CVE-2023-38545: curl curl vulnerability

Should you act?

High exploit likelihood

EPSS ranks it in the top 5% of all CVEs for likely exploitation in the next 30 days. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 78.7%, percentile 99.6: FIRST’s model estimates a 78.7% chance of exploitation activity in the next 30 days, higher than 99.6% of all scored CVEs (very high). EPSS is a probability, not a verdict.
• Needs a user to click or open something. Phishing makes that realistic, but it is not wormable.
• Editorial verdict: Most can relax. curl SOCKS5 Heap Overflow: Hyped, but Narrow in Practice — Pre-announced as the worst curl flaw in a long time, the bug only bites when libcurl uses a SOCKS5 proxy with remote name resolution and an attacker can supply a very long hostname; take 8.4.0 or later in normal patching, and hurry only if that proxy setup applies to you.

What it is

This flaw makes curl overflow a heap based buffer in the SOCKS5 proxy handshake. When curl is asked to pass along the host name to the SOCKS5 proxy to allow that to resolve the address instead of it getting done by curl itself, the maximum length that host name can be is 255 bytes. If the host name is detected to be longer, curl switches to local name resolving and instead passes on the resolved address only. Due to this bug, the local variable that means "let the host resolve the name" could get the wrong value during a slow SOCKS5 handshake, and contrary to the intention, copy the too long host name to the target buffer instead of copying just the resolved address there. The target buffer being a heap based buffer, and the host name coming from the URL that curl has been told to operate with.

Weakness type: CWE-787.

Product: curl curl · Assigned by: hackerone · Published: 2023-10-18

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: required (a user must click/open something)
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 78.69% probability of exploitation activity in the next 30 days; percentile 99.6.

Timeline

• 2023-10-11 — erroneo.us brief: curl SOCKS5 Heap Overflow: Hyped, but Narrow in Practice
• 2023-10-18 — CVE record published by hackerone
• 2026-10-14 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. CVE.org record CVE-2023-38545 cve.org
2. NVD entry CVE-2023-38545 nvd.nist.gov

Other references from the CVE record and advisory

1. curl.se curl.se
2. security.netapp.com security.netapp.com
3. lists.fedoraproject.org lists.fedoraproject.org
4. secpod.com secpod.com
5. support.apple.com support.apple.com
6. support.apple.com support.apple.com
7. support.apple.com support.apple.com
8. support.apple.com support.apple.com
9. seclists.org seclists.org
10. seclists.org seclists.org
11. seclists.org seclists.org
12. security.netapp.com security.netapp.com
13. forum.vmssoftware.com forum.vmssoftware.com
14. github.com github.com
15. github.com github.com
16. github.com github.com
17. cert-portal.siemens.com cert-portal.siemens.com
18. cert-portal.siemens.com cert-portal.siemens.com
19. cert-portal.siemens.com cert-portal.siemens.com
20. cert-portal.siemens.com cert-portal.siemens.com
21. cert-portal.siemens.com cert-portal.siemens.com

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