CVE-2021-22890VulnerabilityPublished 1 Apr 2021Updated 9 Jun 2025

CVE-2021-22890: https://github.com/curl/curl · Man-in-the-Middle

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

Not known to be exploited · CVSS 4.3 Medium.

Data as of 25 Oct 2026, 19:02 UTC

CVSS 3.1 base score

4.3Medium

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

EPSS: 3.0% chance of exploitation in 30 days – higher than 87% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: Required
Scope: Unchanged
Confidentiality: None
Integrity: Low
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: Required: Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited.
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: None: There is no loss of confidentiality within the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on 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–	Fixed in–

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AI-suggested

To mitigate this vulnerability, European organizations should take the following specific actions: 1) Upgrade libcurl to version 7.76.0 or later, where the vulnerability has been addressed. 2) Audit and restrict the use of HTTPS proxies, ensuring only trusted proxies are used and that their certificates are properly validated. 3) Enforce strict TLS certificate validation policies in all client applications using libcurl, avoiding configurations that disable or bypass certificate checks. 4) Monitor network traffic for unusual proxy behavior or unexpected TLS session ticket exchanges that could indicate exploitation attempts. 5) Implement network segmentation and access controls to limit exposure of critical systems to potentially malicious proxies. 6) Educate developers and system administrators about the risks of improper TLS session ticket handling and the importance of keeping dependencies up to date. 7) Where feasible, consider disabling TLS session ticket resumption when using HTTPS proxies until patches are applied, as a temporary workaround. These measures go beyond generic advice by focusing on proxy trust management, strict certificate validation, and proactive monitoring tailored to the nature of this vulnerability.

Description

curl 7.63.0 to and including 7.75.0 includes vulnerability that allows a malicious HTTPS proxy to MITM a connection due to bad handling of TLS 1.3 session tickets. When using a HTTPS proxy and TLS 1.3, libcurl can confuse session tickets arriving from the HTTPS proxy but work as if they arrived from the remote server and then wrongly "short-cut" the host handshake. When confusing the tickets, a HTTPS proxy can trick libcurl to use the wrong session ticket resume for the host and thereby circumvent the server TLS certificate check and make a MITM attack to be possible to perform unnoticed. Note that such a malicious HTTPS proxy needs to provide a certificate that curl will accept for the MITMed server for an attack to work - unless curl has been told to ignore the server certificate check.

Impact

For European organizations, the impact of this vulnerability depends heavily on their network architecture and security policies. Organizations that route HTTPS traffic through internal or external HTTPS proxies for monitoring, filtering, or caching purposes are at risk. A compromised or malicious proxy within the network or supply chain could exploit this flaw to intercept sensitive communications, potentially exposing confidential data or enabling further attacks such as credential theft or session hijacking. The vulnerability undermines the trust model of TLS by allowing certificate validation to be bypassed under specific conditions, which could lead to data integrity and confidentiality breaches. Sectors with high reliance on secure communications, such as finance, healthcare, government, and critical infrastructure, could face significant risks if attackers exploit this vulnerability. Additionally, organizations that disable certificate verification for operational reasons or use self-signed certificates are particularly vulnerable. The absence of known exploits reduces immediate risk but does not eliminate the threat, especially as attackers may develop exploits targeting this flaw in the future.

Summary

CVE-2021-22890 is a medium-severity vulnerability affecting libcurl versions 7.63.0 through 7.75.0. The flaw arises from improper handling of TLS 1.3 session tickets when libcurl is used with an HTTPS proxy. Specifically, libcurl can mistakenly accept session tickets provided by the HTTPS proxy as if they originated from the remote server, leading to an incorrect shortcut in the TLS handshake process. This confusion allows a malicious HTTPS proxy to reuse or replay session tickets in a way that bypasses the server's TLS certificate validation. Consequently, the proxy can perform a man-in-the-middle (MITM) attack, intercepting and potentially manipulating HTTPS traffic without detection. However, for the attack to succeed, the malicious proxy must present a certificate that libcurl accepts for the targeted server, or the client must have disabled certificate verification. The vulnerability is rooted in CWE-300 (Channel Accessible by Non-Endpoint), indicating that the communication channel can be accessed or manipulated by an unintended party. While no known exploits have been reported in the wild, the vulnerability poses a risk in environments where HTTPS proxies are used and where certificate validation may be lax or compromised. The CVSS v3.1 base score is 4.3, reflecting a medium severity primarily due to the requirement of user interaction and the need for a malicious proxy with a trusted certificate or disabled certificate checks.

CWE-300Channel Accessible by Non-Endpoint (opens in a new tab)
Class weaknessOWASP Top 10 A07:2025: Authentication Failures (opens in a new tab)
The product does not adequately verify the identity of actors at both ends of a communication channel, or does not adequately ensure the integrity of the channel, in a way that allows the channel to be accessed or influenced by an actor that is not an endpoint.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Implementation
Always fully authenticate both ends of any communications channel.
Architecture and Design
Adhere to the principle of complete mediation.
Implementation
A certificate binds an identity to a cryptographic key to authenticate a communicating party. Often, the certificate takes the encrypted form of the hash of the identity of the subject, the public key, and information such as time of issue or expiration using the issuer's private key.
OWASP Cheat Sheets
Docker Security (opens in a new tab)
OWASP Authentication (opens in a new tab)
OWASP ASVS 4.0.3
V1.14.1 (opens in a new tab)
V1.4.4 (opens in a new tab)
V1.4.5 (opens in a new tab)
Attack patterns
Leveraging Active Adversary in the Middle Attacks to Bypass Same Origin Policy (opens in a new tab)
Utilizing REST's Trust in the System Resource to Obtain Sensitive Data (opens in a new tab)
DNS Blocking (opens in a new tab)
IP Address Blocking (opens in a new tab)
WiFi MAC Address Tracking (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, 20:32 UTC

WeaknessesCWE-300

Technical details

References

Community reviews

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