Medium severity · published Apr 13, 2026
CVE-2026-33555
An issue was discovered in HAProxy before 3.3.6
5.8
CVSS
0.3%
EPSS
No
On KEV
Description
An issue was discovered in HAProxy before 3.3.6. The HTTP/3 parser does not check that the received body length matches a previously announced content-length when the stream is closed via a frame with an empty payload. This can cause desynchronization issues with the backend server and could be used for request smuggling. The earliest affected version is 2.6.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Confidentiality
None
Integrity
Low
Availability
None
Scope
Changed
Exploitation
Is it being used?
Exploit prediction (EPSS)0.3% · higher than 23% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
HA
HAProxy · HAProxy
2.6 before 3.3.6 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-33555
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-33555
Patch
https://github.com/haproxy/haproxy/commit/05a295441c621089ffa4318daf0dbca2dd756a84
Exploit
https://r3verii.github.io/cve/2026/04/14/haproxy-h3-standalone-fin-smuggling.html
Release Notes
https://www.haproxy.com/documentation/haproxy-aloha/changelog/
Product
https://www.haproxy.org
Release Notes
https://www.mail-archive.com/haproxy@formilux.org/msg46752.html
lists.debian.org
https://lists.debian.org/debian-lts-announce/2026/10/msg00036.html
More from HAProxy
9.8
CVE-2019-19330
The HTTP/2 implementation in HAProxy before 2.0.10 mishandles headers, as demonstrated by carriage return (CR, ASCII 0xd), line feed (LF, ASCII 0xa), and the zero character (NUL,…
9.1
CVE-2026-55203
HAProxy - Integer Overflow in FCGI Demux Record Length Field
9.1
CVE-2023-25725
HAProxy before 2.7.3 may allow a bypass of access control because HTTP/1 headers are inadvertently lost in some situations, aka "request smuggling." The HTTP header parsers in…
8.8
CVE-2020-11100
In hpack_dht_insert in hpack-tbl.c in the HPACK decoder in HAProxy 1.8 through 2.x before 2.1.4, a remote attacker can write arbitrary bytes around a certain location on the heap…
8.2
CVE-2023-45539
HAProxy before 2.8.2 accepts # as part of the URI component, which might allow remote attackers to obtain sensitive information or have unspecified other impact upon…