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Unbounded HPACK integer decoding (no value / octet-count limit)

High
whatyouhide published GHSA-jj2p-32j7-whj2 Jul 5, 2026

Package

erlang hpax (Erlang)

Affected versions

>= 0.1.1 and < 1.0.4

Patched versions

1.0.4

Description

Summary

hpax, the Elixir HPACK (HTTP/2 header compression) decoder used by the Mint/Bandit ecosystem, decodes HPACK variable-length integers with no cap on the decoded value or the number of continuation octets. An unauthenticated remote attacker who can send an HTTP/2 header block to any server using this decoder can supply a single integer built from a long run of continuation octets, forcing the decoder into superlinear (≈O(N²)) bignum arithmetic. Each small, cheaply-crafted request costs the server a large, attacker-controlled amount of CPU (a denial-of-service amplification).

Details

HPACK encodes an integer that does not fit in its prefix by continuing it across following octets: each octet with the high bit set contributes 7 more value bits and signals "more to come", and the first octet with the high bit clear terminates the integer.

The bug is in decode_remaining_integer/3 in lib/hpax/types.ex, reached from the integer entry point decode_integer/2 and ultimately from the public HPAX.decode/2 in lib/hpax.ex. The routine accumulates the running value as int + (value <<< m), where the shift amount m grows by 7 for every continuation octet consumed. The only stopping conditions are the terminating octet and a fallback clause that returns :error on truncated/empty input. Nothing halts decoding because the integer grew too large or because too many octets were consumed: there is no value cap and no octet-count cap.

Because BEAM integers are arbitrary precision, nothing overflows or wraps. Instead, for a run of N continuation octets the decoder builds an integer of O(N) bits, and because it re-adds into an ever-larger bignum on each of the N steps, total arithmetic cost is ≈O(N²). The oversized integer is harmless downstream (an implausibly large length simply fails the binary-size(length) match in decode_binary/1 and decode returns :error); the damage is the CPU and transient memory spent building the bignum before that point.

PoC

  1. Craft a raw HPACK header block fragment: one prefix octet 0xFF (indexed header field, 7-bit prefix all ones, so the integer continues), followed by N continuation octets 0xFF, terminated by a single 0x00.
  2. Pass the fragment to the public API HPAX.decode(payload, HPAX.new(4096)).
  3. Time the decode across increasing N (e.g. 50k, 100k, 200k, 400k octets). Doubling N roughly quadruples the decode time (time / N² stays constant), confirming the superlinear cost; a ~400 KB fragment takes seconds of CPU.

Impact

Any server or application that uses hpax to decode HPACK header blocks from untrusted peers (i.e. any HTTP/2 endpoint reachable by remote clients) is affected. An unauthenticated remote attacker can send small, cheaply-generated header blocks that each consume a large, superlinear amount of server CPU (and transiently memory), degrading or exhausting the service for other users. No privileges or special configuration are required.

References

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

CVE ID

CVE-2026-58226

Weaknesses

Inefficient Algorithmic Complexity

An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached. Learn more on MITRE.

Credits