Brute-force ban bypass via spoofed forwarded address
Published Feb 27, 2026 · Updated Feb 27, 2026
Origin validation errors in the Content Server before 9.4.0 allow remote attackers to bypass login-attempt bans using a spoofed header. The HTTP parser accepts X-Forwarded-For from any client, and do_http_auth combines it with remote_addr to key BanList failures. Authentication and IP banning must be enabled; rotating the header resets the ban key, enabling unlimited password guesses and account access only when a password is found.
Summary
What happened
Origin validation errors in the Content Server before 9.4.0 allow remote attackers to bypass login-attempt bans using a spoofed header. The HTTP parser accepts X-Forwarded-For from any client, and do_http_auth combines it with remote_addr to key BanList failures. Authentication and IP banning must be enabled; rotating the header resets the ban key, enabling unlimited password guesses and account access only when a password is found.
The record
- CVE
- CVE-2026-27824
- Published
- Feb 27, 2026
- Updated
- Feb 27, 2026
- Vendor
- Kovid Goyal
- Product
- calibre
- Classifications
- CWE-307, CWE-346, T1110.001
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Feb 27, 2026CVE publishedPublication date reported by the CVE source.
- Feb 27, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 9.4.0
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- IP Ban Bypass via X-Forwarded-For Header Spoofing PoCproof of concept · demonstrated
Labels summarize the accepted research assessment. They do not indicate a test against your environment.
Technologies
Your stack
See the directory against your own environment.
Your stack
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