CVE-2023-27535VulnerabilityPublished 30 Mar 2023Updated 9 Jun 2025

CVE-2023-27535: https://github.com/curl/curl · Authentication Bypass by Primary Weakness

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

Not known to be exploited · CVSS 5.9 Medium : upgrade to 8.0.0 when convenient.

Data as of 26 Oct 2026, 07:57 UTC

CVSS 3.1 base score

5.9Medium

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

EPSS: 1.8% chance of exploitation in 30 days – higher than 78% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 8.0.0

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

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: High
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: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
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 30 Mar 2023
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

Upgrade curl/curl to 8.0.0.

Affected software

Product	Affected	Fixed in
GitHubcurl/curl`pkg:github/curl/curl`Other threats	Affected <8.0.0	Fixed in 8.0.0

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

AI-suggested

European organizations should prioritize upgrading libcurl to version 8.0.0 or later, where this vulnerability is fixed. For environments where immediate upgrade is not feasible, organizations should audit and restrict the use of FTP connection reuse features, especially in contexts where multiple user credentials are involved. Implement strict validation and isolation of FTP sessions to ensure credentials are not inadvertently shared across connections. Network segmentation and monitoring of FTP traffic can help detect anomalous access patterns. Additionally, organizations should review and harden FTP configurations, disabling unnecessary FTP options such as CURLOPT_FTP_ACCOUNT and CURLOPT_FTP_ALTERNATIVE_TO_USER if not required. Employing secure alternatives to FTP, such as SFTP or FTPS with proper authentication controls, can reduce exposure. Finally, integrate vulnerability scanning and software composition analysis into the development lifecycle to detect outdated libcurl versions and enforce timely patching.

Description

An authentication bypass vulnerability exists in libcurl <8.0.0 in the FTP connection reuse feature that can result in wrong credentials being used during subsequent transfers. Previously created connections are kept in a connection pool for reuse if they match the current setup. However, certain FTP settings such as CURLOPT_FTP_ACCOUNT, CURLOPT_FTP_ALTERNATIVE_TO_USER, CURLOPT_FTP_SSL_CCC, and CURLOPT_USE_SSL were not included in the configuration match checks, causing them to match too easily. This could lead to libcurl using the wrong credentials when performing a transfer, potentially allowing unauthorized access to sensitive information.

Impact

For European organizations, this vulnerability poses a risk primarily to systems and applications that utilize libcurl for FTP operations, especially those that rely on connection reuse for performance optimization. Unauthorized use of incorrect credentials during FTP transfers could lead to unintended data disclosure, exposing sensitive information such as proprietary files, personal data, or configuration details. This is particularly concerning for sectors handling regulated or sensitive data, including finance, healthcare, and government agencies. The vulnerability does not affect integrity or availability directly but compromises confidentiality, which can lead to compliance violations under GDPR and other data protection regulations. Given libcurl's widespread use in various software and embedded systems, the impact could be broad, affecting internal tools, automated scripts, and third-party applications. The medium severity score reflects the requirement for high attack complexity, which somewhat limits exploitation likelihood, but the absence of required privileges or user interaction means remote attackers could potentially exploit this if they can initiate FTP transfers through vulnerable clients.

CWE-305Authentication Bypass by Primary Weakness (opens in a new tab)
Base weaknessOWASP Top 10 A07:2025: Authentication Failures (opens in a new tab)
The authentication algorithm is sound, but the implemented mechanism can be bypassed as the result of a separate weakness that is primary to the authentication error.
OWASP Cheat Sheets
OWASP Authentication (opens in a new tab)
OWASP ASVS 4.0.3
V2.8.5 (opens in a new tab)
V2.8.4 (opens in a new tab)
V9.2.3 (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, 19:20 UTC

WeaknessesCWE-305

Technical details

References

Community reviews

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