h2o: buffer overflow when using the %{error}x log directive
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- No fix listed
- Who is exposed
- Specific setups only
Do first: Check whether your setup matches what the bug needs. An affected custom access-log format is configured (%{error}x, %q with empty query string, or numeric elements). Affected versions are commits up to 4212ce1; fixed in commits 48b7ae1 and above.
- Exploitation?/40Exploitation: unknown (data gap). No CISA SSVC exploitation rating yet
- Exposure12/25Specific setups only, reachable over the network without a login
- Reach?/20Reach: unknown (data gap). No tracked technology or package matched
- Severity8.1/10CVSS 8.1
- No patch5/5No patch listed yet
What would change the score
- Would score about 75 if added to CISA KEV
- Would score about 70 if CISA's SSVC rates exploitation active
CVE-2026-55368 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.1 (high) and affects h2o; no fixed version is listed yet.
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- Published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 25/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Specific setups only
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
The h2o HTTP server has a buffer overflow in its access-log formatter that a remote attacker can trigger without logging in by sending a matching request, which could crash the server. The effect of the one-byte overwrite beyond a crash has not been established.
Read the original description
h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 48b7ae12107fd2b118e4641da49599e9aac8edd8, H2O's access-log formatter could write past its log-line buffer when an affected custom access-log format was configured. The allocation errors affected %{error}x, %q when the query string was empty, and numeric elements whose temporary terminating NUL reached the end of the buffer. A matching request could crash the server, while the effect of the one-byte NUL overwrite beyond a crash was not established. This vulnerability is fixed with commit 48b7ae12107fd2b118e4641da49599e9aac8edd8.
What to do
Check whether your setup matches what the bug needs.
An affected custom access-log format is configured (%{error}x, %q with empty query string, or numeric elements). Affected versions are commits up to 4212ce1; fixed in commits 48b7ae1 and above.Workaround: Disable affected custom access-log formats until a fixed version is available.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- h2o h2o
Version range not reported
Evidence 3
- Advisory
- Repository
- Repository
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-55368
Is CVE-2026-55368 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
What does this mean?
A custom access-log format that uses the %{error}x directive, a %q with an empty query string, or numeric elements can write past the end of its buffer when a matching request is logged. Depending on the format and request, the out-of-bounds writes can crash the server.Source: github.com
Am I affected, and how do I check?
Affected versions are h2o commits up to 4212ce1. To check, look at whether your access-log configuration uses a custom format containing %{error}x, %q with an empty query string, or numeric elements. The fixed commits are 48b7ae1 and above.Source: github.com
How do I fix CVE-2026-55368?
Update h2o to a build that includes commit 48b7ae1 or later, which reserves the required buffer space for all three cases. No fixed version number is given in the facts.Source: github.com
What if there's no patch?
Disable the affected custom access-log formats until a fixed version is available. This is the workaround given in the advisory.Source: github.com