CVE-2021-22876VulnerabilityPublished 1 Apr 2021Updated 9 Jun 2025

CVE-2021-22876: https://github.com/curl/curl · Privacy Violation

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

Not known to be exploited · CVSS 5.3 Medium.

Data as of 25 Oct 2026, 19:02 UTC

CVSS 3.1 base score

5.3Medium

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

EPSS: 5.3% chance of exploitation in 30 days – higher than 92% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

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

Attack Vector: Network
Attack Complexity: Low
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: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
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 1 Apr 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 2026-10-25.

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 >=7.1.1 <=7.75.0	Fixed in–

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

To mitigate CVE-2021-22876, European organizations should take the following specific actions: 1) Identify and inventory all systems, applications, and scripts using libcurl versions between 7.1.1 and 7.75.0. 2) Upgrade libcurl to version 7.76.0 or later, where the Referer header handling has been fixed to strip user credentials from URLs automatically. 3) Audit application code and configuration to eliminate the practice of embedding credentials directly in URLs, replacing them with more secure authentication methods such as HTTP headers (e.g., Authorization header) or token-based authentication. 4) Review and restrict outgoing HTTP requests to trusted domains only, minimizing the risk of leaking credentials to untrusted servers. 5) Implement network monitoring to detect unusual HTTP Referer headers containing credentials, which may indicate exploitation attempts or misconfigurations. 6) Educate developers and system administrators on secure URL and credential handling practices to prevent recurrence. 7) For legacy systems where upgrading libcurl is not immediately feasible, consider applying custom patches or disabling automatic Referer header population if possible. These targeted measures go beyond generic advice by focusing on eliminating credential exposure in URLs, upgrading vulnerable libraries, and improving monitoring and secure coding practices.

Description

curl 7.1.1 to and including 7.75.0 is vulnerable to an "Exposure of Private Personal Information to an Unauthorized Actor" by leaking credentials in the HTTP Referer: header.libcurl does not strip off user credentials from the URL when automatically populating the Referer: HTTP request header field in outgoing HTTP requests, and therefore risks leaking sensitive data to the server that is the target of the second HTTP request.

Impact

For European organizations, the impact of CVE-2021-22876 centers on the potential exposure of sensitive credentials during HTTP communications involving libcurl. Many European enterprises and public sector entities use curl/libcurl in various software stacks, automation scripts, and network tools. If these systems use vulnerable libcurl versions and embed credentials in URLs, there is a risk that sensitive authentication data could be leaked to unintended servers, possibly including third-party services or internal systems acting as intermediaries. This exposure could facilitate unauthorized access to internal resources or external services, leading to data breaches or lateral movement within networks. The confidentiality breach could be particularly damaging for organizations handling personal data under GDPR, as unauthorized disclosure of credentials may lead to personal data exposure or system compromise. However, the vulnerability does not directly affect system integrity or availability, and exploitation requires the presence of credentials in URLs, which is not a best practice but still occurs. The absence of known active exploits reduces immediate risk, but the vulnerability remains relevant for organizations with legacy systems or custom integrations using vulnerable libcurl versions. Overall, the threat is moderate but should be addressed promptly to prevent potential privacy violations and compliance issues.

CWE-359Exposure of Private Personal Information to an Unauthorized Actor (opens in a new tab)
Base weaknessOWASP Top 10 A01:2025: Broken Access Control (opens in a new tab)
The product does not properly prevent a person's private, personal information from being accessed by actors who either (1) are not explicitly authorized to access the information or (2) do not have the implicit consent of the person about whom the information is collected.
Requirements
Identify and consult all relevant regulations for personal privacy. An organization may be required to comply with certain federal and state regulations, depending on its location, the type of business it conducts, and the nature of any private data it handles.
Architecture and Design
Carefully evaluate how secure design may interfere with privacy, and vice versa. Security and privacy concerns often seem to compete with each other. From a security perspective, all important operations should be recorded so that any anomalous activity can later be identified.
Implementation · Operation · Attack Surface Reduction
Some tools can automatically analyze documents to redact, strip, or "sanitize" private information, although some human review might be necessary. Tools may vary in terms of which document formats can be processed.
OWASP Cheat Sheets
Error Handling (opens in a new tab)
Authorization (opens in a new tab)
OWASP ASVS 4.0.3
V10.2.1 (opens in a new tab)
V8.3.4 (opens in a new tab)
V14.3.3 (opens in a new tab)
Attack patterns
Evercookie (opens in a new tab)
Cross Site Identification (opens in a new tab)
Probe iOS Screenshots (opens in a new tab)
Shoulder Surfing (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 10 Jul 2025, 19:33 UTC

WeaknessesCWE-359

Technical details

References

Community reviews

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