new tab)
Injection Prevention in Java (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
Attack patterns
Buffer Overflow via Environment Variables (opens in a new tab)
Server Side Include (SSI) Injection (opens in a new tab)
Cross Zone Scripting (opens in a new tab)
Command Line Execution through SQL Injection (opens in a new tab)
Object Relational Mapping Injection (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 10 Jul 2025, 20:31 UTC

WeaknessesCWE-20

Technical details

References

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