Medium Authentication exploit reference

CVE-2026-6253 — proxy credentials leak over redirect-to proxy

curl might erroneously pass on credentials for a first proxy to a second proxy. This can happen when the following conditions are true: 1. curl is setup to use specific different proxies for different URL schemes 2. the first proxy needs credentials 3. the second proxy uses no credentials 4. while using the first proxy (using say `http://`), curl is asked to follow a redirect to a URL using another scheme (say `https://`), accessed using a second, different, proxy

Published May 13, 2026 · Updated Oct 13, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: curl

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 0.8% (30-day exploitation probability)+8
Technical severity (medium)+3
CISA SSVC: exploitation "poc"+5

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

0.75%

ahead of 54% of scored CVEs · Oct 14, 2026

Affects

Curl

CVSS base

5.9

Medium · 2 source(s)

CISA SSVC assessment CISA

Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.

Exploitation

Proof of concept

none · proof-of-concept · active

Automatable

can an attacker script the kill-chain steps

Technical impact

Partial

partial · total control of the vulnerable component

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

Documented patch — curl.se (NIST National Vulnerability Database)
Documented patch — www.openwall.com (NIST National Vulnerability Database)

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Curl Curl, Haxx Curl).
The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
Apply the fixed version or the patch documented under Remediation.
Search available logs for exploit probes matching the vulnerability class (authentication), errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Curl	Curl	version 7.16.2version 7.19.2version 7.19.1version 7.19.0version 7.18.2version 7.18.1version 7.18.0version 7.17.1 and 153 more version 7.17.0version 7.16.4version 7.16.3version 7.19.3version 7.16.1version 7.16.0version 7.15.5version 7.15.4version 7.15.3version 7.15.2version 7.15.1version 7.15.0version 7.14.1version 7.21.3version 7.26.0version 7.25.0version 7.24.0version 7.23.1version 7.23.0version 7.22.0version 7.21.7version 7.21.6version 7.21.5version 7.21.4version 7.27.0version 7.21.2version 7.21.1version 7.21.0version 7.20.1version 7.20.0version 7.19.7version 7.19.6version 7.19.5version 7.19.4≥ 7.14.1 and < 8.14.2≥ 8.15.0 and < 8.16.1≥ 8.17.0 and < 8.20.0≥ 3b60bb725913ce7339aefef0a14b12df4c24db60 and < 188c2f166a20fa97c2325b2da7d0e5cecc13725fversion 8.19.0version 8.18.0version 8.17.0version 8.16.0version 8.15.0version 8.14.1version 8.14.0version 8.13.0version 8.12.1version 8.12.0version 8.11.1version 8.11.0version 8.10.1version 8.10.0version 8.9.1version 8.9.0version 8.8.0version 8.7.1version 8.7.0version 8.6.0version 8.5.0version 8.4.0version 8.3.0version 8.2.1version 8.2.0version 8.1.2version 8.1.1version 8.1.0version 8.0.1version 8.0.0version 7.88.1version 7.88.0version 7.87.0version 7.86.0version 7.85.0version 7.84.0version 7.83.1version 7.83.0version 7.82.0version 7.81.0version 7.80.0version 7.79.1version 7.79.0version 7.78.0version 7.77.0version 7.76.1version 7.76.0version 7.75.0version 7.74.0version 7.73.0version 7.72.0version 7.71.1version 7.71.0version 7.70.0version 7.69.1version 7.69.0version 7.68.0version 7.67.0version 7.66.0version 7.65.3version 7.65.2version 7.65.1version 7.65.0version 7.64.1version 7.64.0version 7.63.0version 7.62.0version 7.61.1version 7.61.0version 7.60.0version 7.59.0version 7.58.0version 7.57.0version 7.56.1version 7.56.0version 7.55.1version 7.55.0version 7.54.1version 7.54.0version 7.53.1version 7.53.0version 7.52.1version 7.52.0version 7.51.0version 7.50.3version 7.50.2version 7.50.1version 7.50.0version 7.49.1version 7.49.0version 7.48.0version 7.47.1version 7.47.0version 7.46.0version 7.45.0version 7.44.0version 7.43.0version 7.42.1version 7.42.0version 7.41.0version 7.40.0version 7.39.0version 7.38.0version 7.37.1version 7.37.0version 7.36.0version 7.35.0version 7.34.0version 7.33.0version 7.32.0version 7.31.0version 7.30.0version 7.29.0version 7.28.1version 7.28.0	Vulnerable	CVE Program (cve.org) e CISA ADP
Haxx	Curl	≥ 7.14.1 and < 8.20.0	Vulnerable	NIST National Vulnerability Database

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Network

Attack complexity

High

Privileges required

None

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
5.9	CVSS 3.1	Medium	2.2	3.6	134c704f-9b21-4f2e-91b3-4a467353bcc0 via NIST National Vulnerability Database
5.9	CVSS 3.1	Medium	—	—	CISA-ADP via CVE Program (cve.org) e CISA ADP

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H (score shown at the top: 134c704f-9b21-4f2e-91b3-4a467353bcc0)

Weakness MITRE

Weakness class according to MITRE’s CWE catalog.

CWE-522 · Insufficiently Protected Credentials

Other CVEs with this weakness →

EPSS over time FIRST

Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.75%

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
Review authentication logs: logins without MFA, anomalous sessions and recently created accounts.

web/proxy/WAF logsnetwork IDS/IPSauthentication logs

Timeline

What happened to this CVE, newest first.

2026
Affected products added · curl/curl
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
SSVC assessment updated
Oct 13, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 13, 2026
Fix documented
Oct 13, 2026 · NIST National Vulnerability Database
CVSS changed · 5.9
Oct 13, 2026 · NIST National Vulnerability Database
Source status changed · → Modified
Oct 13, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 13, 2026 · NIST National Vulnerability Database
CVE published
May 13, 2026 · curl

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 13, 2026 – Oct 13, 2026

References

4 on the record

curl.se/docs/CVE-2026-6253.json
Vendor Advisory, NIST National Vulnerability Database
www.openwall.com/lists/oss-security/2026/04/29/11
Mailing List, Patch, Third Party Advisory, NIST National Vulnerability Database
curl.se/docs/CVE-2026-6253.html
Patch, Vendor Advisory, NIST National Vulnerability Database
hackerone.com/reports/3669637
Exploit, Issue Tracking, Third Party Advisory, NIST National Vulnerability Database

Elsewhere on this site

Curl
every CVE for this vendor

Authentication
same class

CWE-522
other pages naming this weakness

Related CVEs (same products)

High CVE-2026-92392 HTTP/2 upload Use-After-Free 1d
High CVE-2026-9546 sending old referer Jul 3, 2026
Critical CVE-2026-9079 stale proxy password leak Jul 3, 2026
High CVE-2026-9080 UAF after pause in socket callback Jul 3, 2026
High CVE-2026-9545 exposing HTTP/3 early data Jul 3, 2026
High CVE-2026-8932 incomplete mTLS config matching in conn reuse Jul 3, 2026

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2026-6253 is "proxy credentials leak over redirect-to proxy", rated Medium. It affects Curl (Curl); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 5.9 describes technical severity. EPSS 0.75% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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