Summary
Russh: Unchecked keyboard-interactive prompt count in client auth path
In the russh client keyboard-interactive authentication path, a malicious SSH server could send a USERAUTH_INFO_REQUEST with an attacker-controlled prompt count, and the client would use that raw count directly in Vec::with_capacity(...) before validating that enough prompt data was actually present in the packet.
This is a client-side denial-of-service / resource-exhaustion issue on the keyboard-interactive auth path.
Details
The vulnerable code path is in:
russh/src/client/encrypted.rs
When the client is in CurrentRequest::KeyboardInteractive state and receives SSH_MSG_USERAUTH_INFO_REQUEST, it parses:
nameinstructionslanguage tagn_prompts
Before the fix, the code then did:
let n_prompts = map_err!(u32::decode(&mut r))?;
let mut prompts = Vec::with_capacity(n_prompts.try_into().unwrap_or(0));
That means a malicious server could advertise an enormous n_prompts value even if the packet contained no prompt bodies at all.
The fix rejects inconsistent prompt counts before allocating:
let n_prompts = map_err!(u32::decode(&mut r))?;
let max_prompts = r.remaining_len() / 5;
let n_prompts = n_prompts as usize;
if n_prompts > max_prompts {
return Err(crate::Error::Inconsistent.into());
}
let mut prompts = Vec::with_capacity(n_prompts);
Each prompt needs at least 4 bytes of string length plus 1 byte of echo flag, so remaining_len() / 5 is a safe upper bound. If the declared count exceeds what the packet can actually contain, the packet is malformed and is now rejected instead of being silently truncated.
The tester did not find a same-class server-side bug in the reciprocal USERAUTH_INFO_RESPONSE path. The server already bounds the response count by remaining packet length before allocating.
Affected package and versions:
- package:
russh - earliest affected stable:
0.37.0 - confirmed affected current release:
0.60.2
The tester does not believe this issue affects the other crates in this workspace (russh-config, russh-cryptovec, pageant, or russh-util).
PoC
An in-tree regression test was added:
client::tests::oversized_keyboard_interactive_prompt_count_is_rejected
The test builds a client session in WaitingAuthRequest(KeyboardInteractive) state, feeds it a synthetic USERAUTH_INFO_REQUEST packet with:
- normal
name - normal
instructions - empty language tag
n_prompts = u32::MAX- no prompt bodies
On the fixed code, the client rejects the packet with Error::Inconsistent and does not emit a reply to the caller.
For old-code impact verification, the pre-fix path was also checked separately with a constrained-memory repro. On unfixed upstream/main, the same malformed packet attempted a very large allocation and failed with:
memory allocation of 137438953440 bytes failed
Relevant verification commands:
cargo test -p russh oversized_keyboard_interactive_prompt_count_is_rejected -- --nocapture
cargo test -p russh --lib --no-default-features --features ring oversized_keyboard_interactive_prompt_count_is_rejected -- --nocapture
Impact
Suggested CVSS v3.1:
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H- Score:
6.5
Reasoning:
AV:N: reached by a malicious SSH server over the networkAC:L: the packet format is straightforwardPR:N: no prior authentication requiredUI:R: the victim must initiate a connection and proceed into keyboard-interactive authC:N,I:N: Confidentiality or integrity impact were not demonstratedA:H: the server can drive a very large allocation attempt in the client auth path, which can abort or exhaust client-side resources depending on allocator and platform behavior
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
CVE-2026-48107 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.61.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-48107? CVE-2026-48107 is a medium-severity improper input validation vulnerability in russh (rust), affecting versions >= 0.37.0, < 0.61.0. It is fixed in 0.61.0. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- How severe is CVE-2026-48107? CVE-2026-48107 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of russh are affected by CVE-2026-48107? russh (rust) versions >= 0.37.0, < 0.61.0 is affected.
- Is there a fix for CVE-2026-48107? Yes. CVE-2026-48107 is fixed in 0.61.0. Upgrade to this version or later.
- Is CVE-2026-48107 exploitable, and should I be worried? Whether CVE-2026-48107 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-48107 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-48107? Upgrade
russhto 0.61.0 or later.