CVE-2026-21428VulnerabilityPublished 1 Jan 2026Updated 2 Jan 2026

CVE-2026-21428: yhirose cpp-httplib · CRLF Injection

Affects yhirose cpp-httplibSource CVE Database V5

Not known to be exploited · CVSS 7.7 High : upgrade to 0.30.0 soon.

Data as of 25 Oct 2026, 19:02 UTC

CVSS 4.0 base score

7.7High

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

EPSS: 0.4% chance of exploitation in 30 days – higher than 34% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 0.30.0

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:P

Base metrics

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: None
Vulnerable system integrity: High
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

Threat metrics

Exploit Maturity: POC

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

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, 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: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: None: There is no loss of confidentiality within the Vulnerable System.
Vulnerable system integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the vulnerable system. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the vulnerable system.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.
Exploit Maturity: POC: Based on threat intelligence sources each of the following must apply: · Proof-of-concept is publicly available · No knowledge of reported attempts to exploit this vulnerability · No knowledge of publicly available solutions used to simplify attempts to exploit the vulnerability

Definitions: FIRST CVSS v4.0 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 1 Jan 2026
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

Upgrade yhirose/cpp-httplib to 0.30.0.

Affected software

Product	Affected	Fixed in
GitHubyhirose/cpp-httplib`pkg:github/yhirose/cpp-httplib`Other threats	Affected <0.30.0	Fixed in 0.30.0

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

AI-suggested

1. Immediate upgrade of cpp-httplib to version 0.30.0 or later to ensure proper sanitization of HTTP headers. 2. Implement strict input validation and sanitization on all HTTP headers at the application level to prevent CRLF injection, even if the underlying library is patched. 3. Restrict and monitor outbound HTTP requests from internal servers to limit SSRF impact, using network segmentation and egress filtering. 4. Employ Web Application Firewalls (WAFs) or Intrusion Detection Systems (IDS) configured to detect anomalous HTTP header injection patterns and SSRF attempts. 5. Review and harden server configurations, especially for HTTP/1.1 pipelining support, to minimize SSRF exploitation vectors. 6. Conduct security audits and code reviews focusing on HTTP header handling in custom code interfacing with cpp-httplib. 7. Maintain up-to-date threat intelligence and monitor for emerging exploit attempts targeting this vulnerability. 8. Educate development teams about secure HTTP header handling and the risks of CRLF injection.

Description

cpp-httplib is a C++11 single-file header-only cross platform HTTP/HTTPS library. Prior to version 0.30.0, the ``write_headers`` function does not check for CR & LF characters in user supplied headers, allowing untrusted header value to escape header lines. This vulnerability allows attackers to add extra headers, modify request body unexpectedly & trigger an SSRF attack. When combined with a server that supports http1.1 pipelining (springboot, python twisted etc), this can be used for server side request forgery (SSRF). Version 0.30.0 fixes this issue.

Impact

For European organizations, this vulnerability poses a substantial risk, especially those developing or deploying applications using cpp-httplib versions prior to 0.30.0. Exploitation can lead to SSRF attacks, which may allow attackers to pivot into internal networks, access sensitive data, or interact with internal services not exposed externally. This is particularly critical for sectors such as finance, healthcare, and government, where internal network confidentiality and integrity are paramount. Additionally, organizations using HTTP/1.1 pipelining servers like Spring Boot or Python Twisted in their infrastructure are at increased risk due to the amplification of the SSRF attack vector. The vulnerability could be exploited to bypass perimeter defenses, leading to data breaches, service disruptions, or further exploitation chains. Given the remote and unauthenticated nature of the exploit, the threat surface is broad, affecting any exposed services or applications using the vulnerable library. The absence of known exploits in the wild currently provides a window for proactive mitigation.

Summary

CVE-2026-21428 is a vulnerability classified under CWE-93 (Improper Neutralization of CRLF Sequences) affecting the cpp-httplib library, a widely used C++11 single-header HTTP/HTTPS client and server library. The vulnerability exists in the write_headers function prior to version 0.30.0, where it fails to sanitize CR (carriage return) and LF (line feed) characters in user-supplied HTTP header values. This flaw allows an attacker to inject CRLF sequences into headers, effectively breaking out of the intended header line and injecting additional headers or manipulating the HTTP request body. Such injection can be leveraged to perform SSRF attacks, particularly when the cpp-httplib client communicates with servers supporting HTTP/1.1 pipelining, such as Spring Boot or Python Twisted frameworks. SSRF enables attackers to make arbitrary requests from the vulnerable server to internal or external systems, potentially bypassing firewall restrictions and accessing sensitive internal resources. The vulnerability is remotely exploitable without authentication or user interaction, increasing its risk profile. Although no known exploits are reported in the wild yet, the CVSS 4.0 base score of 7.7 (high severity) reflects the significant impact and ease of exploitation. The issue is resolved in cpp-httplib version 0.30.0 by properly sanitizing header inputs to prevent CRLF injection.

CWE-93Improper Neutralization of CRLF Sequences ('CRLF Injection') (opens in a new tab)
Base weaknessOWASP Top 10 A05:2025: Injection (opens in a new tab)
The product uses CRLF (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs.
Implementation
Avoid using CRLF as a special sequence.
Implementation
Appropriately filter or quote CRLF sequences in user-controlled input.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
Deserialization (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
V1.5.1 (opens in a new tab)
Attack patterns
Command Delimiters (opens in a new tab)
Web Server Logs Tampering (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 8 Jan 2026, 19:10 UTC

WeaknessesCWE-93

Technical details

References

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